qt00129 vol2 final

State of Qatar -Public Works Authority Drainage Affairs

Volume 2 Foul Sewerage Page i

1st Edition June 2005 - Copyright Ashghal

CONTENTS

1 Sewerage Systems Design ....................................................................................... 1

1.1 Standards ..................................................................................................................... 2

1.2 Sources of Information .................................................................................................. 2

1.3 Estimation of Flows ....................................................................................................... 3

1.3.1 Domestic ....................................................................................................................... 5

1.3.2 Industrial ........................................................................................................................ 8

1.3.3 Commercial ................................................................................................................... 8

1.3.4 Institutions such as Schools, Health Centres, Hospitals and Mosques ........................ 9

1.3.5 Infiltration ....................................................................................................................... 9

1.4 Peaking Factors .......................................................................................................... 10

1.5 Hydraulic Design ......................................................................................................... 13

1.5.1 Formulae ..................................................................................................................... 13

1.5.2 Minimum Pipe Sizes and Gradients ............................................................................ 16

1.5.3 Minimum and Maximum Velocities .............................................................................. 16

1.6 Septicity in Sewage, Odour Control and Ventilation ................................................... 17

1.6.1 Explosion and Combustion Risk ................................................................................. 18

1.6.2 Corrosion ..................................................................................................................... 18

1.6.3 Impact on Subsequent Treatment Processes ............................................................. 18

1.6.4 Odours ......................................................................................................................... 18

1.6.5 General Design Guidelines for Odour Control in Sewerage Systems ........................ 19

1.7 Pipeline Materials and Jointing ................................................................................... 24

1.8 Pipe Bedding Calculations for Narrow and Wide Trench Conditions .......................... 24

1.8.1 Bedding Design for Rigid Pipes .................................................................................. 25

1.8.2 Bedding Factors .......................................................................................................... 26

1.8.3 Design Strength........................................................................................................... 26

1.9 Manhole Positioning.................................................................................................... 27

1.10 House Connections..................................................................................................... 28

1.11 Construction Depths ................................................................................................... 28

1.12 Manholes, Chambers, Access Covers, and Ladders .................................................. 30

1.12.1 Inspection Chambers .................................................................................................. 30

1.12.2 Sewer System Manholes ............................................................................................ 30

1.12.3 Elements of Design ..................................................................................................... 30

1.13 Industrial Wastes ........................................................................................................ 31

1.14 Septic and Sewage Holding Tanks ............................................................................. 31

1.14.1 Design of Septic Tanks and Soakaways ..................................................................... 32

1.14.2 Sewage Holding Tanks ............................................................................................... 32

1.15 Oil and Grease Interceptors ........................................................................................ 32

1.16 Flow Attenuation Methods .......................................................................................... 32

1.16.1 Flow Controls .............................................................................................................. 33

1.16.2 Attenuation Storage Tanks and Sewers...................................................................... 33

1.17 Abandonment of Sewers............................................................................................. 39


Page ii Volume 2 Foul Sewerage


2 Pumping Stations .................................................................................................... 39

2.1 Standards ................................................................................................................... 39

2.2 Hydraulic Design ......................................................................................................... 39

2.2.1 Hydraulic Principles .................................................................................................... 40

2.2.2 Pump Arrangements ................................................................................................... 41

2.3 Rising Main Design ..................................................................................................... 42

2.3.1 Rising Main Diameters ................................................................................................ 42

2.3.2 Twin Rising Mains ....................................................................................................... 42

2.3.3 Economic Analysis ...................................................................................................... 42

2.3.4 Rising Main Alignment ................................................................................................ 43

2.4 Maximum and Minimum Velocities ............................................................................. 43

2.5 Pipe Materials ............................................................................................................. 43

2.6 Thrust Blocks .............................................................................................................. 43

2.7 Air Valves and Washout Facilities .............................................................................. 44

2.7.1 Air Valves .................................................................................................................... 44

2.7.2 Vented Non-return Valves .......................................................................................... 44

2.7.3 Wash – Outs ............................................................................................................... 44

2.7.4 Isolating Valves ........................................................................................................... 45

2.8 Flow Meters ................................................................................................................ 45

2.8.1 Application and Selection ........................................................................................... 45

2.8.2 Magnetic Flowmeters .................................................................................................. 45

2.8.3 Ultrasonic Flowmeters ................................................................................................ 46

2.9 Surge Protection Measures ........................................................................................ 46

2.10 Screens ....................................................................................................................... 48

2.11 Pumping Station Selection .......................................................................................... 49

2.12 Pumps and Motors ...................................................................................................... 52

2.13 Sump Design .............................................................................................................. 53

2.14 Suction/Delivery Pipework, and Valves ...................................................................... 55

2.15 Pumping System Characteristics ................................................................................ 56

2.16 Sump Pumps and Over-Pumping Facilities ................................................................ 59

2.17 Power Calculations including Standby Generation ..................................................... 59

2.17.1 Introduction ................................................................................................................. 59

2.17.2 Load Type ................................................................................................................... 59

2.17.3 Site condition .............................................................................................................. 60

2.17.4 Generator set operation and control .......................................................................... 60

2.17.5 Type of installation ...................................................................................................... 60

2.17.6 Type of Control Panel ................................................................................................. 60

2.17.7 Ventilation system ....................................................................................................... 60

2.17.8 Fuel system ................................................................................................................ 60

2.17.9 Starting method .......................................................................................................... 61

2.17.10 Service facility ............................................................................................................. 61

2.17.11 Generator set sizing .................................................................................................... 61

2.18 Switch Gear and Control Panels ................................................................................. 65

2.18.1 Type–tested and partially type tested assemblies (TTA and PTTA) .......................... 65


Volume 2 Foul Sewerage Page iii


2.18.2 Total connected load ................................................................................................... 65

2.18.3 Short circuit level ......................................................................................................... 65

2.18.4 Type of co-ordination .................................................................................................. 66

2.18.5 Form of internal separation ......................................................................................... 66

2.18.6 Bus Bar rating.............................................................................................................. 67

2.18.7 Type of starter ............................................................................................................. 67

2.18.8 Protection device ......................................................................................................... 68

2.18.9 Interlocking facility ....................................................................................................... 70

2.18.10 Accessibility ................................................................................................................. 70

2.18.11 Cable entry .................................................................................................................. 70

2.19 PLC’s SCADA/Telemetry ............................................................................................ 70

2.19.1 PLC ............................................................................................................................. 70

2.19.2 RTU ............................................................................................................................. 71

2.19.3 SCADA and Telemetry Systems ................................................................................. 72

2.20 Lighting ....................................................................................................................... 73

2.20.1 Light Fitting Selection Criteria ..................................................................................... 73

2.21 Maintenance Access ................................................................................................... 77

2.22 Gantry Cranes and Lifting Facilities ............................................................................ 77

2.23 Ventilation, Odour Control and Air Conditioning ......................................................... 78

2.23.1 Ventilation .................................................................................................................... 78

2.23.2 Odour Control .............................................................................................................. 79

2.23.3 Air Conditioning ........................................................................................................... 80

2.24 Structural Design ........................................................................................................ 81

2.24.1 Substructures .............................................................................................................. 81

2.24.2 Superstructures ........................................................................................................... 90

2.25 Site Boundary Wall/Fence .......................................................................................... 97

2.26 Site Facilities ............................................................................................................... 97

3 Documentation ........................................................................................................ 98

3.1 Reference Standards .................................................................................................. 98

3.2 House Connection Survey .......................................................................................... 98

3.3 Building Permit ............................................................................................................ 98

4 Health and Safety .................................................................................................... 99

5 Trenchless Technologies ..................................................................................... 100

5.1 Alternative Techniques ............................................................................................. 100

5.1.1 Pipe jacking (Open/Close Face) ............................................................................... 100

5.1.2 Microtunnelling (Closed Face) .................................................................................. 102

5.1.3 Directional drilling ...................................................................................................... 104

5.2 Planning and Selection of Techniques...................................................................... 104

5.2.1 Initial Planning ........................................................................................................... 105

5.2.2 Selection Criteria ....................................................................................................... 110

5.2.3 Factors Affecting Choice Of Method ......................................................................... 110

5.3 Geotechnical Investigations ...................................................................................... 110


Page iv Volume 2 Foul Sewerage


5.3.1 Geological Strata Overview ...................................................................................... 110

5.3.2 Groundwater Regime ................................................................................................ 110

5.3.3 Soil/Rock properties .................................................................................................. 111

5.3.4 Indicative Scope of Interpretative Reporting ............................................................. 113

5.4 Design ....................................................................................................................... 113

5.4.1 Feasibility Study ........................................................................................................ 113

5.4.2 Pipe Design .............................................................................................................. 113

5.4.3 Shaft Design ............................................................................................................. 114

5.4.4 Ground Movements .................................................................................................. 115

5.5 Environmental Assessment ...................................................................................... 117

5.5.1 Vibration .................................................................................................................... 117

5.5.2 Noise ......................................................................................................................... 117

5.5.3 Dust ........................................................................................................................... 118

5.6 Approvals – Procedures and Formats ...................................................................... 118

5.6.1 Guidance for Design Check ...................................................................................... 118

5.7 Risk Assessment ...................................................................................................... 118

5.8 Trenchless Construction References ........................................................................ 122

5.9 Trenchless Construction Glossary ............................................................................ 123

6 References ............................................................................................................. 125


Volume 2 Foul Sewerage Page 1


1 Sewerage Systems Design

This volume of the Manual covers the design of new and existing sewerage systems, detailing the design standards, parameters and approaches to be adopted. However, this information should not be regarded as prescriptive in all situations, as each design needs to be prepared, reviewed and approved by appropriately skilled and experienced staff, both within the designers’ and the Drainage Afairs (DA) organisations.

The sewerage systems in Qatar are separate in that foul sewage, comprising domestic, commercial and industrial effluent is collected in a separate system to that which collects stormwater runoff and ground waters.

The sewerage system for Qatar collects foul flow discharges from premises, located within the developed areas of its towns and cities, and directs the collected flows to the Sewage Treatment Works (STW).

Sewage flows discharge, generally by gravity, into the sewerage system through house connections to the sewer pipelines and manholes outside the property boundary. This network of branch and trunk sewers directs flows by gravity to pumping stations, which pump flows to the STW.

The flat topography of Qatar discourages long lengths of gravity sewer due to the resulting great depths of construction that would be required. The sewerage systems therefore include many pumping stations, with the result that sewage flows will often be pumped several times before arriving at the STW.

The major sewerage systems and STWs are located in Doha, with similar systems in the smaller towns such as Al Khor.

The Doha Catchments

The Doha sewerage system is contained within three catchments, being the Doha West Catchment Area, the Doha South Catchment Area and the Industrial Area. The system in each catchment is similar, in that it comprises networks of sewers and

manholes, directing flows to numerous pumping stations. The flow from each catchment is then pumped to either the Doha South or Doha West STW.

The Doha South Catchment can be broadly defined as that part of Doha being southeast of the Salwa Road and east of the Industrial Area, along with the central business district within the B Ring Road. The Catchment extends southwards to include Abu Hamour, the Airport area and onwards as far as Wakrah, as well as including Wukair and areas to the north and east of the Abu Hamour area.

The extent of the system and the considerable distances over which sewage is transferred across flat terrain, necessitate some 52 sewage pumping stations. The layout of the network results in foul sewage from certain locations being pumped through as many as six or seven pumping stations before reaching Doha South STW.

Development in the catchment is of predominantly low to medium density, with higher densities in the central business district. In total, some 415km2 of land falls within the catchment that it is predicted will be sewered to Doha South STW. In broad terms, only one quarter of this area is presently developed.

The Doha West Catchment comprises some 250km2 of western and northern Doha. The area also includes North Doha, Rural and Urban Rayyan and the Umm Slal Planning Areas of Qatar. The Catchment lands rise from sea level in the east, to some 35m above sea level in the west. The ground level at Doha West STW is about 45m above sea level.

The sewerage network in the Doha West Catchment is served by a terminal pumping station (PS 32) at the south-west edge of the built-up area, from which sewage is delivered in two parallel rising mains to Doha West STW.

Development in the Catchment is of low to medium density, with some areas completely undeveloped. The major future development area is located at the northern end of Doha Bay, where high-density residential and commercial development is planned.

In order to minimise construction, operation and maintenance costs for pumping stations, new designs should use gravity for the movement of


Page 2 Volume 2 Foul Sewerage


sewage flows. However there are the following practical considerations:

• Depth of trench excavation should generally not exceed 5.0m, or 7.5m maximum in extreme cases, dictated by excavator access and pipeline strength, where possible. It is acknowledged that greater depths are often necessary in Doha, but these should be avoided because of the danger of deep excavations and the difficulty of achieving good compaction in the backfill;

• Gradients should not be flatter than the minimum stated herein, to minimise siltation and septicity.

In theory a separate sewerage system should exhibit no increases in flows from rainfall. However, all systems suffer from infiltration to some extent due to faults and openings in the fabric of the system and illegal connections of stormwater collection systems. Many sewerage authorities deal with such flow increases by incorporating overflows which divert foul flows to watercourses at times of rainfall. However such arrangements are impractical for Qatar due to the lack of watercourses operating all year round, and the resulting unacceptable pollution which would result from discharge of foul flows to wadis with little or no flow.

The extent of infiltration is not fully understood in Qatar, but knowledge will improve with ongoing studies and Drainage Area Plans. In the meantime the sewerage system should avoid the need for overflows, with any increased flows being contained within the sewerage system.

The only overflows permitted are for emergency use only, and only to be located at pumping stations. These emergency overflows are only to operate on failure of pumps, through mechanical or electrical breakdown. Pumps are to be rated to pump all flows expected to be received at the station.

All elements of the sewerage system, including pipelines, manholes, chambers, are to be located on publicly owned lands. Pumping stations and associated facilities shall be on DA owned land. Ideally, access for operation and maintenance of the sewerage system should also be located on publicly owned lands. If not, wayleave agreements should be in place to facilitate such access.

1.1 Standards The following standards are of interest to designers in surface water and foul sewerage systems. This list is by no means exhaustive, but is intended as an easy initial reference. (References are also included at the end of this volume). Volume 1, Section 1.5 also contains the complete list of references for all manuals.

• BS EN 752 – Drain and sewer systems outside buildingsi. This supersedes BS 8005ii, which is withdrawn, and part of BS 8301iii.

Part 1: 1996 Generalities and Definitions

Part 2: 1997 Performance Requirements Part 3: 1997 Planning Part 4: 1998 Hydraulic Design and

Environmental Considerations

Part 5: 1998 Rehabilitation

Part 6: 1998 Pumping Installations

Part 7: 1998 Maintenance and Operations

• BS EN 598: 1995 – Ductile iron pipes, fittings, accessories and their joints for sewerage applications – Requirements and test methodsiv.

• BS EN 1610: 1998 – Construction and testing of drains and sewersv.

• Sewers for Adoption – 5th Edition (WRC)vi.

• BS EN124: 1994 Gully tops and manhole tops for vehicular and pedestrian areas – Design requirements, type testing, marking, quality controlvii.

1.2 Sources of Information The following publications are of interest to designers in surface water and foul sewerage systems. This list is by no means exhaustive, but is intended as an easy initial reference. (References are also included at the end of this volume). Volume 1 Section 1.5 also contains the complete list of references for all manuals.




• Department of the Environment National Water Council Standing Technical Committee Reports, 1981,

• Design and analysis of urban storm drainage -

The Wallingford Procedure, National Water Council UK.

• State of Kuwait Ministry of Planning & Hyder Consulting, 2001, Kuwait Stormwater

Masterplan Hydrological Aspects - Final

Report. Cardiff, (AU00109/D1/015), Hyder Consulting.

• Highways Agency, 2002, DMRB Volume 4 Section 2 Part 5 (HA 104/02) – Geotechnics

and Drainage. Chamber pots and gully tops for

road drainage and services: Installation and

maintenance, London, Highways Agency.

• Water Research Council, 1997, Sewerage Detention Tanks – A Design Guide, UK, WRC.

• Construction Industry Research and Information Association, 1996, Report R159: Sea Outfalls – construction, inspection and

repair, London, CIRIA.

• Building Research Establishment, 1991, Soakaway Design, BRE Digest 365, BRE Watford UK.

• HR Wallingford DC Watkins, 1991, Report SR271 -The hydraulic design and performance of soakaways, Wallingford UK.

• Construction Industry Research and Information Association, 1996, Infiltration

Drainage – Manual of Good Practice, London UK, CIRIA.

• Chartered Institution of Water and Environmental Management, 1996, Research and Development in Methods of Soakaway

design, UK, CIWEM.

• Construction Industry Research and Information Association, 2000, C522

Sustainable Urban Drainage Systems – Design

Manual for England and Wales, London UK, CIRIA.

• Construction Industry Research and Information Association, 2001, C523

Sustainable Urban Drainage Systems – Best

Practice Manual for England, Scotland, Wales, and Northern Ireland, London UK, CIRIA.

• Velocity equations for the hydraulic design of pipes – Wallingford Research.

• HR Wallingford and DIH Barr, 2000, Tables for the Hydraulic Design of Pipes, Sewers and

Channels, 7th Edition, Trowbridge, Wiltshire, UK Redwood Books.

• Ministry of Municipal Affairs and Agriculture, 1997, Qatar Highway Design Manual, January 1997, Qatar, MMAA.

• Construction Industry Research and Information Association, 1996, Design of

sewers to control sediment problems, Report 141, London CIRIA.

• Clay Pipe Development Association Limited, 1998, Design and construction of drainage and sewerage systems using vitrified clay pipes, Bucks, UK, CPDA.

• Report for the hydraulic design of pipes – Wallingford Research.

• Construction Industry Research and Information Association, 1998, Report 177, Dry Weather Flows in Sewers, London, CIRIA.

• Water Research Council, 1994, Velocity

equations, UK, WRC.

• Bazaraa, A.S., Ahmed, S., 1991. Rainfall Characterization in an Arid Area, Engineering Journal of Qatar University, Vol. 4, pp35-50.

1.3 Estimation of Flows The flows in a foul sewerage system are made up of contributions from a number of different sources, including: domestic properties; commercial areas; industrial facilities; institutional contributions from hospitals, schools, etc.; groundwater infiltration; and surface run-off. The contributions to the system from each of these sources must be determined before the required hydraulic capacity of the sewerage can be established. Each of these contributions will follow a different diurnal pattern, with flows varying over a 24-hour period. The design of the system must take these fluctuations into account and be




capable of catering for the peak flows likely to be encountered in any 24- hour period. Diurnal flow patterns will be different on working days, from the patterns on rest days.

The starting point for the design of foul sewerage should be the estimation of the average flow rate or the Dry Weather Flow (DWF). This is calculated from the following formula:

DWF = PG + I + E Equation 1.3.1

DWF = dry weather flow (litres/day)

P = population served

G = average per capita domestic water consumption (l/hd/day)

I = Infiltration (l/day)

E = average industrial effluent discharged in 24 hours (l/day)

The process for establishing flow rates should follow the sequence set out below:

1. Define catchment and sub-catchment boundaries for the area under consideration. This should include all the properties and establishments that contribute to the system and may include future developments as well as existing. The catchment represents the entire upstream area contributing to a point or node in the sewerage system. Generally, catchments are taken to contribute to trunk sewers, while sub-catchments contribute to branches. Thus a catchment may comprise a number of sub-catchments.

2. Determine the numbers and types of dwellings within the catchment and from this, determine the existing and future contributing domestic population and hence the flows from that population to the network. Section 1.3.1 gives detailed guidance on this process. Establish the diurnal flow pattern for the domestic contribution.

3. Identify any existing and proposed industrial establishments in the catchment, together with their daily contributing flow and diurnal flow pattern. Section 1.3.2 gives guidance on this.

4. Identify any existing and proposed commercial establishments within the catchment, together with their working populations and diurnal variations. Section 1.3.3 provides detailed guidance on this.

5. Identify any existing and proposed institutional establishments such as schools, health centres, hospitals and mosques that are within the catchment boundary. Determine the usage of these institutions and derive a diurnal flow pattern for them. Section 1.3.4 provides details of this process.

6. Determine infiltration rates into the sewerage system using the methods described in section 1.3.5. These may increase with time or it may be proposed to rehabilitate the system to reduce infiltration.

7. The flows that are likely to occur in the sewerage system can now be estimated. This is done by adding together the total daily contributing flows from each upstream source to any given point in the network. This is usually done sub-catchment by sub-catchment working down the trunk sewer. It can be done graphically and will establish the maximum likely flow that has to be catered for at the given location. The total daily flow from each contributing source is calculated and summed to give a total daily flow through a given point. This flow is then averaged for a 24-hour day to give an average Dry Weather Flow or DWF. The peak flow for design purposes in upper catchment areas can be taken as 6xDWFviii. From the peak flows the required pipe sizes can be determined. However, it should be noted that the peaking factor would decrease in downstream catchment areas (see section 1.4 for information on peaking factors). Hydraulic design is described in section 1.5.




Figure 1.3.1 -Typical chart showing diurnal

variations in domestic sewage

flows

0.00

0.50

1.00

1.50

2.00

2.50

00:00 04:00 08:00 12:00 16:00 20:00 00:00

Hours

Flo

w,

l/s

Where sewerage systems are very long and the time of flow from top to bottom is significant, peak flows will be heavily attenuated. This is because, for example, locally generated domestic flows in the lower parts of the catchment will have passed downstream by the time the flows arrives from the upper areas. This has the effect of smoothing out the peaks in flows.

1.3.1 Domestic

Domestic flows form the largest proportion of flows in foul sewers. They derive from normal domestic appliances such as sinks, basins, toilets, showers, washing machines, baths, etc., and are dependent on the number of persons in a dwelling. In order to determine suitable domestic contributions to the sewerage system, it is necessary to make certain assumptions. For example, each property is assumed to house a certain number of persons, and this will vary from one type of property to another. The assumption is made that all properties of a given type will contain a given number of persons.

Butler and Daviesix suggest that between 75% and 85% of water used in a dwelling in the Middle East is returned to the sewerage system. Thus, if a property is metered, a good assessment of return to sewer flows can be obtained.

Table 1.3.1 below gives the discharge rates that should be used for the design of foul sewerage systems. Discharges in the table below are averaged over 24 hours in the determination of DWF because the application of peaking factors allows for the diurnal profile.

Table 1.3.1 – Typical Daily Discharges in the ME

Development

type

Discharge

l/day

Unit

Domestic 170 Litres/head/day

Domestic low density high value properties

250 Litres/head/day

Average Infiltration 100 Litres/jhead/day

Infiltration range 0- 250 Litres/head/day

The figures in this table provide general guidance for the design of foul sewerage systems.

The figure to be used for design purposes in Qatar where there is no better information is 270l/h/d, comprising 160 l/hd/day or sewage and 110 l/hd/day infiltration.

Where the area to be served is low density palaces and villas consideration should be given to the use of 200 l/head/day. If the catchment is inland and the ground water table level is low then the infiltration allowance can be reduced or even eliminated.

Design populations of the existing and proposed properties are based on the plots indicated on the Action Plans that can be obtained from the Land Information Centre and the occupancy levels given in Table 1.3.2. The number of discharge units per property is then allotted based on BS 8301, as shown in Table 1.3.2.




Table 1.3.2 – Indicative Occupancy Levels (from

BS 8301)

Plot Description Occupancy

Levels

Discharge

Units

For plots less than 1225m2 6 people 14

For plots equal to and between 1225 and 2500m2

9 people 21

For plots greater than 2500m2

Small Palaces

15 people 35

Larger Palaces

25 people 58

The dry weather flow is then obtained from Figure 1.3.2, which has been reproduced from BS 8310, Figure 2. Where no Action Plan plot or housing information is available, the future area can be assumed as developed at an average of the existing planned plot density.




1

10

100

1 10 100 1000 10000

Discharge Units

Flo

w (

l/s)

Figure 1.3.2 – Conversion of Discharge Units to Flow Rates




1.3.2 Industrial

Estimation of daily discharges from Industrial areas will be dependent on the type of industry occupying the area. The majority of industries in Qatar are “dry” industries such as warehousing and workshops. These will have lower consumption rates than “wet” industries such as concrete or paper manufacture. If possible, metered water consumption rates should be used in design but where these are not available or are impractical to use, the values in Table 1.3.3 can be applied.

Table 1.3.3 – Design Allowance for Industrial

Wastewater Generation

Category Volume (l/s/ha)

Conventional Water - Saving

Lightx 2 .5

Mediumx 4 1.5

Heavy 8 2

Category Volume

Slaughterhousexi 6600 l Per tonne of produce

Drink Productionxi

8400 l Per cubic metre of produce

Laundryxiv 1500 – 2100 l/d Per machine

Tanneryxii 30 – 35 m3 Per tonne of produce

Tanneryxi 7600 l Per tonne of produce

In the above table, light industry may be taken as “dry industries which generally handle materials and goods which do not include washdown facilities. Heavy industries will include factories with washdown facilities and using water in the unit processes. These figures are to be used only in initial land usage planning, and developers must obtain confirmation from end users before final design.

1.3.3 Commercial

Most significant developments include a degree of commercial activity and this should be included in the assessment of discharges to the foul system. This activity can range from a single small office or shop, up to major shopping, hotel or office complexes. Each development type needs to be assessed.

Commercial activities include all those listed above and each may have its own characteristic discharge profile, which will inevitably be different from the standard domestic profile.

Table 1.3.4 gives an indication of the likely discharges from various types of commercial activity.

Table 1.3.4 – Typical flows from commercial premises

Development type Discharge

l/day

Per

Commercial Centresxiii

50 Customer per 12 hour day

Airportxiv 11 - 19 Passenger

Hotelsxv 150-300 Bed

Restaurantsxvi 30-40 Customer

Social Clubsxvii 10 – 20 Customer

Cinemaxviii 10 Seat

Officesxix 750 100m2

Shopping Centresxx 400 100m2

Department Storexxi 2000 Per toilet

Recreationalxxii Centres

80 Customer per 6 hour day

Commercial premisesxxiii

300 100m2

Where possible, the above discharge rates should be checked using installed water supply meters for existing developments. Proposed developments




should be assessed using the figurers given in the table above.

Diurnal profiles should be derived for each type of commercial development and applied to the daily discharge rate from the table.

1.3.4 Institutions such as Schools, Health Centres, Hospitals and Mosques

Table 1.3.5 contains typical values of discharges from various types of institutional premises.

Table 1.3.5 – Typical Institutional Discharges

Development

type

Discharge

l/day

Per

Educational Centresxiii

70 Pupil per 8 hour day

Day schoolsxxiii 50 - 100 Pupil per 8 hour day

Residential schoolsxxiv

150-200 Pupil

Mosquexiii 100 Worshiper per 12 hour day

Sports Centrexxiii 10 – 30 visitor

Retirement Homexxiii

250 Bed

Nursing Homesxxiii 300 - 400 Bed

Assembly Hall 11 - 19 Guest

Prison 300 - 570 Inmate

Hospitalsxxiii 500-750 Bed

Each category of premises will have a different diurnal discharge profile, with day schools only contributing during the school day, and hospitals likely to contribute flows for much of the waking day.

As with other types of development, metered water supply records should be consulted wherever possible to provide an indication of actual consumption figures. A suitable return to sewer factor should then be applied to the results. Sometimes, it may be possible to determine diurnal

profiles by reading water meters at say, hourly intervals throughout the day. The resulting profile is then applied to the daily consumption.

1.3.5 Infiltration

Infiltration describes flows in the foul system, which are not legitimate discharges. Infiltration comprises two components:

• inflows from faulty manhole covers, cross-connections from storm and groundwater control systems, and tidal sources. Inflows can also come from the illegitimate practice of lifting manhole covers to drain surface water during and after storms;

• infiltration of groundwater through displaced and open pipe joints, cracks, fractures and breaks in the fabric of the main sewers and lateral connections, manholes and chambers.

Infiltration causes reduced capacity for legitimate sewage flows, increased requirements for pumping and sewage treatment, and possible structural damage.

Infiltration into foul sewerage systems can be problematic. It generally derives from groundwater entering the pipe network through: poor joints in the pipes; cracks or fractures; defects in manholes; or through private drainage connections. Infiltration generally occurs in areas with a high water table. In coastal areas, infiltration can be saline which can have a detrimental effect on sewage treatment processes and can cause corrosion of metalwork in manholes and pumping stations.

It is normal to allow a figure of 10% of DWF for infiltration. Infiltration should be excluded from the calculation of flows using peaking factors. Thus for a peaking factor, Pf, peak design flow would be given by the equation:

Q = Pf (PG + E) + I

Equation 1.3.2

Where: Q = Peak Design Flow (l/d)

Pf = Peaking Factor

P = Population




G = Daily per capita flow (l)

E = Daily Industrial Flow (l)

I = Daily infiltration flow (l)

A sample calculation sheet for sewers using the above formula is included in Volume 1 Appendix 1

Where local conditions indicate that the figure of 10% DWF for infiltration is too low, then a higher figure may be adopted. However, this must be justified by supporting information, such as the analysis of flow survey results. At the time of drafting this manual, DA suggest that for G in the above formula, an overall figure of 270l/hd/day be used for all domestic flows. This will be revised when flow survey results become available .

Conversely, where the water table is known to be well below the level of the sewerage system, the allowance for infiltration will be less significant locally.

Infiltration is often associated with exfiltration, which is the leakage of foul flows due to faults and openings in the pipework, manholes and chambers. Exfiltration of foul flows results in contamination of the surrounding soils and possible pollution of groundwater.

Since both infiltration and exfiltration involve flows passing through physical defects in the sewerage system fabric, they often occur together in conjunction with fluctuating groundwater levels. This continuing flow mechanism can result in erosion of the surrounds and foundations to pipes and manholes. In serious cases, failure of the asset or ground subsidence has resulted.

The Sewer Rehabilitation Manual provides a detailed explanation of the factors involved in infiltration.

Two CIRIA reportsxxv,xxvi describe various methods for estimating base-flow infiltration. Inflow of stormwater runoff is estimated from the area of development contributing to the flow monitor. Estimation of both components relies on detailed flow and rainfall monitoring, combined with hydraulic modelling to understand the relative contributions of the components in wet and dry weather.

The Infiltration Reduction Procedure contained in the Sewerage Rehabilitation Manual should be

followed, where infiltration is to be reduced. This is an iterative approach to successively focus on sources of excessive infiltration, and to ensure that reduction measures are cost-effective.

It is very evident that removal, or more realistically, significant reduction of infiltration, is a time-consuming and expensive process. It is far more cost-effective to avoid its occurrence in the first place. This can be done by strictly controlling the quality of new and renovated sewerage installations, and by ensuring that best quality materials and construction techniques are used, to provide a long-lasting leak-free system. Such standards should be applied to both private and public sewerage. Property connections should also be correctly made, and abandoned sewers and septic tanks properly sealed.

1.4 Peaking Factors As described in section 1.3, the rate of discharge of sewage from a given property to the sewerage system will vary during the day. The sewerage system must be able to cope with the highest flows likely to occur in the day. Different contributors to the system will have different discharge profiles. For example, shopping areas will generally only contribute flows during the periods when the shops are open, and then the flows will be in proportion to how busy the shops are through the day.

Domestic properties generally show marked morning and evening peaks, which coincide with peak domestic activity. This suggests that foul sewers should be designed to cope with higher than the average, or dry weather flow (DWF), and a common way of designing systems is to cope with a flow of up to six times DWFviii. While this approach may be satisfactory for the smaller sewers at the head of the system, it will tend to over design the larger sewers and ignores the attenuation effects as the flows move downstream.

At the head of a sewerage system, discharges tend to be pulsed, with individual pulses of flow being the discharge from individual appliances. As the pulses flow along the pipe system, the peaks tend to become attenuated and as the flows progress down the system, these pulses combine to form a more consistent flow. The peaking factor will depend on the upstream population and the distance the




sewage has travelled. A number of different ways of determining the peak factor have been proposed which take account of the attenuation downstream with increasing population. There are several formulae for calculation of peaking factorix, for which the Babbit formula is most representative in Qatar

The Babbit Formual (1952) is;

5

5

PPF = ,

Where PF represents the peaking factor, and P is the population in thousands.

However, the formula is not representative at low populations.

Therefore, the upper limit for peaking factors shall be taken as six for populations up to and including 500.. For populations over 500 the Babbit formula shall be used. The minimum value of peaking factor shall be 3.

It is considered that values in excess of six, and below three, are unrealistic for conditions in Doha, but these figures may be revised after a detailed flow survey is carried out (see section 1.3 above).

The variation of peaking factors with population is shown graphically in Figure 1.4.1, which follows




Peaking Factors

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

100 200 500 1000 2000 5000 10000 20000Population

Fa

cto

r

Babbit BSEN 752

Minimum value 3

Maximum value 6

Figure 1.4.1 Plot of Peaking Factors v Population




1.5 Hydraulic Design The hydraulic design of sewerage systems involves achieving a balance between pipe size, pipe gradient and pipe depth, such that self-cleansing velocities are achieved without surcharge, but with the most economical combination of size and depth. Wherever possible, pipe depths should be used that avoid the need for concrete bed and surround.

General Principles

The general principles of foul sewer design are as follows:

• Pipe size should not generally decrease downstream;

• Sewers should be designed to convey peak flows without surcharge;

• Sewers should achieve self-cleansing velocity at least once per day. Note that half-pipe velocity is numerically the same as full-pipe velocity.

• To allow for ventilation of the system, maximum design depth of flow should not exceed 0.75 x pipe diameter or d/D ≤ 0.75.

• Where there is a chance of heavy construction plant tracking over new sewers laid during construction of a site, the minimum depth of cover should be measured from the formation level of the site above the sewers;

• Self-cleansing velocities increase with pipe size (see sections 1.5.1 and 1.5.2 below);

• At manholes, all pipes should be laid such that their soffits are at the same level. Pipes in manholes should not be laid with the inverts level, as this can promote the deposit of solids in minor branches leading to odour problems and blockages;

• Junctions should not enter a sewer at right angles but should enter at an angle of 45° to the direction of flow of the main sewer;

• Sewers should commence at minimum depth upstream and follow ground profiles if possible to minimise excavation. However, it is recognised that in Qatar, due to flat topography, depths will gradually increase

downstream in order to maintain minimum gradients (see section 1.5.1 below). Trunk sewer sections serving larger catchments are likely to become very deep (but see also section 1.11);

• Backdrop manholes should be used where there is a difference >600mm in level between a branch/rider sewer and the main sewer. Backdrops (see also section 1.12.2 below) should not be used to reduce gradients on main sewer lines.

Design Tools

Hydraulic computer models are invaluable tools for understanding the performance of sewerage systems. Hydraulic models are of particular value for:

• Understanding the performance of the complete system, in particular attenuation of flows;

• Understanding the flow regime of complex and interdependent systems, such as those with bifurcations and loops;

• Understanding the flow characteristics of multiple pumping systems, as found in Doha;

• Readily understanding the effects of changes in development on existing systems;

• Simulating modifications to the construction and/or operation of the system.

Hydraulic computer models should use InfoWorks CS software, and be verified against flow and depth measurements carried out on the actual system.

1.5.1 Formulae

1.5.1.1 The Colebrook-White Equation

The Colebrook-White equation allows calculation of velocity of flow in a gravity drain flowing full for any given gradient, diameter, and roughness coefficient, as follows;




( )( )

+−=

gDSDD

kgDSv s

2

51.2

7.3log22

υ

Equation 1.5.1

Where g = acceleration due to gravity, m2/s

D = diameter, m

S = slope or headloss per unit length

sk = roughness coefficient, mm

υ = kinematic viscosity of water (m2/s).

Thus, for a 400 mm diameter pipe with sk = 1.5 ,

and slope 1 in 157, flow temperature 15oC, the velocity will be 1.33 m/s

Using the relationship:

Q=AV

Equation 1.5.2

Where:

Q = flow in the pipe (m3/s)

A = Cross-sectional area of flow

V = velocity of flow

This allows the pipe full discharge to be calculated where:

A=πD2/4

Equation1.5.3 Thus, for the above pipe at full flow, the capacity will be 167 l/s

A sample calculation sheet for sewers using the above formulae is included in Volume 1 Appendix 1

Tables are available from hydraulic research giving values for a wide range of pipe sizes at a range of gradients for various values of ks.

Tables 1.5.1 and 1.5.2 below give recommended values of ks and υ . Both are taken for the Slimed sewers category from Wallingford design tablesxxvii.

Table 1.5.1 - Pipe Roughness ks Values

Material ks Value (mm)

Normal Poor

Concrete (Precast + O Rings) 3.0 6.0

Concrete (Steel Forms) 3.0 6.0

DI (PE Lined) 0.6 1.5

GRP 0.6 1.5

VCP 1.5 3.0

Further values can be obtained by direct reference to Appendix 1 of the Wallingford design tables.

Caution should be exercised in the use of the Wallingford tables. It should be noted that the quality of pipes can vary considerably from one manufacturer to the next, and that condition of pipes can vary with time. Designers should avoid using the optimistic values quoted by some plastic pipe manufacturers, as these invariably refer to new pipes under laboratory conditions. The figure to be used for design purposes shall be 1.5 in all cases

Table 1.5.2 - Kinematic Viscosity υ Values

Temperature, 0C Viscosity, m2/s x 10-6

15 1.141

25 0.897

35 0.727

For detailed sewage modelling applications, the viscosity should be varied for a range of temperatures, but for routine applications, a conservative approach will be to use the lower temperature of 150C.

A graph for proportional velocity and discharge in part-full circular sections is reproduced in Figure 1.5.1. This illustrates the relationship between depth of flow, and velocity. It can be used for estimating the velocity of flow in partially full pipes, and should be used to check velocities for self cleansing velocities at low flow (see table 1.5.4)




Figure 1.5.1 - Proportional Velocity and Discharge in Part-Full Circular Sections




1.5.1.2 Manning’s Equation

Manning’s equation is an empirical formula for uniform flow in open channels. Manning’s equation is:

v=(1/n)R2/3S0½

Equation 1.5.4

Where: n is Manning’s roughness coefficient S0 is bed slope R is the hydraulic radius of the flow

The equation may be useful in pumping station approach hannels and elements of sewage works. However, all pipe calculations must use Colebrook White

Typical values of Manning’s n are given below.

Table 1.5.3 - Typical values of Manning’s n

Channel Material n range

Cement 0.010-0.015

Concrete 0.010-0.020

Brickwork 0.011-0.018

Manning’s equation is only valid for rough turbulent flow conditions.

1.5.2 Minimum Pipe Sizes and Gradients

CIRIA Report R141xxviii defines self-cleansing sewers as follows:

“An efficient self-cleansing sewer is one having a sediment-transporting capacity that is sufficient to maintain a balance between the amounts of deposition and erosion, with a time-averaged depth of sediment deposit that minimises the combined costs of construction, operation and maintenance”.

Foul sewers should be at least 200mm diameter and laid to a gradient of 1 in 60 or 1.67%. This gradient can be relaxed to 1 in 150 (0.67%) where several dwellings are connected to the head of the sewer, and the standard of workmanship during construction is high. Peak flow velocities of

0.75m/sec can be assumed to be self-cleansing in pipes of 150mm diameter.

As sewer sizes increase, so too do self-cleansing velocities, with the result that very large foul sewers require velocities to exceed 1.5m/sec to be self-cleansing. Such velocities in large diameter pipes pose a safety hazard and facilities must be provided to prevent operatives being washed downstream in these sewers.

1.5.3 Minimum and Maximum Velocities

CIRIAxxvi recommends that sewers should be designed to:

1. transport a minimum concentration of fine particles in suspension.

2. transport coarser granular material as bed load.

3. erode cohesive particles from a deposited bed.

In order to minimise the maintenance requirements of any given length of sewer, it is normal to design the sewer to be “self-cleansing”. This means that the sewer is designed to achieve a velocity at least once per day that will carry all solid deposited material along the pipe and not leave any materials deposited in the invert of the sewer.

Table 1.5.4 is based on the simplified CIRIA method of assessing self-cleansing velocities in foul sewers. Surface water sewers require generally higher self-cleansing velocities because of the higher particle densities.

Table 1.5.4 – Approximate Self-Cleansing

Velocities for Foul Sewers

Pipe size

(mm)

Approximate self- cleansing

velocity (m/sec)

200–300 0.75

400 0.77

500 0.82

600 0.86

700* 0.87




Pipe size

(mm)

Approximate self- cleansing

velocity (m/sec)

800 0.88

900* 0.88

1000 0.92

1200 1.03

*700 and 900 are non preferred sizes

Where large diameter sewers (over 1.0m diameter) are laid to steep gradients, very high flow velocities occur. For example,; a 1000mm pipe laid to 1:100 gradient with a depth of flow of 750mm will have a discharge velocity approaching 3.4m/sec, which is unacceptable in foul sewers. The designer should implement energy dissipation measures in such cases. It should be emphasised that scour in pipes at these velocities is not a significant problem with modern materials, but if velocities become very high, odour emissions can be increased and noise can become a problem.

As a general rule, it is preferable to aim to achieve self-cleansing velocity at least once per day. The designer should aim to achieve a velocity at the design flow (i.e. peak flow) of between self-cleansing and 2.0m/s, with 2.5m/s as an upper limit.

In small sewers, less than 600mm diameter, it is not necessary to include measures to limit flow velocity. The use of backdrop manholes for this purpose is discouraged. However, backdrop manholes may be justified where there is a significant difference in level between a branch sewer and trunk sewer. In this case, the economics may justify the construction of a backdrop to minimise excavation for the branch sewer trench. The discharge from a backdrop into a manhole requires careful design to prevent flows from washing over the benching.

Backdrops for large diameter sewers are complex structures that may involve the creation of vortices to dissipate energy, for which specialist design is required. These are often purpose-made in stainless steel. A typical example is included in the standard drawings, Volume 8.

1.6 Septicity in Sewage, Odour Control and Ventilation

In rising mains and shallow gravity sewers, respiration of bacteria in wastewater and slimes present on submerged sewer walls rapidly depletes any dissolved oxygen or nitrates causing anaerobicity (septicity)xxix. One of the main impacts of septicity is the formation of sulphide by the bacterial reduction of inorganic sulphate present in the wastewater. Some of the sulphide will combine with metals in the sewage. The remainder will be present in ionised and unionised form, as below.

S2- ⇔ HS- ⇔ H2S

Only the un-ionised form is released to the atmosphere. The proportion of sulphide present in the un-ionised form is dependent upon the pH value of the sewage and is about 50% at a pH value of 7. For example, a liquid concentration of 2mg/l of sulphide at pH 7.0 would be in equilibrium with a gaseous H2S concentration of 300ppm (ml/m3). At a pH value of 8.0 this would decrease to about 60ppm.

Septicity can have an impact on health and safety, corrosion, subsequent treatment processes and odours. Hydrogen sulphide is a toxic gas. WHO guidelines for dose-effect relationships for H2S are given in Table 1.6.1xxx.

Table 1.6.1 - Health Impacts of Hydrogen

Sulphide

H2S Level

(ppm)

Health Impact

1000-2000 Immediate collapse with paralysis of respiration

530-1000 Strong central nervous system stimulation, followed by respiratory arrest

320-530 Risk of death

150-250 Loss of olfactory sense

50-100 Serious eye damage

10-20 Threshold for eye irritation




UK Occupational exposure limit (OEL) concentrationsxxxi of hydrogen sulphide and other gases associated with septic conditions are given in Table 1.6.2.

Table 1.6.2 - Exposure Limits for H2S and Other

Gases

Gas Long term OEL (8-hour) (parts per million)

Short term OEL (15

minute) (parts per million)

Hydrogen sulphide 5 10

Methyl Mercaptan (methanethiol)

0.5 -

Ethylmercaptan (ethanethiol)

0.5 2

Ammonia 25 35

Methylamine 10 -

Ethylamine 10 -

Dimethylamine 10 -

1.6.1 Explosion and Combustion Risk

The WRC report ‘Enclosed Wastewater Treatment Plants’xxxii considers the potential risk of the development of flammable concentrations of gases arising in a STW. The possible gases considered are given below in Table 1.6.3.

The lower explosive limit for hydrogen sulphide is 40000ppm. This concentration is unlikely to be achieved under normal operation, and risk is therefore minimal.

Table 1.6.3 - Flammable Gases in Sewers

Gas Lower explosive limit % v/v in air

Upper explosive limit % v/v in air

Carbon Monoxide

12.5 -

Hydrogen sulphide

4.0 (40000 ppm) 46

Petroleum 100 ppm

Methane 5.3 15

Spontaneous combustion of sulphur around the edge of lifted manhole covers has been reported in Doha. In this instance, the reaction of hydrogen sulphide with iron oxide at the manhole cover has led to its catalytic oxidation to sulphur, which is spontaneously combustible. Operational procedures may be required to reduce this risk. Although these are beyond the scope of normal design functions, it is important that the designer is aware of such issues and to include mention of them in the design HARA’s.

1.6.2 Corrosion

Hydrogen sulphide is associated with the corrosion of concrete and mortar as the result of its bacterial conversion to sulphuric acid. High levels of hydrogen sulphide may develop below covers, with consequent impact on the structure of the tank or manhole as has been found at a number of sites. Metal work and electrical equipment is also vulnerable to H2S corrosion.

Selection of construction materials for tanks, manholes, covers, and fittings should take into account the potential for corrosion.

1.6.3 Impact on Subsequent Treatment Processes

The discharge of septic sewage can increase the rate of development of sulphide in the primary sedimentation stage sewage and sludges.

High levels of septicity have been associated with poor settleability of activated sludges.

High levels of septicity or sulphates have been associated with poor gas yields from mesophilic anaerobic digestion processes.

1.6.4 Odours

The discharge of septic sewage can be a significant source of odours at the discharge point, whether to an intermediate pumping station or to the inlet of a STW. Threshold levels for various odours are listed in Table 1.6.4.

The odour threshold level of hydrogen sulphide measured in a laboratory is about 0.5 parts per billion (ppb). The level above which odour problems




can occur is typically ten times this value. Background H2S levels are often in the range 2-5ppb.

Table 1.6.4 – Odour Threshold Levels

Gas Odour threshold (parts per billion)

Hydrogen sulphide 0.5

Methyl Mercaptan (methanethiol)

0.0014-18

Ethylmercaptan (ethanethiol)

0.02

Ammonia 130-15300

Methylamine 0.9-53

Ethylamine 2400

Dimethylamine 23-80

1.6.5 General Design Guidelines for Odour Control in Sewerage Systems

The design of sewerage systems to reduce the development of septicity is the subject of a number of guidesxxxiii. Guidelines include:

Rising mains

• Minimise lengths of pumping mains, and use lift pumps rather than long rising mains to minimise retention under anaerobic conditions( there is no satisfactory minimum length of rising main which can be quoted for design purposes. Even a retention time of 30 minutes is sufficient to develop anaerobic conditions. );

• Minimise turbulence at the discharge point;

• Discharge into the gravity sewerage system at low level to avoid turbulence and consequent release of odours;

• Location of discharge point should NOT be immediately prior to hydraulic drops or sharp bends;

• Manhole covers at discharge points may need to be sealed.

Pumping stations

• Minimise turbulence at inlet to sump. Use submerged, rather than overflow weirs;

• Use level detectors to minimise the volume of sump used under normal flow conditions;

• Use frequent pumping regimes to minimise retention time in sump, and also spread odour load more thinly over the day;

• Maximise benching to give self-cleansing conditions and ensure no accumulation of grit. Guidelines are given in BS 8301xxxiv;

• Ensure any screenings or grit can be removed, or are washed back into main flow of sewage;

• Active/passive odour control unit may be required depending on the sensitivity of the site, size of installation, and other factors such




as degree of septicity of sewage under normal flow conditions.

Gravity sewers

• Maintain self-cleansing velocities;

• Avoid turbulent flow (including sharp bends and drops);

• Minimise length of siphons (which will act as rising mains);

• Ensure there is ventilation of the sewer (by provision of vents);

• Design to ensure no accumulation of grit or debris.

Storage Tanks and Shafts

• Minimise turbulence of discharges to tanks and shafts (discharge at low level);

• In sensitive areas (i.e. next to houses) odour control may be needed to treat displaced odours when levels rise.

Refer also to section 2.23 of this volume, and section 1.5 of volume 5

The formation of sulphide in rising mains and gravity sewers has been the subject of extensive studies xxxv, xxxiii.

The concentration can be estimated from the following equationxxxvi:

Cs=K tCOD[(1+0.004D)/D]1.07(T-20)

Equation 1.6.1

Where:

Cs = concentration of sulphide (mg S/l) Kc = constant, usually taken to be 0.00152 t = anaerobic retention time (mins) D = diameter of rising main (cm) T = temperature of sewage (°C) COD = COD of sewage (mg/l) In gravity sewers, there is a balance between sulphide formation when flow is stagnant, and sulphide release and oxidation during turbulent flow. In practice, little sulphide should be formed in a well-ventilated, self-cleansing, partially-filled gravity sewer used for domestic sewage. Where problems do occur, they are typically associated with sewers of shallow gradients where accumulation of grit, silt

and slimes causes localised septicity at points where turbulence is insufficient to remove such debris.

An indicator of the likelihood of septicity in a gravity sewer is the ‘Z formula’ with the effect of different values of Z as indicated in Table 1.6.5.




Z as calculated below is a dimensionless parameter that indicates the potential for sulphide generation.

Z = 3(EBOD) x P

S0.5Q0.33 b

Equation 1.6.2

Where:

EBOD = 5 day BOD (mg/l) multiplied by a temperature factor 1.07 (T-20)

T = sewage temperature (co) S = slope of sewer (m/100m) Q = wastewater flow (l/s) P = wetted pipe wall perimeter (m) b = surface width of the stream (m)

Table 1.6.5 - Values of Z, Indicating Sulphide

Generation Potential

Value of Z

Likely condition

<5000 Sulphide rarely present

7500 Low concentrations of sulphide may be produced

10000 Sulphide may develop sufficiently to cause odour and corrosion problems

15000 Frequent problems with odours and significant corrosion problems

More detailed equations have been developed by Pomeroy, Jensen and others for gravity sewers, linking re-aeration rates with sulphide formation release and oxidation. These are incorporated in a computer model (SPACA) developed by Hyder Consulting. Modelling can enable the areas where septicity develops to be identified allowing effective targeted remedial measures to be taken.

The model divides the sewerage network into a series of nodes (for example junctions or manholes) and pipes (gravity or pumped). Details of the sewers are required (length, slope and diameter) and the wastewater (including flow rate, COD and pH value). The model calculates the amount of sulphide produced or lost along each section and carries out a mass balance across the system. The model also calculates the amount of chemicals required to prevent septicity.

Sewerage Septicity Investigations

Where a sewerage system is already in use, site sampling can be carried out at the pumping station and at the discharge point. Measurements that should be made are:

• COD; • BOD; • Temperature; • Flow rate of wastewater; • pH value; • Respiration rate of sewage; • Sulphide (liquid and gas phase); • Redox potential; • Dissolved oxygen concentration; • Chloride concentration (or conductivity).

In addition, modelling would require information on:

• Length, material and diameter of rising mains; • Length, material, diameter and slope of gravity

sewer; • Pumping regime/flow profile; • Details of receiving sewer; • Location and odour control arrangements for

manholes and chambers; • Ventilation of house connections; • Design horizon.

Septicity Control Using Chemicals

DA policy is to introduce septicity control facilities at all new pumping stations as required.

It should be stressed that septicity control using chemicals is only acceptable in Qatar if no other methods are suitable.

Addition of chemicals is used to prevent odour problems in the sewerage system, at the STW inlet works and in sludge systems. The annual cost of chemicals can be significant, and optimisation of dose rate should be carried out, e.g. dosing may not be necessary during cooler weather. In addition to the cost, the chemical should be selected with consideration given to the subsequent treatment of the sewage, e.g. disinfection by UV irradiation may be affected by residual iron in the effluent. Iron salts are also reported to increase combustibility of dried sludge.

Many of the chemicals used, such as iron salts, high purity oxygen, alkali and oxidising agents such as permanganates and hydrogen peroxide are potentially hazardous. Appropriate precautions are required in their handling and storage, such as bunded tanks, eyewashes or safety showers.




Measures are also required to ensure that chemicals do not deteriorate during storage, e.g. due to exposure to sunlight, moisture, or heat.

The chemicals most commonly used for septicity control in the sewerage system or receiving wastewater treatment works are:

Calcium Nitrate is used widely to prevent septicity in sewerage systems. Micro-organisms present in the sewage and in the slimes on the sewer wall will use nitrate as an alternative oxygen source under anoxic conditions. Sodium nitrate can also be used.

High doses of nitrate can be added at the start of the sewerage system, and there will be no loss along the sewerage system. Excess nitrate may lead to rising sludge due to denitrification in the primary sludge.

Some oxidation of sulphide in sewage and sludges may also be achieved by nitrate addition. A required ratio of between 10:1 to 30:1 of nitrate to sulphide has been reported. Addition of nitrate with anthroquinone has recently been proposed to oxidise sulphide in sludges.

Nitrate salts are supplied and stored as a liquid and dosed as a liquid to the pump sump at the start of a rising main. Average daily dose rates are calculated from the aerobic respiration of the sewage, but assume that the rate of nitrate uptake is 40% of that under fully aerobic conditions. The amount of nitrate required for rising mains of different diameters is given in Table 1.6.6. These values are derived assuming that the demand for nitrate nitrogen is 40% of that derived previously; that 2.85 grams of oxygen are available for every gram of nitrate nitrogen, and that calcium nitrate is supplied at a concentration of 110.6g/l N.

The uptake of nitrate results in a slight reduction in BOD. If sufficient nitrate is provided, the sewage will remain fresh.

Table 1.6.6 - Nitrate Dosing Requirements for

Different Pipe Diameters

Diameter(mm)

Nitrate required per 1000 m length

(kg/d as N) l/d assuming 110.6gN/l

350 11.0 99.9

500 18.2 164.2

1000 52.2 471.9

Iron nitrate acts in the same way as calcium nitrate when dosed at the start of a rising main. The iron component also combines with sulphides as they form, and hence dosage rates in practice may be approximately half that calculated for calcium nitrate.

Iron nitrate is, as with other iron salts, an acidic chemical requiring appropriate storage and handling.

Iron salts (sulphate, chloride and nitrate) have been used very effectively to control odours. Iron salts combine with sulphide in the sewage to form a number of insoluble iron sulphides (FeS, Fe2S3, Fe3S4 and FeS2). Ferric salts are more effective than ferrous salts. However a mixture of ferric and ferrous salts in the presence of dissolved oxygen may be the most effectivexxxv:

Fe2_ + 2Fe3+ + 4HS- → Fe3S4 + 4H+

The required dose rate decreases with increasing pH value and increases at acidic pH values, with little effect expected at pH values much below 6. At pH 7.0, the dose rate is 2.4mgFe / mgS.

Iron salts are added as a liquid at the discharge point of a rising main or to a septic flow, such as sludge liquor prior to return to the sewage flow. Dosage rates for rising mains containing sewage at pH 7.0, temperature 30oC, with a COD of 600mg/l are given in Table 1.6.7.

Table 1.6.7 – Dosing Rates for Iron Salts

Diameter (mm)

Iron required per 1000m length, assuming 2.4mg/l as Fe

(kg/d as Fe)

350 19.4

500 29.2

1000 68.2

Iron salts are acidic and corrosive and require care with storage and handling. Iron salts attack metals,




and appropriate materials are required for bunded tanks, dosing pumps and pipework, together with appropriate safety equipment such as safety showers and eyewashes.

Although effective at precipitating sulphide, iron salts have no impact on the concentration of other odorous chemicals such as volatile fatty acids or on the degree of septicity of the sewage. They therefore may be less effective than septicity prevention systems for reducing odour.

Addition of iron salts to sewage may:

• Increase the mass, volume and thickness of primary sludge;

• Reduce concentration of phosphate below the required concentration for secondary treatment;

• High doses may adversely affect the settleability of the primary sludge;

• Give some solids deposition within the sewer;

• Affect subsequent ultraviolet disinfection processes;

• Increase the combustibility in subsequent thermal drying processes.

Oxygen supplied and stored as a liquid and then dosed into a side stream of sewage as a high purity gas has been used in rising mains and sewers elsewhere in the Middle East to prevent septicity. However, the amount that can be dosed is limited by the saturation concentration of dissolved oxygen, being about 34mg/l at 30oC. The injection of excess oxygen or air into rising main sewers can give rise to gas pockets, which may adversely affect pump regimes. Excess oxygen also exacerbates microbiologically induced corrosion.

Under aerobic conditions, sulphide will be oxidised (predominantly by microbial action) to thiosulphate and sulphuric acid, with some chemical oxidation to sulphur. The rate of oxidation in the sewage stream depends on the numbers of oxidising bacteria present in the sewage and can be in the range of 1 (fresh sewage) to 15mgS/l.h. Some reduction of BOD and COD is seen. Oxidation can occur within the sewage stream, where it will reduce the risk of subsequent odour problems. Where the oxidation

to sulphuric acid occurs in the slimes on exposed sewer walls or sumps, corrosion of the sewer fabric can occur.

The uptake of oxygen results in a corresponding reduction in BOD. If sufficient oxygen is provided, the sewage will remain fresh.

Dose rate is calculated to match the respiration rate of micro-organisms in sewage (typically 12mg/l.h) and wall slimes (assumed to be 1.9g/m2.h at 30oC). This can be calculated for a length of rising main of radius r metres and length, L metres:

gO2/h = ((2prLx 1.9)+ (pr2Lx12))1.07(T-30)

Equation 1.6.3

Overall respiration rate (mg/l) of sewage and slimes in rising mains of different diameters is given in Table 1.6.8.

Table 1.6.8 - Respiration Rates of Sewage and

Slimes

Pipe

diameter

(mm)

Respiration rates (mg/l.h at 30oC)

Total DO demand rate mgO2/l.h Slimes Wastewater

350 22.1 12 34.1

500 15.5 12 37.5

1000 13.7 12 15.7

The amount of oxygen required per 1000m for mains of different diameters using the above respiration rates is given in Table 1.6.9.

Table 1.6.9 - Oxygen requirements

Diameter (mm)

Oxygen required per 1000m length (kg/d)

350 78.7

500 129.4

1000 371.9




The concentration of oxygen at the injection point is determined by dividing the daily oxygen requirement by the daily flow rate of sewage. The maximum amount of oxygen that can be injected is limited by its saturation concentration (about 34mg/l at 30oC and atmospheric pressure).

Alkali addition, such as lime or caustic soda, can be used to increase the pH value of sewage or sludge. At raised pH values, the release of H2S, other acidic sulphurous compounds and volatile fatty acids will be suppressed. Addition is generally to pH 8.0 or 8.5, as higher values can lead to a significant release of ammonia. Use of alkali will become less effective if dosed sewage is diluted with neutral sewage downstream.

Chlorine/hypochlorite acts as an oxidant and also as a bactericide, reducing oxygen demand, and slime growth, but is not widely used because of the high chlorine demand of the sewage, and the reluctance to add chlorine to the sewage flow.

Hypochlorite is used as an oxidant in wet-scrubbing of odorous air:

HS- + 4Cl2 + 4H20 → SO42- + 9H+ + 8Cl-

HS- + Cl2 → S + H+ + 2Cl-

Hydrogen peroxide oxidises previously formed sulphide to sulphur and water, and provides dissolved oxygen.

Peroxide dosed at the inlet to the rising main provides dissolved oxygen to satisfy the oxygen demand of the sewage and slimes. Dosage rates can be calculated as for oxygen, with available oxygen calculated as 0.48gO2 per gH2O2.

Peroxide can be dosed at the top of the rising main to oxidise sulphide present in the sewage. Dose rate is assumed to be 1 gram of H2O2 per gram of S oxidised at a pH value of less than 8.5.

Chlorine dioxide has been used as an oxidising agent, mainly with sludges and sludge liquors.

Potassium permanganate has been used successfully as an oxidising agent to reduce sulphide levels in sludge liquors and sludges.

Enzyme based chemicals have been promoted for septicity control. These appear to act by reducing fat levels or by inhibiting the activity of sulphate reducing bacteria. Their effectiveness is very site specific, and long-term effectiveness is not known.

1.7 Pipeline Materials and Jointing

The preferred material for use in gravity foul sewers (in Qatar) is vitrified clay pipe (VC), up to 1000mm diameter.

VC pipes are manufactured to 1200 dia in the Middle East. However, Glass Reinforced Plastic (GRP) is preferred for diameters in excess of 1000mm.

uPVC is not acceptable on DA projects.

HDPE is not preferred, but may be used as a sliplining where trenchless methods (see section 5) are necessary for installation, using concrete jacking pipes. Such instances may occur because the high strength concrete pipes necessary for withstanding jacking forces do not have adequate chemical resistance to withstand the aggressive nature of the sewage. The concrete pipe thus provides the required strength, and the lining is chemically resistant.

All materials and jointing should be specified in accordance with QNBS. See also Volume 1 section 4.3.

1.8 Pipe Bedding Calculations for Narrow and Wide Trench Conditions

Pipes can be categorised as rigid, flexible and intermediate:

(a) Rigid pipes support loads in the ground by virtue of resistance of the pipe wall as a ring in bending;

(b) Flexible pipes rely on the horizontal thrust from the surrounding soil to enable them to resist vertical loads without excessive deformation;

(c) Intermediate or semi-rigid pipes are those pipes that exhibit behaviour between those in (a) and (b).

Vitrified clay pipes are examples of rigid pipes while steel, ductile iron, UPVC, MDPE and HDPE pipes may be classified as flexible or intermediate pipes,




depending on their wall thickness and stiffness of pipe material.

The load on rigid pipes is concentrated at the top and bottom of the pipe, thus creating bending moments. Flexible pipes may change shape by deflection and transfer part of the vertical load into horizontal or radial thrusts, which are resisted by passive pressure of the surrounding soil. The load on flexible pipes is mainly compressive force, which is resisted by arch action rather than ring bending.

The loads on buried gravity pipelines are as follows:

(a) The first type comprises loading due to the fill in which the pipeline is buried, static and moving traffic loads superimposed on the surface of the fill, and water load in the pipeline;

(b) The second type of load includes those loads due to relative movements of pipes and soil caused by seasonal groundwater variations, ground subsidence, temperature change and differential settlement along the pipeline.

Loads of the first type should be considered in the design of both the longitudinal section and cross section of the pipeline. Provided the longitudinal support is continuous, has uniform quality, and the pipes are properly laid and jointed, it is sufficient to design for the cross section of the pipeline.

In general, loads of the second type are not readily calculable and they only affect the longitudinal integrity of the pipeline. Differential settlement is of primary concern especially for pipelines to be laid in newly reclaimed areas. The effect of differential settlement can be catered for by using either flexible joints or piled foundations. If the pipeline is partly or wholly submerged, there is also a need to check the effect of flotation of the empty pipeline.

The design criteria for the structural design of rigid pipes is the maximum load at which failure occurs while those for flexible pipes are the maximum acceptable deformation and/or the buckling load. The approach for rigid pipes is not applicable to flexible pipes. For all DA projects, the designer must refer in the first instance to the manufacturer’s literature, to ensure that the design is in compliance with recommendations.

Please refer to Volume 1 Appendix1

1.8.1 Bedding Design for Rigid Pipes

The design procedures for rigid pipes are outlined as:

(a) Determine the total design load due to:

• the fill load, which is influenced by the conditions under which the pipe is installed, i.e. narrow or wide trench conditions;

• the superimposed load which can be uniformly distributed or concentrated traffic loads; and

• the water load in the pipe.

(b) Choose the type of bedding (whether granular, plain or reinforced concrete) on which the pipe will rest. Apply the appropriate bedding factor and determine the minimum ultimate strength of the pipe to take the total design load;

(c) Select a pipe of appropriate grade or strength.

Details of design calculations, tables, etc, are contained in Appendix 1, Volume 1 - General.

1.8.1.1 Narrow Trench Conditions

When a pipe is laid in a relatively narrow trench in undisturbed ground and the backfill is properly compacted, the backfill will settle relative to the undisturbed ground and the weight of fill is jointly supported by the pipe and the shearing friction forces acting upwards along the trench walls. The load on the pipe would be less than the weight of the backfill on it and will be determined as in ‘narrow trench’ conditions.

1.8.1.2 Wide Trench Conditions

When the pipe is laid on a firm surface and then covered with fill, the fill directly above the pipe yields less than the fill on the sides. Shearing friction forces acting downwards are set up, resulting in the vertical load transmitted to the pipe being in excess of that due to the weight of the fill directly above the pipe. The load on the pipe will then be determined as in ‘wide trench’ condition.




1.8.2 Bedding Factors

The strength of a precast concrete or vitrified clay pipe is given by the standard crushing test. When the pipe is installed under fill and supported on bedding, the distribution of loads is different from that of the standard crushing test. The load required to produce failure of a pipe in the ground is higher than the load required to produce failure in the standard crushing test. The ratio of the maximum effective uniformly distributed load to the test load is known as the '‘bedding factor'’ which varies with the types of bedding materials under the pipe and depends to a considerable extent on the efficiency of their construction, and on the degree of compaction of the side fill. Typical bedding factors are given in Table 1.8.1 below

Table 1.8.1 – Bedding Factors

Condition BF

Class D (Pipe laid on trench bottom) 1.1

Class F (Pipe laid on Granular Bedding) 1.5

Class B (1800 Granular Bedding) 1.9

Class S (3600 Granular Bedding) 2.2

Class A (Pipe on Concrete Cradle) 2.6

1.8.3 Design Strength

For design, it is required that the total external load on the pipe will not exceed the ultimate strength of the pipe multiplied by an appropriate bedding factor and divided by a factor of safety.

The design formula is as follows:

s

mt

eF

FWW ≤

where We = total external load on pipe,

Wt = ultimate strength of pipe,

Fm = bedding factor,

Fs = design safety factor of 1.25 for ultimate strength of pipe




1.9 Manhole Positioning The sewerage system should be designed to facilitate flows by gravity in a branched arrangement of small local sewers connected to larger district sewers, connected to the major trunk sewers.

All public sewers should be located in Government owned lands, to permit access for construction and maintenance and to facilitate connection from private premises.

Manholes and sewers should be sited with due regard to public utility services. Sewers in roads and highways should be located in accordance with the Standard Services Reservations Drawings as published by the Road Affairs. These are included as drawings SR1 and SR2 in Volume 8 of this Manual.

The location of manholes in the sewerage system is dictated by a number of factors:

� For non man-entry sewers maximum spacing between manholes is governed by jetting lengths. Jetting hoses are 100m and allowance has to be made for the vertical drop to invert level. In sewers over 600mm dia. jetting can be carried out from each end. Greater spacing may only be provided in special cases, where due consideration is given to maintenance, and subject to DA approval. Required spacing is summarised in table 1.9.1 below.

Table 1.9.1 – Maximum Manhole Spacings

Diameter (mm)

Distance (m)

To 600 80

Above 600 120

• Manholes should, where possible, not be constructed close to kerb lines;

• Manholes should be constructed at the head of each system, and at every change of diameter, direction and/or gradient;

• A manhole should be constructed at every significant sewer junction (a significant sewer junction is one where the connecting sewer

serves more than five properties), including all rider sewers;

• Manholes should not be constructed in locations on bends in the highway, which may cause vehicles to skid;

• Manholes should be accessible at all times;

Manholes and chambers will form the main points for access to the sewerage system for operation and maintenance. They should therefore be located with adequate access for maintenance vehicles.

Where new manholes are to be constructed in existing highways, close liaison is required with the Roads Department. Although the Standard Services Reservation Drawings should be followed where possible, care must be taken to ensure that the locations of all existing utilities in the vicinity are known, and that the proposed manhole location will not interfere with such utilities. Manholes should not be located such that they would prevent access to utility equipment or in an emergency situation.

Building over or near to a sewer, and associated manholes and chambers will not be permitted. Building over sewers, or directly adjacent to them, causes major problems with access for maintenance and renewal of sewerage assets. In extreme cases, demolition of premises could be required.

The land along the line of the sewer for construction and access for maintenance and replacement is called the easement. No other developments should be permitted within the confines of the easement.

Where access to a sewer is restricted on both sides, the easement width required is a minimum of 6m, being normally 3m either side of the centre line of the pipeline. This distance is considered to be the minimum practical working width to allow access of construction plant, and storage of excavated material and pipe sections during maintenance operations. Where the depth from finished ground level to invert exceeds 3m, or the sewer diameter exceeds 600mm, the easement widths required are the greater of two times the depth to the invert of the sewer, plus the pipe diameter, or ten times the diameter of the sewer. Thus the easement for a 1m pipe at 5m depth will be 11m. For this reason, it is essential that excavation depths be kept to a minimum.




Foundations and basements of buildings adjacent to easements shall be designed to ensure that no building load is transferred to the sewer. Trenches or pipelines shall not be constructed within a notional 45-degree line of influence spreading below the footing, or 1.5m from the face of the building, whichever is greater.

These requirements refer to permanent easements required in connection with pipe-laying and subsequent maintenance. They exclude temporary storage areas, and the like, used during construction.

1.10 House Connections A house connection is defined as the connection from a premise (domestic, commercial, industrial, institutional, etc.) to transfer foul flows to the public sewerage system.

For every house connection, a terminal manhole (Manhole number 1), in accordance with the Standard Drawings, should be provided and should be positioned within the boundary of the premises at a distance not exceeding 2.0m from the boundary line. The required depth of MH1 is 1.2m The terminal manhole is intended to form a demarcation of maintenance responsibility, and to protect the public sewerage system from damage or blockage due to indiscriminate discharge of sewage by the occupants of the connected premises. The terminal manhole also acts as a seal to prevent the emission of gases from the public sewer, potentially causing nuisance to the occupants of the premises.

Before a connection is made, the capacity of the existing sewerage system should be checked to confirm that it has sufficient capacity to accommodate the flow from the premises.

All house connections should comply with the following general principles:

• They should be designed and constructed to enable foul flows to pass to the public sewer without flooding or surcharge;

• They should be of 150mm minimum internal diameter for a typical 6-person villa development. For a large palace, office

complex, multi-storey apartment block, hotel, etc, the minimum diameter shall be 200mm;

• They should laid at minimum gradients, sufficient to avoid deposition under all flow regimes. It is desirable that a gradient of 1:60 for 150mm diameter pipework be used for design purposes, although this may not always be possible in flat areas. In such cases, the gradient may be reduced to 1:180;

• They should be constructed to watertight standards in accordance with the standard drawings and specifications.

The private sewerage system between the premises and the terminal manhole shall be designed and constructed in accordance with the general requirements of the Manual. The private sewerage system shall be designed and constructed as a separate system, capable of accepting foul flows only. Illegal connections allowing the entry of storm water runoff shall not be made to the foul sewerage system.

House connection to existing pipelines should be made either to the nearest manhole or to a Y-junction incorporated into the pipeline. It should be noted that Y–branches will not generally be accepted by the DA and should only be used as a last resort, where access for manhole construction is impractical. Y-branches and saddles are not to be added to existing pipelines to avoid the permanent damage resulting from such modifications to the sewer.

Where several premises are being connected or the sewer is deep, then consideration should be given to the provision of a rider sewer. The rider sewer would be constructed parallel to the public sewer, at shallower depth and discharge into a manhole.

Typical details of house connections, rider sewers, etc are shown on the Standard Drawings, Volume 8.

1.11 Construction Depths The topography of Qatar is virtually flat, thus affording little or no opportunity to use natural land slopes to promote gravity movement of sewage. All gradients in pipelines must therefore be provided by the slope of the pipeline within the trench. The geology is also predominantly rock, requiring




excavation by rock breaker, which is slow, noisy and expensive.

Faced with these natural constraints, designers have no choice other than to employ pumping stations to lift the sewage into the downstream gravity system, once the trench depth begins to approach around 5m. Open cut excavations in excess of 5 m should generally be avoided where possible, on safety grounds. However, It is recognised that this is generally costly to achieve, and so the policy in Qatar is to allow open cut up to 7.5m depth, but not more.

Whilst this is a general rule, the economics and practicalities should also be considered when determining excavation depths. The use of shallower trenches will result in the need for more pumping stations. The cost of providing shallow trenches and more pumping stations versus the cost of deeper trenches and less pumping stations should be subject to a lifetime cost (NPV) comparison at feasibility stage before embarking upon final design.

Trenchless options should be considered for deep sewers

The economics of deep excavations are governed by the following factors:

• Depth of trench for safety, although rock geology encourages stability of trench sides;

• Depth of trench for reach of machines for breaking out and removing excavated materials;

• Availability of suitable trench supports;

• Width of trenches allowing for battering back for stability;

• Strength of pipelines to withstand imposed loads from backfill.

The costs and timescales for excavation then need to be balanced against the construction and O&M costs for pumping stations. Comparison should be carried out for all options on a NPV basis within a 15-year timeframe. Options may include several combinations of sewer depth and numbers of pumping stations, with perhaps deep tunnelled interceptor sewers, which may or may not require a

terminal pumping station. Both gravity sewers and pumping stations will have operational costs, and should be subjected to the NPV process alike in order to provide a true cost comparison.

Other aspects such as noise from pumping stations, and the consequences of flooding due to station failure also need to be considered.

The minimum cover depth from Finished Ground Level (FGL) to top of pipe or surround should be as per Table 1.11.1. However, designers should note that these values will often need to be exceeded in upstream sewers to allow adequate falls for house connections from larger developments, and to avoid other utilities in congested areas.




Table 1.11.1 - Minimum Sewer Cover Depths

Location Minimum Cover

Urban areas (paved) 1200mm to FGL

Rural Areas (unpaved) 900mm to FGL

Other Pipeline Services 500mm clearance preferred

300mm for twin rising mains in common trench

200mm absolute minimum clearance

1.12 Manholes, Chambers, Access Covers, and Ladders

These installations are required to access sewers for testing, inspection, maintenance, repair and removal of debris. Every sewer length on the public system should be accessible without the need to enter premises or cross property boundaries.

Manholes and chambers generally fall into two categories, being:

• Inspection Chambers; and

• Sewer System Manholes.

1.12.1 Inspection Chambers

These structures are of shallow depth (generally 1.2m) and are intended for use on sewerage systems within property boundaries, and for the terminal manhole (MH 1) of the house connection. Please refer to standard details Volume 8

These chambers are generally used for inspection of sewer pipelines and clearance of blockages.

1.12.2 Sewer System Manholes

These structures are of a depth to suit the levels of the sewer pipelines, and are the means of access into the public sewerage system.

The arrangement and dimensions of manholes depend on the diameter of the connecting sewers and their depth to invert below finished ground level.

Backdrop Manholes

Backdrop manholes will be required where there is a sudden drop in sewer level. In most cases this will arise when a shallow branch or rider sewer enters a deeper trunk sewer. Although the general philosophy of sewer design dictates that pipes should enter manholes with level soffits where possible, this will not be economical where shallow sewers meet deeper sewers (the branch sewers should be constructed at minimum depth to avoid excessive cost of excavation).

Standard drawings of backdrop manholes are included in Volume 8. For smaller backdrops of less than 1m fall, an inclined backdrop may be used, but vertical drops are required for greater falls, to avoid excessive excavation.

Designers should refer also to the discussion in sections 1.5 general principles, and 1.11 regarding sewer depths.

1.12.3 Elements of Design

Manholes and chambers shall generally be constructed in accordance with the standard drawings contained in Volume 8.

Minimum cover size should provide sufficient access to admit persons with normal hand tools and cleaning equipment, and to admit persons wearing breathing apparatus in emergencies. Maximum cover size should be limited by the weight which persons can safely lift.

Access shafts should be sufficiently large for persons to go down to the sewer in comfort (with breathing apparatus in emergencies) and yet be small enough for the nearness of the walls to give a sense of security.

Where the invert of the manhole or chamber is more than 6 m from the cover level, intermediate platforms shall be provided at regular intervals. Headroom between platforms should not be less than 2.1m and not greater than 6m. The platform should be fitted with handrailing and safety chains around the access opening to protect persons from




falling. The location of openings in successive platform shall be offset to prevent dangers of free falling. (In certain circumstances, separate access may be required to allow equipment and materials to be lowered directly to the pipe invert).

Inverts and benching should be neatly formed. The ends of pipes should finish flush with internal faces of the manholes. The channel inverts should be curved to that of the connecting pipes and carried up the full diameter of the pipes in flat vertical surfaces, matching the cross-sections, levels, and gradients of their respective sewers.

The benching should be formed from plane surfaces, sloping gently towards the sewers. Benching slopes should not be too steep to cause persons to slip into the sewer, nor too flat to accumulate sediment. A suitable gradient for benching is 1 in 12.

1.13 Industrial Wastes Foul flows from industrial and commercial premises have the potential to contribute major flows and polluting loads to the main sewerage system. Such flows need to be managed as part of the overall sewerage and treatment management process. This is best done by initial licensing of the industrial and commercial premises, followed by ongoing monitoring of their effluents to ensure that they are complying with their licenses.

The license application should include the following information:

• Name, address, type of business and number of employees;

• Main business process, water usage process and pollution process;

• Water usage (daily and peak flows) effluent flows (daily and peak flows), discharge pattern (regular, intermittent, weekend to weekday patterns);

• Pollutants in effluent (BOD, COD, SS, chemicals, temperature, etc) and concentration of each pollutant.

Each application should be checked by a site visit to confirm the supplied information and make

arrangements for further checks, such as flow measurement and effluent sampling. In many cases, some form of pre-treatment will be needed at the premises to ensure that discharges comply with the license standards. Large industrial premises may require their own complete flow balancing and treatment facilities to meet license requirements for discharge flows and effluent quality.

The waste discharge license should include the following stipulations:

• Average and peak flow rates;

• Maximum concentrations for a specified range of pollutants;

• Flow measurement and sampling facilities;

• Reporting requirements.

The licensing process should ensure that all significant industrial and commercial discharges are defined and understood, in terms of their location, volume and polluting effects.

Please refer to Volume 1 Appendix 5 for example of industrial waste discharge permit application

1.14 Septic and Sewage Holding Tanks

Septic and sewage holding tanks are used to store and treat foul flows from premises, prior to connection to the main sewerage system (ie if the premises are complete before the sewerage system), or where no sewage system is available. They comprise an underground tank for anaerobic treatment followed by a soakaway tank or pipe system to encourage effluent flows to disseminate to the surrounding ground.

Since they only provide partial treatment, these tanks are a major source of groundwater pollution and therefore should not be constructed where main sewerage is available. For existing developments, house connections from manhole number 1 to the main sewerage system should be made at the earliest opportunity, and usage of the septic and sewage holding tank stopped.




Septic tanks should preferably be designed for a minimum hydraulic retention time of one month to allow anaerobic treatment of the organic content.

1.14.1 Design of Septic Tanks and Soakaways

The following information will be required for designing the septic tank to serve the development:

• Maximum and minimum population to be served;

• Water consumption and discharge rates;

• Any special conditions affecting the composition of the sewage, such as grease, oil and detergents, which would adversely affect the treatment process;

• The need for additional oil and grease traps;

• Ground soil condition and depth of water table, as these will affect the percolation of effluent through the soakaway.

For larger developments, multiple tanks and soakaways may be necessary.

Further information on the design of septic tanks and soakaways is contained in Volume 6 - Developers Guide.

1.14.2 Sewage Holding Tanks

Where the groundwater level is high, soakaways will not be permitted, as they will be considered as ineffective in percolating the effluent into the surrounding ground. At such sites, it will be necessary to provide sewage holding tanks. The tank shall be watertight to prevent the ingress of water, and shall be suitably constructed and protected against corrosion. The tank shall be designed against the effects of uplift from groundwater pressures.

The tank shall have a minimum of two days storage of sewage discharged from the development, based on the population and per capita flow.

However, it is preferable to have up to 30 days if possible, depending upon the size of the catchment,

to enable less frequent emptying and associated tanker traffic.

Further information regarding sewage holding tanks is contained in Volume 6 - Developers Guide.

1.15 Oil and Grease Interceptors

Oil and grease interceptors are usually located underground and are used to reduce, or remove light liquids such as oil, petrol, grease, and other floating solid pollutants. Regular and planned maintenance, by removal of floating matter, is required if they are to function efficiently.

Oil and grease interceptors are required to treat all foul and surface water flows from such establishments as:

• Hotels, Restaurants and catering premises;

• Petrol stations and fuel storage facilities;

• Garages and workshops;

• Paint and chemical manufacture and storage.

Further information on the design of oil and grease interceptors is contained in Volume 6 - Developers Guide.

In addition to underground interceptors, all above-ground storage of polluting liquids should have retention bunds installed around all storage tanks. The retention volume of the bund should exceed that of the tanks by 10%. The bund should be of durable construction, for example reinforced concrete, suitably protected against natural elements and the retained liquid. The bund should be fitted with valved drainage for removal of rainwater. No drainage connection is to be made to main sewerage or drainage systems.

1.16 Flow Attenuation Methods

The role of flow attenuation is to reduce peak flows by the temporary storage of wastewater within the system. Flow attenuation is often used to reduce flooding and overload of pumping stations, and to reduce discharge and pollution from overflows.




Flow attenuation is most usually achieved by providing additional storage, the most common being the on-line or off-line tank. Storage tanks are normally sited underground.

On-line storage systems attenuate flows by utilising existing capacity within the system or by constructing oversize sewers. The off-line storage systems involve storage tanks adjacent to the sewer with connections to and from the sewer.

Excessive use of storage can lead to problems in the downstream sewerage system and at the STW, due to deterioration of the sewage during storage. Prolonged in-sewer storage can potentially lead to higher STW effluent loads (particularly total suspended solids and ammonia) and/or poor biomass performance. Pumped storage systems may increase the risk of septic conditions, with resulting odour, corrosion and health hazard problems. Combined with high ambient temperatures, such conditions exist in Qatar, and therefore considerable care will be needed to avoid prolonged storage of sewage.

1.16.1 Flow Controls

Flow controls are used to limit the flow passing forward to the downstream system, by backing up flows into the upstream storage tank. The most common controls are described below.

Orifice Plates and Pipes

These are the simplest controls, being usually circular or square apertures sized to produce a restriction in flow. Orifice plates may be fixed in location or mounted in guides for easy removal. Because of the solids content of the flow, it is widely accepted that they are not suitable where the minimum dimension is less than 200mm. Smaller sizes are prone to blockage. Their use is therefore limited to higher flow ranges.

The formula used for sizing a circular or square submerged orifice is:

Qs = Cd A√√√√(2gH)

Equation 1.16.1

Where:

Cd is the coefficient of discharge, commonly around 0.7

A is the orifice area

H is the hydrostatic headloss across the orifice plate, or difference between upstream and downstream water levels.

For short lengths of pipe, the friction losses can be neglected and the above formula used.

Penstocks

Penstock settings can be sized as for orifices, but should be used with caution, as the setting may be altered after installation. Use of orifices is therefore preferred. Penstocks also have the disadvantage of requiring periodic maintenance, which involves confined space entry.

Other Devices

Head discharge relationships for the various market products (such as Hydrobrakes) should be obtained from the manufacturers. It is preferable for the rating curve to have a steep gradient in order for the pass-forward flow to remain near constant from onset of spillage through to maximum weir flow.

Operational Issues

A common problem with all controls is the tendency to block from time to time. Access for rodding or other means of clearance must be provided at the design stage. It should be remembered that the chamber may become flooded and therefore the clearance facility must be operable from outside the chamber. Measures such as flap gates, vent tubes and fixed pressure hose connections have been used with some success. Some of the proprietary flow controls are also fitted with bypass pipes, to allow draining down of the chamber in the event of a blockage.

1.16.2 Attenuation Storage Tanks and Sewers

Attenuation facilities usually comprise underground storage tanks, equipped with flow control devices on their outlet to limit peak flows from the tank.

Layouts




Tank arrangements fall into two main categories, namely on-line and off-line, of which there are many further sub-classes. Figure 1.16.1 shows several alternative layouts

On-line tanks are storages constructed along the route of the pipe in question, and share the same hydraulic gradient. On-line tanks (with perhaps the exception of emergency storage) always drain flows to the downstream sewer by gravity. On-line tanks would normally be preferred to off-line from an operational point of view, but require certain hydraulic conditions to be satisfied in order to present a viable option.

Off-line tanks are constructed along a route separated from the main sewer, and may return flows to the main sewer by gravity or pumping, again depending upon the hydraulic conditions.




1. On-line Storage

2. On-line Storage + Flow Control

Storage Tank

Storage Tank

3. Off-line Storage + Gravity Return

Storage Tank

4. Off-line Storage+ Screened Overflow+ Gravity Return

5. Off-line Storage+ Screened overflow+ Gravity Return

6. Off-line Storage+ Gravity Return+ variable flow control

7. Off-line Storage+ Pumped Return+ screened overflow

Storage Tank

Flow Control

Storage Tank

Flow Control

Non-returnValve

Overflow

& Screen

Overflow & Screen

Flow Control

Storage Tank

Outfall

Outfall

Outfall

Outfall

Overflow & Screen

Overflow

& Screen

Storage Tank

Flume

Flow Control

Flume

Pump

Figure 1.13.1 Alternative Tank Layouts

Figure 1.16.1 – Alternative Storage Tank Layouts




Materials and Construction

Materials for tank construction may be concrete, GRP, plastic or coated steel. In-situ reinforced concrete is the most obvious choice for construction of specific designs, but certain applications will lend themselves to the use of proprietary products, e.g. large diameter pipes, precast concrete box culverts and modular, thin-walled plastic or GRP tanks with mass concrete surrounds. Designs using plastics should ensure adequate resistance to jetting pressures. All underground structures should have adequate resistance against uplift due to groundwater pressures.

On-line Storage

On-line storage is shown in cases 1 and 2 in Figure 1.16.1. This is the simplest type of arrangement, and should be used wherever possible. Hydraulic conditions will determine the viability. The tank will need to operate within the hydraulic regime of the existing system. On-line tanks of any size will not be practical in very flat sewers, due to the large surface area requirement. Thus although they are the preferred arrangement, their use is limited in Qatar due to the flat topography. It is unlikely that they would be situated in upstream areas, but they could be of use adjacent to pumping stations where a significant headloss is available between the sewer invert and the Pump Station Top Water Level (TWL).

On-line tanks become more practical with increased gradient, but at greater depths, due consideration will need to be given to the greater pressures developed at the downstream ends, e.g. at pipe joints. In such cases, consideration may be given to the use of backdrops and cascades of tanks.

An on-line tank will operate by surcharging as the flow approaches the predetermined pass-forward flow. This flow may be the capacity of the downstream sewer, whereby a flow control is required to limit the pass-forward flow. In both of the above cases, care should be taken to ensure a self-cleansing velocity to prevent sediment build up. In large diameter tanks with low base flows, this may be difficult. In such cases, a dry weather flow channel should be provided. It is recommended in Sewerage Detention Tanks – A Design Guide,

WRC, 1997xxxvii that the longitudinal slope of the tank is kept to a minimum of 1:100 in on-line tanks and that sidewall slopes into the centre channel are

a minimum of 1:4. Care should be taken with benching in on-line and off-line tanks - this should be steel trowel finished with granolithic topping to prevent accumulation of solids.

Off-line Storage

Off-line storage with gravity return is shown in Cases 3 to 6 in Figure 1.16.1. This would typically be preferred where construction could proceed without the need for over-pumping, or if insufficient length is available for on-line storage. The storage may be provided in a single tank, an over-sized pipe/box-culvert or groups of pipes. Care should be given to flow distribution at the upstream end, and the order of preference in filling. As the tank may not be 100% filled on a regular basis, selection of a preferential flow channel will reduce the need for desilting operations.

Where screens are installed, as in Cases 4 to 6, it is preferable to retain the screenings in the main flow where possible, to prevent accumulation in the tank. However, flow control measures should be devised to prevent screenings entering the tank from the downstream end. A further refinement of this is shown in Case 6, where a variable flow control is provided, linked to a gauge of the downstream sewer reserve capacity.

Operational Issues

Operation and maintenance of such underground structures present particular health and safety issues for access and maintenance. These aspects include:

• Blockage of flow control devices - access needs to be provided to safely enter the structure and for clearance tools and removal of debris. Where a blockage has resulted in sewage being retained for some time, clearing the blockage suddenly may have an unacceptable impact on downstream facilities, such as pumping stations and STW. Designs therefore need to consider facilities for gradual emptying or removal of effluents;

• Removal of sediment - access needs to be provided to safely enter the structure, and for clearance tools and removal of debris;

• Design to optimise removal of sediment to: minimise time and effort needed inside




underground structures; modifications to the structure of the tank to allow sediment to be removed from ground level; use of low friction coatings to discourage accumulation of sediment; modification of inlet design to increase scour; steepening of benching gradients and installation of dry weather flow channels to encourage self-cleansing; use of mechanical plant and flushing mechanisms to periodically remove sediments A useful design check is provided in Table 1.16.1.




Table 1.16.1 - Storage Tank Design Checklist

Consider maintenance & cleaning operations

Consider the erection/removal of falsework in confined spaces during construction (use

false soffits or pre-cast slabs for roof sections)

Design benching to be self-cleansing

Ensure sufficient access of adequate size are incorporated (NB can plant be removed

Consider the type of screen required

Design out any possible maintenance hazards

Ensure adequate ventilation is achieved

Is odour control required?

Consider retention times of the tank

How long does it take to empty the tank? Consider follow on storm events

Provide a facility for overpumping of the tank

Are overflows required?

Provide penstocks on the tank inlets/outlets to enable flows to be diverted or isolated

Provide a penstock protected bypass pipe

Is a flow control required on the tank outlet/bypass pipe?

Reinstatement of area, consider future access requirements

Does the site need to be purchased?

HARAS complete?

EIA complete ?

Consider type of covers (think about manual handling, and security of access)

Incorporate a sufficient number of davit sockets

What telemetry is required?

On-line or Off-line tank?

Are welfare facilities required?

Is a gravity discharge achievable? Otherwise pumps will be required.

Is a power supply needed?

Is a water supply needed for washing down?

Planning permission is required for all control kiosks and permanent accesses to the

Is a standby generator required?

DA and RD Discharge consents for emergency overflow

What is required in the way of control kiosks/buildings

Ensure that access for a tanker is possible

Place screens on inlet to tanks on off-line tanks




1.17 Abandonment of Sewers

Disused sewers and drains have the great potential to allow unwanted flows, such as groundwater to enter the system through deteriorating faults in the system fabric. They therefore need to be removed from the system to prevent structural deterioration, unauthorised use, and ingress of groundwater and infestation by rodents.

Disused sewers shall be removed or, where this is impracticable, they shall be filled in accordance with the materials and details contained on the Standard Drawings in Volume 8.

2 Pumping Stations

2.1 Standards The standards and sources of information to be used are listed in sections 1.1 and 1.2.

2.2 Hydraulic Design The overall design philosophy of the pumping system needs to be a balanced design with due consideration of functional, environmental and economic aspects. For pumping systems in the vicinity of sensitive receivers, reliability of the system is of key concern. Bypass or overflow of raw sewage, even in emergency situations, should be avoided where possible.

Particular attention should be paid to the following issues:

• Design flow;

• Standby power supply or temporary storage;

• Standby pumps;

• Overflows and emergency bypass;

• Twin rising mains;

• Availability of QGWEC power supply;

• Land area available and proximity to housing or public areas;

• Access to the proposed site.

Since the pumping station will probably be serving an area of new development, it is likely that the initial flows to the station will be much smaller than those expected for the full design. Flows will increase in the following years to reach the design capacity of the station. If the inflows are greatly below the pump output, the result will be excessive periods of inactivity of the station, with the potential for premature failure of equipment. Such infrequent operation of pumps will also result in retention of sewage in the rising main, raising problems with septicity, corrosion and effects on the receiving STW.




Consideration should therefore be given to the sizing and numbers of pumps to match the likely build-up of incoming flows. Where possible, similar pumps should be installed, on duty and assist basis, with similar standby pump(s). The use of similar pumps will avoid any changes in pumping regime due to the rotation of duty pumps for operational reasons.

Consideration should also be given to installing twin rising mains. One main would be used in the early years of the scheme to achieve satisfactory maximum flow velocities and hence minimise siltation. When flows increase, then the second main would be brought into use.

Although not strictly required for the early years of a scheme, it would not be economic to construct one rising main and then construct the second within a short period, say five years. The additional costs and disruption of digging a second trench, together with operational and safety requirements of working adjacent to a “live” rising main, would be avoided.

2.2.1 Hydraulic Principles

A pumping system may consist of inlet piping, pumps, valves, outlet piping, fittings, open channels and/or rising mains. When a particular system is being analysed for the purpose of selecting a pump or pumps, the head losses through these various components must be calculated. The station loss (i.e. the loss on the suction and delivery pipework from the sump to the common header) should also be considered. The frictional and minor head losses of these components are approximately proportional to the square of the velocity of flow through the system and are called the variable head.

Friction losses should be determined using the Colebrook–White Formula.

Losses in fittings at the station, and outside of it should be determined using the formula:

δH = kv2/2g

Equation 2.2.1

Where δH denotes the fitting headloss (m), k is the loss coefficient, v the velocity (m/s) and g is the gravitational constant, 9.81m/s2.

Indicative values of k are given in Table 2.2.1below.




Table 2.2.1 – Indicative Minor Loss Coefficients,

k, for Various Fittings

Fitting Coefficient k

Standard 900 bend 0.75

Long Radius 900 bend 0.4

Standard 450 bend 0.3

Tee - line to branch 1.2

Tee – flow in line 0.35

Taper up 0.5

Sharp Entry 0.5

Bellmouth Entry 0.1

Sudden Exit 1.0

Non-return valve* 1.0

Gate Valve, fully open* 0.12

*Note that for valves it is advisable to obtain

manufacturers data on headlosses. System head

calculations would normally be carried out using valve

open figures.

It is also necessary to determine the static head required to raise the liquid from suction level to a higher discharge level. The pressure at the discharge liquid surface may be higher than that at the suction liquid surface, a condition that requires more pumping head. These two heads are fixed system heads, as they do not vary with rate of flow. Fixed system heads can be negative, if the discharge level or the pressure above that level is lower than suction level or pressure. Fixed system heads are called static heads.

The Total Dynamic Head (TDH) for a system is the sum of the major and minor friction losses plus the static head. The duty point for a pump selection will be the required flow at the TDH.

A system head curve is a plot of total system head, variable plus fixed, for various flow rates. It may express the system head in metres and the flow rate in cubic metres per second. Procedures to plot a system-head curve are:

1. Define the pumping system and its length;

2. Calculate the fixed system head;

3. Calculate the variable system head losses for several flow rates;

4. Combine the fixed head and variable heads for several flow rates to obtain a curve of total system head versus flow rate.

The flow delivered by a centrifugal pump varies with system head. Pump manufacturers provide information on the performance of their pumps in the form of characteristic curves of head versus capacity, commonly known as pump curves. By superimposing the characteristic curve of a centrifugal pump on a system-head curve, the duty point of a pump can be determined.

The curves will intersect at the flow rate of the pump, as this is the point at which the pump head is equal to the required system head for the same flow.

The recommended values for coefficient of Colebrook–White Roughness Factor (Section 1.5.1 above) ks for use in rising mains are contained in Table 2.2.2 below. Note also the values indicated in Table 1.5.1, which refer to gravity sewers.

Table 2.2.2 – Recommended Values of

Colebrook-White Roughness Factors

(ks) for use in Rising Mains

Mean Velocity in m/s ks (mm)

Up to 1.1m/s 0.3mm

Between 1.1m/s and 1.8m/s 0.15mm

The discharge capacity for multiple pumps will not be simply the sum of the discharge capacity of individual pumps because the system-head curve for multiple pumps will be different from that of a single pump.

2.2.2 Pump Arrangements

The number of pumps to be installed depends on the station capacity and the range of flows. The maximum discharge rate from a pumping station, when all duty pumps and rising mains are in use should be slightly greater than or equal to the maximum incoming flow to the station. Pumps should be selected with head-capacity characteristics that correspond as closely as possible to the overall station requirements.




Standby capacity is required so that should any of the pumps in the station be inoperable due to routine maintenance or mechanical failure, the operation of the station can still be maintained. For instance, in a station where a single duty pump provides the duty output, a second pump of equal capacity is mounted. Where three duty pumps of equal capacity are required to meet the maximum design flow conditions, a fourth pump of similar capacity is provided as standby.

It is not desirable to have pumps of different sizes for operation and maintenance reasons, unless the flow ranges vary widely throughout the day. To cater for slow build-up of flow in the early years of operation, phased installation of pumps, or the use of a smaller diameter impeller should be considered.

2.3 Rising Main Design

2.3.1 Rising Main Diameters

The minimum diameter of pumping mains is controlled by the need to avoid blockage, and therefore should not be less than 100mm. Where sewage is screened or macerated before pumping the minimum diameter should not be less than 80mm.

The maximum and minimum diameters are sized to maintain flow velocities for all stages of pumping within the ranges specified in Section 2.4.

2.3.2 Twin Rising Mains

The use of twin rising mains should be considered on a case by case basis. The main factors for consideration include the design elements, risk assessment and cost benefit analysis.

Considerations for the design elements comprise the rate of build up of flow, the range of flow conditions, the range of velocity in the mains, the availability of land for the twin mains and associated valve chambers as well as the complications in pump operation.

A thorough risk assessment should be carried out which should include the likelihood of mains bursting, the consequence of failure, area affected, sensitive receivers affected (such as beaches), and

the feasibility of temporary diversion or tankering away.

A cost benefit analysis should include all tangible factors (such as cost of pipework, land cost, energy cost, etc) and intangible factors (such as nuisance, closure of beaches, etc).

Twin rising mains should be considered in the following circumstances:

• To accommodate a wide range of flow conditions, such that the velocity in the mains can be kept within acceptable limits. For instance, a pumping system serving a new development may have very low initial flows with a slow build up of flow;

• To provide continued operation for a major pumping system when one of the mains is damaged and where the failure of the system would have serious consequence;

• To minimise adverse environmental impacts to sensitive areas;

• To facilitate future inspection and maintenance of major pumping systems, while the normal sewage flow can be maintained.

When twin mains are found to be preferred, it is advisable to use both mains as duty rather than one as duty and the other as standby, from an economical and operational point of view. Should one of the duty mains be taken out of operation, the remaining one would still be able to deliver a higher quantity of flow at a higher velocity. The occurrence of overflow or bypass can be minimised or even eliminated. Septicity in the standby mains would also pose an operational and maintenance problem.

2.3.3 Economic Analysis

As the size of the rising main increases, the velocity and the system head will decrease, with savings in the cost of pumping. The increase in the capital cost of rising mains will be offset by the power cost of pumping. However, it is also important that the velocity in the mains should be within a suitable range to minimise the deposition of solids. Excessive hydraulic head losses are to be avoided.




The selection of a suitable size for the rising mains should be based on economic analysis of capital cost and recurrent cost of the pumping system including the power cost. A trial and error approach should be adopted in order to arrive at an optimal solution while maintaining the velocity within acceptable limits.

Therefore, combinations of different sizes of rising mains and the system head should be evaluated, taking into account both the capital cost and the energy cost of pumping.

2.3.4 Rising Main Alignment

The alignment of the rising main should discourage surge in its flow conditions. Where possible the rising main should be laid with continuous uphill gradient of not less than 1:500, and with gentle curves in both horizontal and vertical planes. Long flat lengths of rising main should be avoided therefore pipes should be laid with rise and falls of 1:500, rather than flat. Air release valves should be provided at high points and as the profile of the main dictates. Washouts should be installed at low points. The arrangement and locations of valves should be planned together with the alignment of the rising mains.

2.4 Maximum and Minimum Velocities

The maximum velocity in rising mains should not exceed 2.0 m/s, The desirable range of velocity should be 1m/s to 2m/s with due consideration given to the various combinations of number of duty pumps in operation. (This is because lower velocities cause siltation, and higher velocities increase surge problems and power usage).

2.5 Pipe Materials Pipe materials for use in pumping stations should always be Ductile Iron (DI).

Rising mains outside pumping stations may be ductile iron or Glass Reinforced Plastic (GRP) with concrete protection, however DI is preferred.

2.6 Thrust Blocks Thrust blocks are concrete blocks designed to prevent pipes from being moved by forces exerted within the pipe by the flow of water hitting bends, tapers, and closed, or partially closed valves. In the design of pressurised pipelines, thrust blocks are essential on flexibly jointed pipelines where any pipe movement would open up the joints in the line and cause water leakage. Restraint straps may also be required for above-ground pipework.

Thrust blocks are also necessary near valves where a flexible joint is located to facilitate removal of the valve for maintenance purposes. The size of block is dependent upon the angular deflection, flow, size of pipe and the pressure of water inside the pipe. The designer should also refer to the pipe manufacturers’ literature.

The following design assumptions are to be adopted:

• Thrusts developed due to changes in direction of pipeline, dead end or change in diameter should be considered. Force due to change in velocity head can normally be assumed as negligible unless there is a drastic change in pipe diameter;

• Thrust blocks should be designed for the condition of no support being available from the backfill, i.e. to be cast against undisturbed ground;

• For most cases, thrust blocks will be designed to transfer forces directly onto undisturbed ground using direct bearing, the acceptable bearing pressure being confirmed by geotechnical investigation. If the adjacent ground has insufficient bearing capacity, the block may need to be designed using ground friction or piling to transfer thrusts to a more competent soil layer. Consideration should also be given to the presence of adjacent services and the possibility of future disturbance during maintenance operations. Complex thrust blocks may be required to avoid transfer of forces and consequential damage to adjacent services;




• For pipes with flexible joints such as DI pipes with socket and spigot joints, all the thrust is assumed to be taken up by the blocks.

Static thrusts may be calculated using the formulae as follows:

For blank ends:

Fs = 100 A P

Equation 2.6.1

Where:

Fs = the static thrust (KN)

A = the cross sectional area (m2)

P = the Pressure (bar)

For Bends:

Fs = 100 A P(2 sin θ/2)

Equation 2.6.2

Where θ is the angle of deviation at the bend.

Dynamic thrusts for water or sewage may be calculated using following:

Fd = 2A V 2 sin θ/2

Equation 2.6.3 Where

Fd = the dynamic thrust (KN)

V = the velocity (m/s)

As stated above, this force is negligible in normal cases, but if significant, then the total thrust should be taken as the sum of static and dynamic thrusts.

The above procedures will be satisfactory for most routine applications. For further guidance, please see CIRIA Report R128xxxviii. It is recommended that this reference is used for more complex applications, such as where thrust forces are in excess of 1000KN or loose material is encountered.

2.7 Air Valves and Washout Facilities

These facilities are required to minimise the adverse effects of surge and to facilitate the operation and maintenance of the rising main.

2.7.1 Air Valves

Air-relief valves are installed at locations of minimum pressure. Air is sucked into the air-relief valve when pipeline internal pressure is below atmospheric. Upon subsequent pressure rise, the admitted air is then expelled. Air valves should be installed at all high points., Additional air valves should also be placed at 800m spacings on long sections of straight grade.

Each air valve will operate independently and therefore several valves may be required along the pipeline if there are numerous rises and falls in the vertical profile of the rising main.

2.7.2 Vented Non-return Valves

An air valve combined with a vented non-return valve allows air enter the pipeline freely on separation of the water column, but controls the expulsion of air as the column rejoins. This has the effect of creating an air buffer between the column interfaces, thus reducing the impact velocity of the rejoining column and the surge potential of the system.

2.7.3 Wash – Outs

The purpose of the washout system is to drain the rising main for maintenance works. The washout should be installed at low points of the pipeline profile, and needs to be located carefully, taking into account that sewage will be discharged. For long rising mains with few low points, wash-outs should be strategically located at suitable intervals, generally 800m, to reduce the time required for emptying the main in an emergency. Location should be adjacent to a suitably sized gravity sewer for draindown where possible If a direct connection to a suitably sized sewer is not available, the washout chamber should be provided with a sump




so that the drained contents of the rising main may be tankered away.

2.7.4 Isolating Valves

For long rising mains, isolating valves should be included to allow sections of the rising main to be isolated and emptied within a reasonable time. In-line sluice or gate valves are often used as isolating valves. The isolating valve installation may incorporate washout facilities.

2.8 Flow Meters

2.8.1 Application and Selection

The variety of choices facing the designer confronted with a flow measurement application is vast. For example, types of flow meter using the positive displacement principle include rotary piston, oval gear, sliding vane, and reciprocating piston. Each type has advantages and limitations and no single type combines all the features and all the advantages.

Differential pressure meters have the advantage that they are the most familiar of any meter type. They are suitable for gas and liquid, viscous and corrosive fluids. However their usable flow range is limited and they require a separate transmitter in addition to the sensor.

Some of the most important parameters for flowmeters are accuracy, flow range, and whether the medium is sewage or water. Meter selection should be made in two steps. First by identifying the meters that are technically capable of performing the required measurement and are available in acceptable materials of construction; and second, by selecting the best choice from those available to cover special measurement features such as reverse flow, pulsating flow, response time and so on.

2.8.2 Magnetic Flowmeters

Magnetic-type flowmeters use Faraday’s law of electromagnetic induction for measurement. When a conductor moves through a magnetic field of given field strength, a voltage level is produced in the

conductor that is dependent on the relative velocity between the conductor and the field. Faraday foresaw the practical application of the principle to flow measurement, because many liquids are adequate electrical conductors. So these meters measure the velocity of an electrically conductive liquid as it cuts the magnetic field produced across the metering tube. The principal advantages include no moving components, no pressure loss, and no wear and tear in components.

Magnetic flowmeters offer the designer the best solution for pumped sewage flow. With nothing protruding into the flow of sewage, the chances of a blockage, if installed correctly, are non-existent. Magnetic flowmeters should always be installed with full-pipe conditions.

Care should be taken during design to provide sufficient straight lengths of pipeline up-stream and down-stream of the flowmeter, in accordance with the manufacturers installation instructions. As a general guideline, 12 pipe diameters of straight pipe on the inlet, and 6 pipe diameters on the outlet will ensure that the flowmeter is able to achieve the specified accuracy. If the amount of space available is restricted then the minimum length usually accepted by manufactures is inlet run of 5 pipe diameters and outlet run of 3 pipe diameters.

The following International and British Standards are a good source of information on flow meter selection and installation, and can be quoted in specifications:

• BS EN ISO 6817xxxix, 1997: Measurement of Conductive Liquid Flow in Closed Conduits;

• BS 7405xl, 1991: Guide to Selection and Application of Flowmeters for the Measurement

of Fluid Flow in Closed Conduits.

Flow meters should be pressure tested and calibrated by the manufacturer, and certified to a traceable international standard. As a minimum, the overall accuracy should be better than ±0.5% of the flow range. The repeatability of the result should be within ±0.2%.

In addition to the calibration certificate, the flow meter manufacturers should provide the following:

i. Isolated 4-20mA dc and pulse outputs;




ii. Programmable in-built alarm relays for empty pipe, low and reverse flows;

iii. In-built digital display for flow rate, totals and alarms;

iv. Transmitter enclosure shall be protected to IP67;

v. Calibration and programming kit.

The earthing rings should be included according to the individual manufacturer’s instructions. The sensor lining should be neoprene or an equivalent material of similar or improved properties, suitable for the application of pumped sewage flow. In below-ground flow meter chamber installations, the installed equipment should be submersible to the maximum chamber depth.

2.8.3 Ultrasonic Flowmeters

Ultrasonic meters are available in two forms: Doppler and transit-time. With Doppler meters, an ultrasonic pulse is beamed into the pipe and reflected by inclusions, such as air or dirt. The Doppler meter is frequently used as a “clamp on” device which can be fitted to existing pipelines. It detects the velocity only in a small region of the pipe cross section and as such its accuracy is not good. The single or multi-beam transit-time flow meters project an ultrasonic beam right across the pipe at an acute angle, first with the flow, and then opposite to the flow direction. The difference in transit time is proportional to flow rate. This type of ultrasonic meter is considerably more expensive but offers better accuracy. Unlike the Doppler meter, it requires a relatively clean fluid.

The main use of this type of flow meter in pumped sewage flows is in retrospective installation where the pumping main cannot be broken into for operational reasons. A clamp-on ultrasonic flow meter can be used to give reasonably accurate flow measurement.

For new installations, the lower cost of in-pipe ultrasonic flow meters could make them a viable alternative to magnetic flow meters for large diameter pipe installations.

2.9 Surge Protection Measures

Surge (or water hammer) is an oscillating pressure wave generated in a pipeline during changes in the flow conditions.

There are four common causes of surge in a pipeline:

• pump starting;

• pump stopping/power failure;

• valve action;

• improper operation of surge control devices.

The most likely one of these is the sudden stopping of pumps caused by a power failure.

A surge analysis should usually be carried out unless the system is simple. This is best carried out using approved software such as “Flowmaster”.




An approximate calculation for a simple pipeline is:

∆∆∆∆P = a x ∆V

g

Equation 2.9.1

Where: ∆P = Pressure change (m)

a = pressure wave velocity (m/s)

∆V = flow velocity change in 1 cycle (m/s)

g = acceleration of gravity (9.81m/s2)

The above equation can be used for calculation of both negative and positive pressures

The simple cycle time can be calculated with the formula:

Cycle time = 2 x pipeline length

Wave velocity

Equation 2.9.2

Table 2.9.1 – Indicative Surge Wave Velocity

Values for Selected Pipe Materials

Pipe Material Velocity (m/s)

Ductile Iron 1000–1400

Reinforced Concrete 1000–1200

Plastic 300–500

If the surge pressure approaches zero or the pipeline maximum pressure, a full surge analysis should be carried out. When surge analysis is complete, suitable surge suppression devices should be selected by consultation with the manufacturer.

Surge Suppression Methods

Surge suppression could be achieved using one of the following devices. The most appropriate device will depend on the individual circumstances of the installation:

• Flywheel;

• Pressure vessel with bladder;

• Dip-tube surge vessel;

• Surge tower.

Air valves should not be depended upon as a sole method of surge control, but their operation under surge conditions should be carefully considered.

Flywheels

Flywheels absorb energy on start-up, slowing the rate of velocity change in the pipeline. In reverse, when the pump is stopping, the flywheel releases energy again, slowing the rate of velocity change. Together these two actions reduce the peak surge pressure.

As the flywheel must be located on the drive shaft it is not suitable for submersible pumps or close-coupled pumps. However, they are simple devices for wet well/dry well pumps and are preferred where possible.

If submersible pumps have been chosen, a larger pump running at a slower speed may have the effect of a flywheel.

Because the flow continues through the pump after the stop signal, the effect on the stop and start levels should be carefully considered.

Pressure Vessels

Pressure vessels for surge suppression are tanks partially filled with a gas (air or nitrogen). Usually the liquid is contained in a bladder with gas on the outside to prevent the liquid absorbing the gas or coming into contact with the inside of the pressure vessel, and this is the preferred type. The bladder material should be carefully selected for use in the conditions experienced in Qatar.

Refilling is usually from a high-pressure cylinder and care should be taken to avoid over pressurisation of the bladder. Bladders should not lose pressure in normal operation, but they can fail, leading to absorption of the gas into the liquid, and a drop in pressure.

Vessels without a bladder are charged with air pressure from an air compressor, either manually or automatically. There is therefore additional machinery and an additional maintenance requirement. This type of surge vessel is not recommended.




On pump start-up, liquid enters the vessel, compressing the gas until it equals the liquid pressure. When the pump stops, the gas pressure forces liquid back out into the pipe system, both actions slow the rate of pressure change, which reduces the peak surge pressure.

To dampen oscillations, a non-return valve may be fitted to the surge vessel outlet pipe, to allow unrestricted flow into the pipeline, and a bypass around the NRV fitted with an orifice plate to restrict the flow back into the vessel.

Dip Tube Surge Vessels

A dip tube surge vessel is pressure vessel, the top portion forming a compression chamber limited by a dipping tube with a shut off float valve.

This type of vessel is particularly appropriate for use on rising mains with flat profiles.

Surge Towers

A surge tower is a vertical tank or pipe fitted into the pipeline, open to atmosphere and the energy storage is by the static head of the liquid in the tower.

Surge towers are only practical for systems with relatively low heads and surge pressures, but can pose an odour risk.

Due to the design of a surge tower, there is no routine maintenance required to ensure the surge tower keeps operating correctly.

It is unlikely that surge towers would be appropriate for use in Qatar.

Air Valves

Air valves are required on the pumping mains to release air, but they should not be used as a surge protection measure.

However, air valves, particularly if fitted with a vented non-return valve or in-flow check valve, may assist in surge control, and their operation must be carefully considered.

Air valves require regular maintenance because if the air valve does not function correctly, large or negative surge pressures could result, with consequent damage to equipment or personnel.

If air is allowed into the rising main on pump stop/trip through an air valve, the pump control system should be designed to prevent a restart until the transient pressures have stabilised.

Control of the pumps is usually by start/stop level signals, but where surge on start-up may have a significant effect, the use of ‘soft’ starters should be considered.

2.10 Screens Screen Selection

Screens should generally be provided for pump protection, unless they are small (<20l/s) submersible stations with small inlet sewers. Screens should incorporate the following features:

• Screen chambers should be separate from the wet wells;

• Coarse screens should be fitted in the screen chambers at the inlet to pumping stations to protect the pumping equipment. They should remove coarse screenings, but allow screenings less than 75mm to pass forward to the STW;

• L-shaped or coarse basket screens should be provided;

• The screens should be set in guides with lifting facilities at ground level so they can be manually removed and cleaned;

• Minimum of one duty and one standby screen should be provided;

• Mechanically raked screens should be considered for large pumping stations, typically >1000l/s;

• Fine screening is not required at the pumping station, but is required at the treatment works to remove debris that may affect the sewage treatment process.

Screen Installation

The manual duty and standby screen should be installed in the incoming channel, so that the standby screen can be lowered into position to




protect the pumps while the duty screen is removed and cleaned.

Mechanically raked screens should be installed in a channel or similar flow-line, which can be completely isolated from the rest of the system and drained for maintenance. A manually raked bypass screen shall be provided.

Mechanical screens shall be housed in ventilated and odour controlled enclosures.

Screens should be provided with actuated penstocks (or valves) before and after each screen for operational and maintenance isolation.

All mechanically raked screens should have an automatic cleaning mechanism, which will clean the screen of accumulated debris and screenings, depositing them in a collection trough or channel above the highest possible water level.

Screenings Handling

Manually removed screenings should be placed in a covered container until removed from site to avoid odour problems.

Mechanically removed screenings should be washed, compacted and deposited into a covered container to avoid odour problems.

2.11 Pumping Station Selection

Sewage pumping station type selection should be carefully considered for each scheme. In general, submersible pumping stations are generally selected for flows up to 100l/s, and wet well/dry well stations for larger flows. However, each station should be treated on its own merits and the following considerations assessed:

• Initial and final design flow;

• Total head on the pumps;

• Rising main profile and the requirements for surge protection (dry well pumps usually have a greater moment of inertia than submersibles);

• Requirement for Variable Speed Drive (VSD): (submersible motors are not always adequately rated for use with VSD);

• Space available for pumping station (submersible stations usually require less space);

• Proximity of housing or public areas (opening submersible pump wells may create odour nuisance).

An alternative to wet well submersible pumps and dry well pumps is the dry well submersible. These should normally be considered only where an existing dry well installation is being uprated and there is insufficient space to install a conventional dry well pump and motor.

Particular attention should be paid to motor cooling and cabling if dry well submersibles are to be considered.

The designer should present three alternative pump suppliers for tender purposes.

Submersible pumping stations

Submersible pumping stations should incorporate the following features:

• Minimum of one duty and one standby pump;

• Non-return valve and gate valves for isolation of each pump;

• Valves to be in a separate, easily accessible chamber adjacent to the pump sump;

• Air reaction operation level controls as follows:

- High level alarm (also float);

- Pump start;

- Pump stop;

- Low level pump protection (also float).

• Ultrasonic level controls should not be used for sewage;

• Air reaction level equipment should include stainless steel dip pipe and duty/standby compressors.

Where the available pumps have unsuitable duties for the full range of flows, the use of variable speed drives should be considered. However, due to the




additional heat generated in the motor, the approval of the pump manufacturer should be obtained before variable speed drives are used.

Submersible Pump Sump Design

The CIRIA guide ‘The hydraulic design of pump sumps and intakes’ by M. J. Prosserxli should be referred to when designing pump sumps. Some pump manufacturers also provide guidance on the design of sumps for their pumps. Sump design should be in accordance with the following criteria:

• Sumps should be designed so that the dimensions satisfy the requirements for the minimum sump volume to ensure the maximum rated pump starts per hour for the motor and switchgear are not exceeded;

• Sumps should be designed to provide a uniform steady flow of water into any pump without creating swirl or entraining air. Unsteady flow can lead to fluctuating loads, vibration, noise and premature failure. Swirl can affect the flow capacity, power and efficiency. It can also result in local vortices that introduce air into the pump, also leading to fluctuating loads, vibration, noise and premature failure;

• Sumps should be designed to prevent the accumulation of sediment, scum and surface flotsam;

• Sump corners should be benched to 45°. Minimising the sump floor area and residual volume will increase the velocity into the pumps and improve scouring;

• The use of flushing devices to improve scour in pump sumps should be considered;

• The velocity in the pump riser pipe at the design duty should be as high as practicable to reduce the risk of solids deposition. However, the velocity should not normally exceed 2.5m/s to avoid significant headloss and risk of pipe erosion;

• The water surface in the sump should be as free from waves and turbulence as possible to provide a strong and reliable echo for ultrasonic level controls;

• At the designed stop level there should still be sufficient water surface area without obstructions to provide a good echo return.

Submersible Pump Installation

When submersible pumps are installed, the following should be considered:

• There should be sufficient space between them to prevent interaction between the pump suctions. This will depend upon the type of pump being used and the manufacturer should be consulted on configurations at draft design stage; A rule of thumb is to use an initial spacing between pump centres of twice the pump diameter. Further guidance is given in table 2.11.1 below.

Table 2.11.1 Approximate Minimum

PumpSpacingsxlii

Flow (l/s) Spacing (mm)

100 700

200 1000

300 1200

400 1350

500 1500

600 1700

700 1800

800 1900

900 2050

1000 2175

• There should also be sufficient space for someone to stand beside each pump, should work be required in the sump;

• Pump mounting stools and duckfoot bends should be securely bolted to the structural concrete of the sump and not the benching;

• Discharge non-return and isolating valves should be located outside the sump in a valve chamber;




• Pump guide rails should rise close to the underside of the sump covers above the pumps;

• The covers should have a clear opening large enough to allow the removal of the pump while on the guide rails;

• Support points for the pump power cables and lifting chain should be provided under the pump covers, which should be easily accessible from the surface.

Wet/Dry Well Pumping Stations

Wet well/dry well pumping stations should incorporate the following features:

• Normally, two sumps with 2 duty and 1 standby pump for each sump, for the ultimate flow;

• Non-return and two gate valves for each pump isolation;

• Where possible, the discharge manifold should be below ground level to minimise additional pipework and friction losses;

• Where wet well/dry well pumping stations are being uprated, dry well submersible pumps could be considered;

• Operation level controls (air reaction) as follows:

- High level alarm (plus float);

- Pump start;

- Pump stop;

- Low level pump protection (plus float).

• Air reaction level equipment should include stainless steel dip pipe and duty/standby compressors.

Where the available pumps have unsuitable duties for the full range of flows the use of variable speed drives should be considered. However due to the additional heat generated in the motor, the approval of the pump manufacturer should be obtained before variable speed drives are used.

Wet Well Design

The CIRIA guide ‘The hydraulic design of pump sumps and intakes’ by M. J. Prosser should be referred to when designing wet wells, which should incorporate the following features:

• Wet wells should be designed to provide a uniform steady flow of water into any pump without creating swirl or entraining air. Unsteady flow can lead to fluctuating loads, vibration, noise and premature failure. Swirl can affect the flow capacity, power and efficiency, it can also result in local vortices that introduce air into the pump also leading to fluctuating loads, vibration, noise and premature failure;

• Wet wells should be designed to prevent the accumulation of sediment, scum and surface flotsam;

• Wet well corners should be benched to 45°. Minimising the sump floor area and residual volume will increase the velocity into the pumps and improve scouring;

• The use of flushing devices to improve scour in wet wells should be considered;

• The water surface in the wet well should be as free from waves and turbulence as possible to provide a strong and reliable echo for ultrasonic level controls;

• At the designed stop level there should still be sufficient water surface area without obstructions to provide a good echo return;

• Wet wells should be designed so that the dimensions satisfy the requirements for the minimum sump volume to avoid excessive pump starts;

• The pump suction pipes should be installed through the wet/dry well dividing wall with a downward bend and bellmouth to position the pump suction as close to the sump floor as possible to assist in sediment removal;

• There should be sufficient space between the bellmouths to prevent interaction between the pump suctions.




Dry Well Design

Dry well design should incorporate the following features:

• The pumps should be installed along the wet/dry well dividing wall with sufficient space between them to allow access for maintenance and repair;

• The pump distance from the dividing wall will be set by the length of the protruding stub pipe, suction valve and pump inlet pipe;

• Drive shafts should be supported from concrete beams spanning the dry well;

• Consideration should also be given to access around the pumps and valves. Platforms and walkways should be installed to provide access to all equipment at a suitable level for safe operation, maintenance and repair;

• The general floor level should be higher than the sump level to reduce the size of pump plinths and the need for access platforms;

• Careful thought should also be given to the shipping route for removing equipment;

• Access to the dry well and machinery should be by staircase so that tools and equipment can be carried in and out safely;

• Lifting arrangements for the pumps and valves shall be provided (see also section 2.21 and 2.22);

• The dry well floor should slope gently towards the dividing wall and then to one side where a sump pump should be installed to keep the floor as dry as possible;

• The sump pump should be installed in a small well, large enough to accommodate the pump and should discharge back through the wall into the wet well. Consideration should be given to the sump pump discharge to avoid backflow from the wet well to the dry well;

• A high level alarm should be installed in the dry well to give a warning of flooding before damage to machinery occurs.

Pump Installation

For the most compact arrangement, a close-coupled pump can be mounted horizontally with the discharge upward, however this results in the motor being low in the dry well and at risk from flooding. The most common arrangement is for a vertical pump shaft with the motor above. This will require a bend between the suction valve and the pump suction. The bend should be fitted with a handhole and valve to enable the pump to be drained prior to maintenance. Further bends may be required to direct the pump or manifold discharge upwards. Where space allows, installation of the discharge manifold at the pump level, with the discharge directly through the side wall should be considered.

Pipes should be sized to achieve sensible velocities, and the risk of cavitation through insufficient NPSH should be considered when designing suction pipework. Pumps must be selected to ensure satisfactory operation when only one pump is operation in a new rising main.

2.12 Pumps and Motors Centrifugal Pumps

These are the most common type pumps for foul sewage and are available in a variety of forms. The pump operates by passing the liquid through a spinning impeller where energy is added to increase the pressure and velocity of the liquid. Submersible pumps are centrifugal pumps.

Sewage pumps should have an open type impeller with a minimum passage of 100mm. Impellers with smaller passages are likely to suffer from frequent blockage due to the nature of sewage debris.

Dry well centrifugal pumps should normally have a maximum running speed of 980rpm. Submersible pumps may run at 1450rpm (4 pole motor), but pumps operating at 2900rpm (2 pole motor) will suffer excessive wear and premature failure, and should not be used.

Pump Motors

Motors on submersible pumps should be certified for use in Zone 1 explosive atmospheres unless operating continuously submerged. Pumps operating in dry conditions should have a casing designed to provide adequate cooling in the operating conditions.




Pump motors should normally be fed from 415 volts, 50 hertz, 3-phase power supply. For larger motors 690V or 3.3KV motors can be used.

Because additional heat is generated in the motor when used with a variable speed drive, the approval of the pump manufacturer should be obtained before VSDs are used.

For dry well and screw pumps where the motors are installed vertically or at a steep angle, they should be specifically designed for that purpose, with adequately rated end thrust bearings.

Where flywheels are installed, the motor rating shall be suitably uprated.

2.13 Sump Design The CIRIA guide ‘The hydraulic design of pump sumps and intakes’ by M.J. Prosserxli should be referred to when designing sumps or wet wells.

Sumps should be designed to provide a uniform steady flow of water into any pump without creating swirl or entraining air. Unsteady flow can lead to fluctuating loads, vibration, noise and premature failure. Swirl can affect the flow capacity, power and efficiency. It can also result in local vortices that introduce air into the pump also leading to fluctuating loads, vibration, noise and premature failure.

Sumps should also be designed to prevent the accumulation of sediment and surface scum.

Most sumps and wet wells at standard pumping stations will probably be uniform in section and can be designed to avoid turbulent flows.

Modelling

For non-standard pumping stations, which may have high flows, multiple pumps or complex shapes, or where turbulent flows, vortices, swirl or air entrainment are more likely to occur, modelling should be considered.

For pumping stations, a physical model built to scale can be very effective in identifying flow problems and in some cases modelling by computational fluid dynamics (CFD) methodology may have benefits. Modelling is the process of replicating the hydraulic

performance of drainage, pumping and treatment systems by constructing models of the intended installations. These models need to be verified before use to provide confidence that they adequately represent the actual performance of the system.

The verified model is then used to test system performance under its proposed use. The model must be capable of modification to test various physical configurations and operating regimes for the installation, to produce the optimum solution for actual construction.

Traditionally, physical models were favoured, especially for coastal/estuary/river systems and complex pumping installations. In recent years mathematical models have superseded physical models. Mathematical models are exploiting increased computer hardware and software capability, and are more efficient than physical models in time and effort.

Physical Models

Physical modelling consists of constructing a reduced scale, geometrically similar model of a proposed system, and operating the model to simulate full-scale flow conditions. Model tests can provide the designer with the assurance that the proposed scheme operates satisfactorily, or allows him to improve the flow conditions and achieve a better design.

Changes in the model can be made by trial and error, and are usually based on the experience and intuitive understanding of the engineer conducting the tests. The amount of modification which can be undertaken on a physical model is limited, and therefore the initial model should be as accurate as possible.

Factors to be considered in deciding on the need for physical models include:

• The similarity of the proposed scheme to existing satisfactory designs. As well as the designer’s own experience, much information is available from manufacturers’ published reports and design guides. However, it should be recognised that most large scale and/or complex designs will be unique, and hence modelling will be needed;




• The size and cost of the proposed scheme. Bearing in mind that physical modelling can take many months with corresponding high costs, then designers of small schemes should seek to adopt standard and well-proven designs for small schemes. Large schemes, such as terminal pumping stations with multiple pumps and complex inlet arrangements would merit modelling;

• The time available for modelling. In some cases the scheme can be well under way to completion before the possible need for modelling is realised. Even at such late stages, modelling can save much time and cost in modifying construction works.

For pumping stations, all of the intake should be modelled, including the approach works, the inlets and the sump itself. Upstream pipelines may need to be included.

All hydraulically significant details such as screens, penstocks, support channels and benching, should be included in the model. No components above maximum water level need be modelled.

Model construction should be in durable and waterproof materials, with clear perspex being the best for viewing purposes. Model size should be as large as costs allow. Scales can vary from perhaps 1:4 for very small sumps, up to 1:50 for large intakes to reservoirs or tanks. For sump models, 1:25 would be the smallest desirable scale.

Physical testing could typically take between one and six months for construction, testing and reporting.

Sump Volume

Pump sumps should have a minimum sump volume calculated to ensure that in the worst flow conditions any pump installed does not exceed the maximum allowable starts per hour. The CIRIA guide ‘The hydraulic design of pump sumps and intakes’ by M.J. Prosserxli should be referred to when designing sumps or wet wells.

The minimum sump volume is the volume between the start and stop levels of the duty pump and for a single pump the worst case occurs when the inflow is exactly half of the pumping rate.

To calculate the minimum sump volume for a specific pump the formula contained in the above CIRIA guide is:

T = 4V/Qp

Equation 2.13.1

Where:

T is the cycle time for the pump, e.g. if the recommended maximum starts per hour for a pump is 10, then the cycle time will be 6 minutes (60/10 = 6)

V is the volume of sump between the start and stop levels in m3

Qp is the pumping rate in m3/minute

Therefore if Qp is 1.2m3/min (20l/s) and the maximum number of starts is 10/hour, the volume required will be:

V (m3) = 6(min) x 1.2(m3/min) / 4

V = 1.8m3

For 10 starts per hour this could also be expressed as:

V = 1.5 x Qp

The sump volume when multiple pumps are installed is calculated as for a single pump, where the minimum sump volume is the capacity between the start and stop level for each pump. However, additional capacity is required to allow a vertical distance of 150mm between the start or stop levels of consecutive pumps.

With sewage there is a possibility of septicity, therefore there are restraints on the maximum volume of the sump related to the retention time of the liquid in that sump.

Maximum and minimum start / stop levels

The minimum stop level should be the level at which the pump can be stopped and restarted without losing suction or as specified by the pump manufacturer.

To avoid turbulence and odour release at foul sewage pumping stations, the lowest pump stop




level is usually set at the invert of the incoming sewer, the last section of which is laid to a steep fall to avoid the sewer being used as the sump.

The minimum start level should be the required distance above the stop level to provide the minimum sump volume.

Allowable pump starts per hour

The maximum allowable starts per hour should be as specified by the pump or motor manufacturer. In the absence of any specified figure the following are suitable guidance figures:

Less than 100kW - 15 starts/hour

100kW < 200kw - 10 starts/hour

>200kW - 8 starts/hour

Stop / start levels for single and multiple pump operation

The start and stop levels for single pump operation should be set within the maximum and minimum start / stop levels defined in the previous section, provided that the minimum sump volume is attainable.

The start level for each additional pump should be set a suitable height above the previous pump to prevent accidental pump starts caused by surface waves or level sensor errors.

The stop level for each additional pump should be set at the required distance below the start level to provide the minimum sump volume for that particular pump. The stop level will normally be just above the previous duty pump stop level.

The effect of flywheels should be considered in determining stop/start levels because the flywheel increases the pump start-up and stop times.

Pump duty level

The pump duty level for a single pump should be the midpoint between the pump start and stop levels. For multiple pump installations it should be the midpoint between the top water level (last duty pump start level) and the bottom water level (first duty pump stop level).

Pumps should also operate within their performance curve at both top and bottom water levels under single or multiple pump operation.

2.14 Suction/Delivery Pipework, and Valves

Pipework

Only superior materials are acceptable for use in pumping station pipework. The pipework installation should incorporate the following features:

• Sufficient bends and flange adapters to allow easy dismantling and removal of pumps, non-return valves or other major items of equipment;

• Each dry well pump should be installed with suction and discharge isolation valves to permit isolation of the pump from the wet sump and discharge pipework for maintenance;

• Each submersible pump should be installed with a discharge isolation valve to permit isolation of the pump from the discharge pipework for maintenance;

• Each pump should also be fitted with a non-return valve to prevent reverse flow back through the pump when stopped;

• Valves should be positioned to permit the removal of each pump and non return valve without draining either the wet well or discharge manifold, and allow the other pumps to continue operating normally;

• Suction isolating valves for dry well pumps should be bolted directly to a flanged pipe securely fixed through the sump wall;

• Discharge isolation valves should be bolted directly to a flange on the discharge pipe or manifold;

• Discharge non-return valves should be bolted directly to the discharge isolation valve. They should be installed in horizontal pipework with a short length of pipe and a flange adapter on the pump side to allow dismantling;




• Where the pump delivery pipework joins the pumping station discharge manifold, the entry should be horizontal;

• At the opposite end of the pumping station discharge manifold, a valved connection back to the sump should be provided for draining the discharge pipework, or flushing the sump;

• Consideration should be given to providing an isolating valve on the pumping main before it leaves the pumping station/chamber and before any over pumping connection, to allow the pumping station to be fully isolated and the fixed pipework drained for repair;

• All flexible couplings should be restrained on both sides by securely fixed equipment, thrust blocks or tie straps across the coupling to prevent displacement of the coupling under pressure.

Valves

Valves should incorporate the following features:

• Isolation valves for sewage should be of the double-flanged wedge-gate type with a bolt-on bonnet. When fully open, the gate should be withdrawn completely from the flow. The valve should have an outside screw rising stem and the handwheel direction of operation should be clockwise to close. Station valves should have metal seats;

• Valves greater than 350mm diameter should be fitted with actuators. Where installed in chambers they could be fitted with non-rising stems to limit the headroom required;

• Reflux valves for sewage should be of the double flanged, quick action single door type, designed to minimise slam on closure by means of heavy doors, weighted as necessary. The door hinge pin/shaft should extend through the side of the body and be fitted with an external lever to permit back flushing;

• Reflux valves should be provided with covers for cleaning and maintenance without the need to remove the valve from the pipeline. The covers should be large enough so that the flap can be removed and the valve can be cleaned;

• The non-return valves should have proximity switches to prevent dry running and allow a change of duty (standby on high level will then start);

• All reflux valves should be installed in the horizontal plane;

• Butterfly valves should not be used with sewage.

2.15 Pumping System Characteristics

NPSH, Vibration, Cavitation and Noise

Net Positive Suction Head (NPSH) is used to check the pumping installation for the risk of cavitation.

Cavitation is the formation and collapse of vapour bubbles in a liquid. Vapour bubbles are formed when the static pressure at a point within a liquid falls below the pressure at which the liquid will vaporise. When the bubbles are subjected to a higher pressure they collapse causing local shock waves, if this happens near a surface, erosion can occur.

Cavitation will typically occur in the impeller of a centrifugal pump, where it can cause noise and vibration as well as affecting the pump efficiency. If allowed to persist it can lead to damage to the pump or even breaking away of foundations.

NPSH is the minimum total pressure head required in a pump at a particular flow/head duty. It is normally shown as a curve on the pump performance sheet.

NPSH = Pa – Vp + Hs – Fs

Equation 2.15.1

Where:

Pa = atmospheric pressure at liquid free surface

Vp = vapour pressure of liquid

Hs = height of supply liquid free surface, above eye of pump impeller

Fs = suction entry and friction losses




In order to avoid cavitation, the NPSH available should be at least 1m greater than the NPSH required by the selected pump at all operating conditions.

When calculating NPSH, absolute values for atmospheric and liquid vapour pressures are used.

Pump Duty Point

Each pump has a performance curve where the flow is plotted against head.

Each pipework system has a friction curve where the friction head is plotted against flow.

The system curve is obtained by adding the static head to the friction losses and plotting the total head against the flow.

The pump duty point is where the pump performance curve and the system curve cross. It shows the flow that a particular pump will deliver through the pipework system at a particular total head at the pump duty level.

In multiple pump installations, it is essential that the operating conditions of a single pump running are carefully checked to ensure that the pump will operate at maximum and minimum static heads satisfactorily, and without risk of cavitation.

The duty point should be used when considering the suitability of alternative pumps for a particular duty by comparing the efficiency and power requirements for each pump at the duty point.




Figure 2.15.1 – Characteristic Curve for Multiple Pumps

Characteristic curve for new pipe




2.16 Sump Pumps and Over-Pumping Facilities

Sump pumps should be provided for all dry wells and wet wells at pumping stations. For dry wells they should be used to remove any water that may collect at low level. For wet wells, they should be used to empty the wet well prior to man entry.

Over-pumping facilities should be provided where there is a single sump and access may be required for repair of pumps/screens/etc. A suction chamber should be provided before the pumping station, with a penstock to isolate all flows into the pump sump. A connection into the pumping main should be provided for the over-pumping discharge. Consideration should be given to providing an isolating valve on the pumping main before the over-pumping connection to allow the pumping station to be fully isolated and the fixed pipework drained for repair.

Sump Pump Installations

Sump pumps should incorporate the following feature:

• Sump pumps should discharge to the wet well above the water level to prevent gas release;

• Discharge pipes should be fitted with a non-return valve and isolating valve, in an easily accessible position;

• The sump pump should be fitted with a discharge connection and guide rail to allow the pump to be easily removed from the sump for cleaning or unblocking;

• Where a temporary sump pump is to be used, a power supply point and discharge connection should be provided. Both should be located at a high level in the dry well, and be easily accessible from the access walkways.

Sump pumps should be installed in a sump of sufficient dimensions for the proposed pump and allow a suitable level controller to operate within the sump, the minimum depth should be 300mm.

The sump pumps should be sized for the possible leakage of glands and seals. A guide should be 0.5l/s for each leakage point, with a minimum of 5l/s. An assessment should also be made of any possible inflow from outside the dry well (i.e. rain and flooding).

2.17 Power Calculations including Standby Generation

2.17.1 Introduction

A standby power generator set is essential in applications where the loss of the power supply can not be accepted due to critical loads. The generator set configuration and sizing will vary from one application to another dependent on the load type, operation characteristics, site condition, and application requirements.

The sizing and selection of the generator set should take into consideration the aspects raised in the following sub-sections.

2.17.2 Load Type

In some applications, the total connected load in the pumping station will need to be powered from the generator set in case of power failure, while in other locations only the essential load will need to be kept running (partial loads). The designer should consider the requirements according to the site characteristics and the proposed application, to size the required generator set. The following points are to be investigated at the initial stage to select the type of generator that is required:

• Voltage level according to load voltage level (415v, 3.3kv, 6.6kv, 11kv);

• Total generator connected load;

• Individual load characteristics such as kilowatt rating, maximum allowable voltage dip by the motor manufacturer, starting method, sequence of operation;

• Load type - inductive or capacative;

• Load profile.




2.17.3 Site condition

The site condition should also be examined and the following data collected and submitted to the generator set manufacturer to be considered in the sizing process:

• ambient temperature;

• elevation above sea level;

• humidity;

• wind direction and dust contamination in air;

• nearby residential areas for sound level consideration.

2.17.4 Generator set operation and control

The generator set operation and control varies from application to application depending on the following points:

• Number of units to be controlled;

• Manual or automatic synchronisation;

• Manual or automatic start-up;

• Manual or automatic changeover switch between main local authority incomer and main generator set incomer (control panel outgoing feeder).

2.17.5 Type of installation

Standby generator sets can be installed by different means according to the site requirement and unit size. The type of installation can be categorised in the following ways:

• Building installation: The unit will be installed inside a building suitable to accommodate all the units and their ancillaries. This type of installation is recommended in large or major pumping stations, or treatment plants;

• Weatherproof enclosure: The unit is mounted inside a weatherproof enclosure on a trailer suitable for transportation between different sites;

• Soundproof enclosure: The unit is installed inside a soundproof enclosure, mounted on a trailer suitable for transportation and operation in residential areas;

• Skid mounted unit: For temporary site work (e.g. construction site).

2.17.6 Type of Control Panel

The control panel can be unit mounted (on the generator set unit) or remotely mounted (inside the control room).

The control panel is used to operate and monitor the unit in case of power failure. Panels have many options depending on the type of operation required, and the mode of operation (one unit, two units, automatic start, manual start, etc).

2.17.7 Ventilation system

Unit ventilation and the cooling system are critical parts of the overall system performance and capability. The ventilation system is required to keep the surrounding atmosphere temperature as per the specified ambient temperature, to avoid any temperature rise due to heat generation from the engine. The ventilation system should be by the means of forcing air out of the room using a fan installed at a level above the highest point in the generator (e.g. roof mounted or wall mounted). The air will be delivered through air louvers mounted at the lowest permissible level to avoid sand ingress from the surrounded area and at the same time to guarantee airflow across the generator set body.

In addition to the room ventilation, the generator should have an engine driven fan. This will draw air through sand trap louvers in the wall, and over the alternator and engine, discharging the air through a set mounted radiator and wall mounted outlet louvers.

2.17.8 Fuel system

The fuel system usually consists of a main storage tank, daily fuel tank, fuel transfer system, and fuel line between tanks and the generator set:




• Main storage tank. This will be required in applications where the fuel consumption at site is very high due to a large number of units installed, or due to the difficulty in providing daily supply of fuel to the site. In that case, the storage facility of the main storage tank should be sufficient for three days consumption. The bulk tanks should normally be mounted partially below ground level within bunds to enable the day tank to empty under gravity back to the bulk tank in the event of a fire;

• Daily fuel tank. The daily fuel tank should be suitable for eight hours full load operation, and normally mounted on a stand beside the generator set to enable gravity feed to the engine;

• The fuel transfer system. A fuel transfer system is required between the main tank and daily tank to keep the daily tank full and ready for operation. The tank level should never fall below a minimum level. The system consists of transfer pumps, level sensor, control panel, valves (solenoid valves, actuated valves, hand operated valves) and flow meter to monitor the units consumption, as well as the delivery supply to the main tanks.

• A thermal ‘cut-off’ link must be mounted above the engine, arranged to close both a valve on the fuel line between the day tank and the engine, and also a dump valve to drain the day tank back to the bulk tank in the event of a fire.

2.17.9 Starting method

The generator starter method is usually one of the following methods:

• Air starting method. This type of starting is suitable for large generator sets requiring a high starting torque, especially medium and low speed engines (750RPM, 600RPM). This usually consists of:

a) Air operated starter unit (sized by the generator set manufacturer);

b) Air tank vessel (suitable for six starts before refill);

c) Electrically operated air compressor unit (capable of refilling the tank within 15 minutes);

d) Diesel operated air compressor with the same capacity working as backup for the electrical air compressor;

e) Air piping between air vessel and starter unit.

• Electrical starting method. This type of starter is suitable for small loads, transportable and enclosed units, which work at high speeds (1500RPM). The starting method consists of an electrically operated starter, battery, and charging alternator. A battery charger is required to keep the battery fully charged and ready for operation in cases where the unit is rarely operated. The battery type should be maintenance free for high reliability starting;

• Starting aid. Some applications require immediate starting and load handling without any delay due to critical load type. To get the generator set ready for such an application the unit should be equipped with a jacket water heater to keep the engine warm and ready for load immediately after starting without any delay for warming the engine before applying the load.

2.17.10 Service facility

The generator set building should be equipped with an overhead crane capable of lifting the heaviest part likely to be encountered during maintenance of the generator set. The main inlet and outlet louvers and building shall be designed such that the complete generator set can be installed and removed through the louver openings. For container or enclosure units, a lifting facility should be provided for offloading and transporting the unit. The enclosure should be capable of having the side and roof dismantled and removed for ease of maintenance and parts replacement.

2.17.11 Generator set sizing

The following procedure can be used to size the generator set according to the available data from pump motors and other loads (e.g. lighting/other




non-motor load) as well as the sequence of operation, and starting of the motor:

1) Starting KVA (SKVA) calculation

- Calculate lock rotor current (LRA) = for DOL x Full load current

- Calculate the SKVA = (LRA V * 1.732)/1000

2) Effective SKVA

Use Table 2.17.1 as a guideline for calculating the effective SKVA.

Suppose that we have three motors, which will start and run in sequence (motor-1, motor-2 and motor-3).

Using the highest effective SKVA calculated and the required voltage DIP (10%, 20%, and 30%) as specified by the motor manufacturer, the generator set can be selected from the data sheet provided by the generator set manufacturer.




Table 2.17.1 – Guide to Generator Set Sizing – Effective SKVA

Step Motor 1 Motor 2 Motor 3 Comments

1 Motor load (KW) A B C KW motor/ motor efficiency

2 Starting KVA (SKVA) X Y Z LRA*V* 1.732 /1000

3 Total motor load connected before the required motor start

in sequence

0 A

A+B

4 Total motor load connected after motors have been

started in sequence

A A+B A+B+C

5 (Step3/Step4)*100 0 (A/(A+B))*100 ((A+B)/(A+B+C))*100

6 Using step-5 result, obtain compensation for motor

already started from fig.2.17.1

D E F From Fig. 2.17.1

7 Multiply (step-2xstep-6)

X*D

Y*E Z*F

8 Obtain the reduce voltage factor from fig.2.17.2

Q R S From Table 2.17.2

9 Effective SKVA ( Step-7 x Step-8)

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

40 50 60 70 80 90 100

multiplier

Figure 2.17.1 - Reduced voltage starting factor

Table 2.17.2 - Reduce voltage starting factor

Type Multiply SKVA BY

Star/Delta 0.33

Auto transformer 80% , 65%, 50%

0.68, 0.46 , 0.29

DOL 1.0

Solid state Estimate 300% of full load KVA




Worked example:

The following motors required a standby generator

i. 90 kw 3-phase motor, soft starter , voltage dip 30%

ii. 70 kw 3-phase motor, star/delta ,voltage dip 30%

iii. 45 kw 3-phase motor , direct online (DOL), voltage dip 30%

1- Calculated Locked rotor current

90 kw motor = 6 x 90,000 = 939 amp √3 x 415x0.8

75 kw motor = 6 x 75,000 = 730 amp √3 x 415x0.8

45 kw motor = 6 x 45,000 = 469amp √3 x 415x0.8

2- Calculated SKVA

90 kw motor = 939 x 415 X 1.732 = 674.9 1000

75 kw motor = 730 x 415 X 1.732 = 524.7 1000

90 kw motor = 469 x 415 X 1.732 = 337.1 1000

Table 2.17.3 – Generator Set Sizing – Worked Example

Step

Motor1 Motor 2 Motor 3 Comments

1

Motor load KW

90

70

45

KW motor/ motor efficiency

2 Starting KVA (SKVA) 674.9 524.7 337.1 LRA*V*1.732/1000

3 Total motor load connected before the

required motors start in sequence 0 90 160

4 Total motor load connected after motors have been started in sequence

90 160 205

5

(Step-3/step-4)*100 0 56.3 78

6 Using step-5 result obtain compensation for already start motor

1 1.15 1.25 from Fig. 2.17.1

7 Multiply (step-2xstep-6)

674.9 603.4 421.4

8 Obtain the reduce voltage factor 3 0.33 1 from Table 2.17.2

9 Effective SKVA ( Step-7 xstep-8) 337.5 199.1 421.4 NB Motor 1 is a solid state starter

The selected generator will be sized for the highest effective SKVA @30% Voltage dip = 421.4KVA.




2.18 Switch Gear and Control Panels

Low voltage switchgear and control panels form the link between the electrical load, such as; motors, lighting, actuator valves, air conditioning equipment and the power generation source (main authority supply, generator set).

The design of the switchboard should take into consideration the points discussed in the following sub-sections.

2.18.1 Type–tested and partially type tested assemblies (TTA and PTTA)

According to BS EN60439-1xliii the low voltage switchgear (assembly) and its component parts shall be made in a way that it can be safely assembled and connected. Assure that this configuration of assembly and its components are safely operated without any risk to the operator or equipment. Some of the risks that can affect the operation to be considered include:

1. Direct and indirect contact with live parts;

2. Temperature rise;

3. Electrical Arc;

4. Overload;

5. Insulation failure;

6. Mechanical failure.

To achieve a type-tested assembly (TTA) the following performance requirements should be verified:

• Temperature – rise limits;

• Dielectric properties;

• Short circuit withstand strength (main circuit);

• Effectiveness of protective circuit;

• Short circuit withstand strength of the protective circuit;

• Clearance and creepage distance;

• Mechanical operation test;

• IP degree of protection.

The partially type-tested assemblies (PTTA) are assemblies that contain both type-tested and non type-tested arrangements (derived by calculation from the type-tested arrangements compliant with tests required for TTA).

2.18.2 Total connected load

The control panel sizing and design to cover the demand of the total load connected, including the standby load.

2.18.3 Short circuit level

The short circuit level calculation carried out according to the total connected load and power source from the local authority electricity network. The short circuit level is one of the most important criteria in switchboard design. Its importance arises from the need to protect the equipment with the correct protection device, suitable for the specific level of short circuit, so that no damage or harm can affect the equipment or human safety. Care must be taken in the design stage to control the fault level. If the total connected load is too high, the total connected load to the switchgear can be split into two or more assemblies to reduce the fault level.

The short circuit level can be calculated according to the following steps.

Step-1 Determine the transformer full load amperes:

I(fl) = KVAx1000E (l-l) x1.732 Equation

2.18.1

Where:

I(fl) = transformer full load

KVA = transformer capacity volt ampere

E (l-l) = line to line voltage

Step-2 Find the transformer multiplier




Equation 2.18.2

Where:

Z (T) = transformer impedance

Step-3 Determine the transformer let through short circuit current

I s.c = I (fl) x Multiplier

Equation 2.18.3

Where:

I s.c = transformer let through short circuit current

Table 2.18.1 shows some examples of expected and standard fault level.

Table 2.18.1 – Example of Expected and

Standard Fault Level

Short circuit level Type of application

16KA/1sec Distribution board

(≤250 Amp)

35KA/1sec Motor Control Centre

(≤400 Amp)

50KA/1sec OR 50

KA/3sec

Motor Control Centre

(≤2000 Amp)

80KA/1sec OR

80KA/3sec


(≤3000 Amp)

120KA/1sec OR

120KA/3sec


(≤5000 Amp)

2.18.4 Type of co-ordination

Electrical component co-ordination according to IEC 97-4-1xliv, provides two types of protection. Manufacturers test components such as contactors and circuit breakers in unison to confirm what will happen under short circuit conditions.

According to IEC 947-4-1, the co-ordination between the electrical components can be categorised into the following two types:

Type – 1: co-ordination (personal safety only);

Type – 2: co-ordination (personal/components safety).

The designer, where possible, should select type-2 co-ordination to assure full protection of personal safety as

well as the electrical components. In the event of a short circuit, this type of co-ordination will ensure that the components are reusable after fault clearance. Type-1 co-ordination only guarantees personal and electrical installation safety, and the equipment may not be able to resume operation without repair or replacement of the affected part.

2.18.5 Form of internal separation

The form of separation should be according to BS EN60439-1xliii or suitable equivalent. The designer should consider Form-4 (see Figure 2.18.1) in all designs for high personal safety and equipment protection.

In the case of multiple incomers and/or feeders, Form-4 should be considered for ease of maintenance without the need for interruption to other equipment as would be the case with Form-2

In case of multi-incomer and outgoing starters/feeders, Form-4 should be considered for ease of carrying out maintenance without interruption to other equipment, in case of isolation of certain feeders.

The Type to be used can vary between Type-3 and Type-7 as shown in Figure 2.18.1, diagram (1& 2).

According to the project requirements or budget limitations, Form-2, Type-2 (diagram-3, Figure 2.18.1) should be considered in some applications, such as unit mounted control panels (e.g. scrubber units, sludge drying beds) where the shutdown of the unit is mandatory to carry out maintenance on the unit.

I s.c = I (fl) x Multiplier

Multiplier = 100

%Z (T)




Figure 2.18.1 – Form and Type of Internal

Separation

Form-4 type-3: Diagram-1

Cable gland

Internal Separation

Enclosure

Terminal forexternalconductor

Bus bar

Functionunit

Form-4-Type-7: Diagram-2

Internal

Separation

Enclosure

Terminal for

external

conductor

Bus bar

Function

unit

Form –2 – Type-2: Diagram-3

Internal

Separation

Enclosure

Terminal forexternalconductor

Bus bar

Functionunit

Cable gland

2.18.6 Bus Bar rating

The bus bar rating should be suitable to carry the total connected load. As mentioned previously, consider any future loads by increasing the size of the bus bars and also consider the suitability of extension at both ends.

2.18.7 Type of starter

The designer should consider the following points when choosing the starter type to be used.

Motor size

The motor size (kW) will determine if a standard starter can be used (direct on line DOL or start delta starter Y/D), or if a more advanced type of starter such as a soft starter is required. The main issue to consider is the starting current. The greater the (kW) rating, the greater the starting current required. A high starting current has an overall effect on the system stability and other equipment installed. The following ratings can be considered as general guidelines only. The designer should apply knowledge and experience to justify the starter method to be used.

Table 2.18.2 – Guideline Starter Methods for

Motor Ratings (kW)

Motor rating KW Starting method

≤ 5kw Direct online (DOL)

5 ≥ kW ≤25 Star delta (Y/D)

>25kw Soft starter ( solid

state drive) (S/S)

Motor duty and application

The motor duty will vary according to its application. The following table gives examples of such duties.

Table 2.18.3 – Example Motor Duties and

Applications

Duty type Application example

Continuous run at

constant load and

speed

Potable water

Short run at

constant load and

Sewage pumping station




speed

Continuous run at

variable load and

speed

Irrigation network

Intermittent

periodic duty

Injection system

Motor Application

The type of motor starter can also be selected according to the motor application as mentioned in Table 2.18.3, as a high number of starts per hour will cause even a small motor to overheat. An example of a suitable starter for each application is presented in Table 2.18.4.

Table 2.18.4 - Example Starter Methods for Duty

Types

Duty type Starter

Continuous run at

constant load and

speed

DOL, Y/D, S/S

Short run at

constant load and

speed

DOL, Y/D

S/S if sufficient cooling

time between operations

Continuous run at

variable load and

speed

VSD

Intermittent

periodic duty

D.C starter, DOL

Notes: DOL: direct online, Y/D: star/delta , s/S:

soft starter, VSD: variable speed drive

Voltage level

Starter type can be varied according to the voltage level. In the medium voltage range (e.g. 3.3kv) the starting current will be very low when compared with a lower voltage (e.g. 415v). In this case, the use of a direct contact starter would be acceptable.

Cost considerations

The cost of the starter should also be considered when compared to the motor size and application. As an example, a soft starter could be used to reduce the starting current for a 10kW motor. Taking into account the cost of the soft starter and

comparing it to the cost of the motor, the starter could cost more than the motor however.

Star delta starters can for most applications be considered more economically viable than a soft starter, therefore balance the motor cost against soft starter cost.

2.18.8 Protection device

The designer should categorise all loads connected to the switchgear according to critical status in the process and effect on operator safety. Table 2.18.5 provides examples.

Table 2.18.5 – Examples of Protection Required

for Load Types

Load type Type of protection

Protective device

Main incomer feeder

(local authority/ generator set)

Overload, short circuit, restricted earth fault, phase losses, phase reveres.

- main MCCB or ACB

Pump, grinder

Overload, short circuit, earth leakage, phase losses, phase reveres, under voltage, motor stall, winding temperature.

1- conventional protection device (OLR), MCCB

2- Electronic protection devices

3- motor manager protection unit

Valve actuator

Overload, short circuit, earth leakage.

Conventional protection device

(OLR), ELCB

Instrument

(level/ flow/ pressure)

Overload, short circuit, earth leakage


(OLR), ELCB

Building services

(lighting/ sockets)

Overload, short circuit, earth leakage, phase losses , phase reverses.


MCB, ELCB, Fuses




Note: ELCB = Earth leakage circuit breaker

OLR = Over load relay

MCCB = Moulded case circuit breaker

ACB = Air circuit breaker

Type of protection

1. Short circuit protection:

This type of protection is required to protect the equipment against short circuit (with three phase, two phase or single phase), which can occur due to: insulation failure or damage, or by an incorrect switching operation. Short circuits are associated with electrical arcs and can therefore pose a fire risk.

2. Overload protection:

This type of protection is required to protect the equipment against overload current which is due to operational over current present for an excessive period of time. This over current will raise the motor winding or cable temperature above the permissible level and shorten the service life of the insulation. The task of overload protection is to allow normal operational overload current to flow, but to interrupt these currents before the permissible loading period is exceeded.

3. Under/over voltage protection:

This type of protection is required to protect the equipment against over/under voltage which is present due to main power supply instability (e.g. transformer tap changing/load fluctuating) or unstable supply from a standby generator (due to large load connected, faulty governor or voltage regulator). Operation with an under-voltage condition will draw more current from the supply, this over current will raise the motor winding or cable temperatures above the permissible level and shorten the service life of the insulation. The same will be the case with over-voltage which will effect the insulation of the motor or cable leading to insulation failure. This type of protection can be applied at the main incomers of the switchgear by a special relay to sense the voltage supply and trip the main incomers if the set limits are exceeded.

4. Phase losses/phase reversal protection:

This type of protection is required to protect the equipment against phase loss from the main supply, or phase reversal which can happen in the event of main supply reconnection or reconnection of the motor after maintenance. Operation with phase loss will raise the motor winding temperature due to an unbalanced current in the motor winding. In the case of phase reversal, the motor direction will be reversed, which will result in equipment damage or faulty operation (pump vibration, high sound levels etc). This type of protection can be applied at the main incomers of the switchgear or motor feeder by a special relay to sense the phase status (direction/availability) and trip the main incomers/feeder when a fault occurs.

5. Earth leakage protection:

This type of protection is required to: protect the equipment and personnel in the event of indirect contact; give additional protection in the event of single phase direct contact; earth fault protection; and protection against fires resulting from earth fault leakage current.

This type of protection can be applied at the switchgear outgoing feeders (motor / distribution board) by a special relay which senses the earth leakage current through a summation current transformer, the unbalanced current from the transformer will release a mechanism that will trip the breaker when a fault occurs.

6. Motor protection relay (electronic relay):

This type of protection is used to protect the motor against many faults that can affect the motor operation and safety. The actual protection type can be varied according to the motor application (critical/normal) and size (cost). The following types of protection can be achieved by a motor protection relay:

• Over / under current;

• Phase loss/ unbalance/reversal;

• Ground fault;

• Locked rotor;




• Motor stall.

This type of protection can be applied at the motor terminals. The fault signal from the relay will release a mechanism that will trip the breaker when a fault occurs. Fault indication will usually be displayed on a LCD screen or by indication LED’s.

2.18.9 Interlocking facility

An interlocking facility is required where more than one incomer is used in the switchgear required. Some examples are as follows:

• Supply from two transformers/local authority supply;

• Supply from two incomers - one from transformer/local authority supply, and one from standby generator(s) panel;

• Supply from three incomers - two from transformers/local authority supply, and one from standby generator(s) panel.

The interlock facility should guarantee the safety of operation by not allowing under any condition the connection of two different incomers to the same bus bar section (transformer/transformer) or (transformer /generator) or main bus bars with the bus coupler closed.

2.18.10 Accessibility

The panel access for cable termination and maintenance can be arranged in the following format:

• Front access (suitable for installation area with limited space at the back of the MCC);

• Back access (suitable for installation area with available space at the back of the MCC, minimum one metre);

• Front/back access.

2.18.11 Cable entry

Cable entry to the MCC can be arranged in the following format:

• Bottom entry (suitable for MCC fixed at the top of cable/MCC trench);

• Top entry (suitable for MCC with cables such as feeders and incomers installed at ground level or above the MCC top level). Top entry panels are not preferred and should only be used in special circumstances.

Cables should be sized and installed in accordance with the IEE (Electrical Wiring) Regulations and QGEWC Regulations, and de-rated in accordance with the Electrical Research Association Report No. 69-30xlv.

Instrument, alarm, and control cables should be segregated from power cables.

The designer should consider the following when selecting cable routes:

• Number, size and function of cables;

• Access for installation and maintenance;

• Interface with other equipment, e.g. cable routes should not prevent other equipment being removed for maintenance;

• Risk of mechanical damage ;

• Means of support;

• Effect of installation method on de-rating factors;

• Hazardous area classification.

2.19 PLC’s SCADA/Telemetry

2.19.1 PLC

PLC stands for Programmable Logic Controller. The PLC is a microprocessor-based device which is programmed to perform certain controlling tasks. The PLC is the brain of the overall process. It can receive analogue and digital signals from the process devices, analyse them and send digital and analogue signals to control these devices or activate certain alarms.




PLCs were originally used for controlling purposes. Almost all PLCs are now equipped with signal transmitters (i.e. include some RTU features) that are capable of transmitting data to the network operation centre.

A redundant PLC system with hot standby configuration is highly recommended for critical applications where uninterrupted control is required. The power supply for the PLC system is usually 24Vdc or 110Vac. In case of power failure, the equipment should be backed up by a UPS system, which can supply the PLC with up to eight hours of power depending on the importance of the process.

The modular type CPU (Central Processing Unit) in the PLC is capable of: solving application logic; storing the application program; storing numerical values related to the application processes and logic; and interfacing to the I/O systems.

The PLC carries out PID control, which is a significant task. PID (Proportional-Integral-Derivative) control action allows the process control to accurately maintain a setpoint by adjusting the control outputs. For example, pump flowrate setpoint is maintained by the following:

• Proportioning Band: is the area around the setpoint where the controller is actually controlling the process. The output is at some level other than 100% or 0%. The band is generally centred around the setpoint (on single output controls), causing the output to be at 50% when the setpoint and the flow rate are equal;

• Automatic Reset (Integral): corrects for any offset (between setpoint and process variable) automatically over time by shifting the proportioning band. Reset redefines the output requirements at the setpoint until the process variable (flowrate) and the setpoint are equal;

• Rate (Derivative): shifts the proportioning band on a slope change of the process variable. Rate, in effect applies the ‘brakes’ in an attempt to prevent overshoot (or undershoot) on process upsets or start-up. Unlike Reset, Rate operates anywhere within the range of the instrument. Rate usually has an adjustable time constant and should be set much shorter than

reset. The larger the time constant, the more effect Rate will have;

• Modulated Simplex I/O system: is the preferred solution for safe process since the duplex (redundant) I/O system is usually expensive, and the modulated simplex I/O configuration guarantees that any failure of a single I/O card will not cause the relevant I/O rack to fail. For instance, if a rack contains three I/O cards, which controls three pumps (two duty, one standby), the failure of one card will cause the whole pumping process to fail. In Modulated Simplex I/O systems however, it will cause the failure of one pump, which will be classed as the standby pump, and the other two pumps will continue run normally.

2.19.2 RTU

RTU stands for Remote Telemetry Unit. This unit delivers remote information back to network operation centres. Operations staff can access remote sites that have RTUs, via a web browser, SNMP (Simple Network Management Protocol) Manager, and XML (Extensible Markup Language). If an ethernet connection is not available, then the RTU's may be accessed via PSTN (Public Switched Telephone Network), normal dialup and even SMS (Short Message Service) messaging.

Earlier generation RTUs were hardwired and supported limited functionality’s such as data transfer and alarming. The new generation RTUs are equipped with powerful processors that allow the RTU to control certain instruments and devices, and to receive/transmit analogue and digital I/O (input/output) signals.

The microprocessor based RTU have a proven track record within the water and wastewater industry, a robust modular construction, and are constructed for ease of maintenance and repair. These are intelligent devices, capable of handling data collection, logging, report by exception, data retrieval and pump sequence control programs.

RTU’s equipped with RS232/485 links are recommended for interconnection to standalone control systems, standard equipment packages and PLCs (Programmable Logic Controller). A dedicated




serial port should be provided for connecting a hand-held programming unit or PC.

The RTU software enables the RTU to process local input equipment information, before transmitting it to the master station to reduce transmission overheads. A report by exception operation is necessary for cost effective communication. The report is triggered by change of state of digital values, analogues reaching threshold values or varying by specified amounts. The RTU also reports when polled, and when the memory buffer is full.

2.19.3 SCADA and Telemetry Systems

Supervisory Control And Data Acquisition (SCADA) is an industrial measurement and control system consisting of a central host or master (usually called a master station, master terminal unit or MTU); one or more field data gathering and control units or remotes (RTU’s); and a collection of standard and/or custom software used to monitor and control remotely located field data elements. Contemporary SCADA systems exhibit predominantly open-loop control characteristics and utilise predominantly long distance communications, although some elements of closed-loop control and/or short distance communications may also be present.

Systems similar to SCADA systems are routinely seen in factories and treatment plants. These are often referred to as Distributed Control Systems (DCS). They have similar functions to SCADA systems, but the field data gathering or control units are usually located within a more confined area. Communications may be via a local area network (LAN), and will normally be reliable and high speed. A DCS system usually employs significant amounts of closed loop control.

SCADA systems on the other hand generally cover larger geographic areas, and rely on a variety of communication systems that are normally less reliable than a LAN. Closed loop control in this situation is less desirable.

The main use of SCADA is to monitor and control plant or equipment. The control may be automatic, or initiated by operator commands. The data acquisition is accomplished by the RTU's scanning the field inputs connected to the RTU (it may be also

called a PLC - programmable logic controller). This is usually at a fast rate. The central host will scan the RTU's (usually at a slower rate). The data is processed to detect alarm conditions, and if an alarm is present, it will be displayed on special alarm lists.

Data can be of three main types:

• Analogue data (i.e. real numbers) will be trended (i.e. placed in graphs);

• Digital data (on/off) may have alarms attached to one state or the other;

• Pulse data (e.g. counting revolutions of a meter) is normally accumulated or counted.

The trending function can be a powerful diagnostic tool for use by the operators or maintenance personnel. The data stored and archived can be viewed over any period of historic time, which allows fault patterns, which would otherwise go unnoticed to be detected. For stormwater stations the data can be analysed to determine how the station coped with storms. Based on this data, modifications can be made to the operation of the station to improve its response during such incidents.

The primary interface to the operator is a graphical display (mimic) which shows a representation of the plant or equipment in graphical form. Live data is shown as graphical shapes (foreground) over a static background. As the data changes in the field, the foreground is updated, e.g. a valve may be shown as open or closed. Analogue data can be shown either as a number, or graphically. The system may have many such displays, and the operator can select from the relevant ones at any time.

A further function of the SCADA system is the production of maintenance data and management reports. For example, SCADA systems can be easily configured to produce maintenance requests for equipment that has run a set number of hours, or if its’ performance has been declining over time. If a standalone maintenance system is already in place, SCADA systems can feed information directly to the maintenance software.

For managers, SCADA systems can produce detailed reports on subjects such as power or chemical usage. Combined with the trending facility




that is also inherent within SCADA, and by inputting cost data, it can produce cost forecasts for a wide range of process consumables.

2.20 Lighting The designer should follow the guidelines and information given below to design a proper lighting system. The British standards specified within and the CIBSE lighting guidexlvi should be considered during the design.

2.20.1 Light Fitting Selection Criteria

Light fittings are selected according to the following criteria and application.

2.20.1.1 Installation Location

The location of the light fittings to be designed has a large affect on the type of luminaire to be specified. Generally, the following categories can be considered:

1. Internal Lighting

Internal lighting fittings are required in places such as:

a. Motor control centre rooms (MCC);

b. Control and SCADA monitoring rooms;

c. Substation (11kv & transformer);

d. Pump rooms;

e. Off-loading bay & walk ways;

f. Kitchen and toilets;

g. Administration offices;

h. Machinery rooms (compressor, generator, chemical storage, and chemical dosing system room).

2. External lighting

a. Building (external wall mounted fittings);

b. Internal road lighting (inside station boundary);

c. Water storage tank lighting;

d. External installed machinery (settlement tanks, inlet works aeration tanks);

e. Pump wet wells and screen chambers.

2.20.1.2 Environmental Conditions

In many industrial applications the environmental condition is hostile or hazardous as explained below.

1) Hostile conditions - damage to light fittings can occur due to:

a. High ambient temperatures;

b. Windy and vibrating environments;

c. Corrosive atmosphere (hydrogen sulphide gases, high humidity);

d. Wet atmosphere (water ingress);

e. Dusty atmosphere.

2) Hazardous conditions - The operation of light fittings in certain environments can cause fire or explosion due to gas generation or fumes (methane, etc).

A risk assessment on the source of ignition and type of explosive atmospheres should be carried out using the methodology suggested in BS EN 1127-1xlvii for all potentially hazardous areas such as screen chambers and wet wells.

2.20.1.3 Luminance Level Required (Lux)

The luminance level required varies from one area or application to another. The luminance level should generally be in accordance with the CIBSE lighting guidexlvi. The relevant levels are replicated below for convenience in Table 2.20.1.




Table 2.20.1 – Luminescence Levels for Various

Service Areas

Service area Luminance level (lux)

Internal area (inside building)

Motor control centre room 300

Control / SCADA room 500

11kv switchgear room 300

Transformer bay 150-200

Kitchen 150

Toilets 150

Store 200

Offloading bay / walkway 100-150

Pump house 150-200

Cable gallery 150-200

Administration offices 300

Machinery room 150-200

External area (Inside station boundary)

Internal Road lighting 50- 100

Tank area 50

Building (external wall and door

entrance)

70

External installed machinery 100

2.20.1.4 Type of Light Fitting

Light fitting types that can be used in different locations can be categorised as follows.

1. Fluorescent fitting

The fluorescent fitting is a combination of lamps and luminaries. The fittings are available with different lamp sizes (18w, 36w, 58w), arrangements (3x18w, 4x18w, 2x36w, 2x58w) and installation type (surface mounted, recessed mounted). This type of fitting is ideally suited to internal installation use. It can be

used in most locations with some changes in the body material, IP rating and lamp wattages.

2. Flood lights

Flood lights are used mainly for external building area lighting such as tank areas, and machinery areas (grit removal, settling tank, aeration tanks etc). The lighting installation can be wall mounted on external buildings or post mounted in working machinery areas, or ground level mounted and directed to the tank walls in case of tank area lighting. The fittings should be a minimum of IP65; and the body should be suitable for the environment of the application (corrosion resistant, UV protected).

3. High bay lights

High bay lighting should be used in pump rooms when the bay heights are above six meters. The high bay lamps can provide lighting for maintenance purposes, in the case of regular inspections and access to the pump house. Side mounted (4-meter height) fluorescent fittings can be used due to the extended start-up time of high bay lamps.

4. Emergency lights

Emergency lights are used in case normal lighting fails or the power supply fails. They give light in emergency situations such as a fire, to provide escape-route sign lighting and emergency-exit sign lighting as per BS 5266xlviii. The type and installation of emergency lighting should consider the following points:

• Escape route signs shall be mounted above building exit doors at 2 - 2.5m above floor level;

• Escape route lighting such as Corridors, gangway and stairs shall have a horizontal luminance on the floor (centreline of escape route) of not less than 0.2lux;

• Emergency lighting in large open areas such as open plan offices should have an average horizontal luminance for escape purposes of not less than 1.0lux;

• Emergency lighting in Motor control centre rooms and operator control rooms (SCADA) should have an average horizontal luminance not less than 2.0lux.




Emergency light system

There are two types of emergency light system:

a. Self-contained;

b. Centrally powered.

Luminaire mode of operation

There are two modes of operation as follows:

• Maintained: lamp used as normal when the building is occupied. The power supply is from the normal source directly or indirectly;

• Non-maintained: lamp off as long as the normal power supply is available. The lamp will energise from the emergency power supply automatically in the event of normal power failure.

Types of emergency lighting

The following types of emergency lighting luminaire are commonly used:

• Self-contained separate luminaire (maintained/non-maintained);

• Normal luminaires modified to contain a battery pack and conversion unit (maintained);

• Normal luminaires fed from a central battery system with conversion unit (maintained);

• Normal luminaires with a separate lamp for use with a battery pack, inverters, rechargeable unit (non-maintained);

• Normal luminaires with a separate lamp for emergency use, fed from a central battery system (non-maintained)/(sustained luminaire);

• Normal luminaires fed from a central power source (maintained/ non-maintained).

5. Roadway lighting

The design of roadway lighting should be according to BS 5489-3xlix. For lighting required for pumping station roads, the selection of the suitable light fittings, post heights and post spacing will be according to the level of lux required. The light fitting body and canopy material should be suitable for the installation location and environmental

conditions. Usually, three types of lamp are commonly used. These are; high-pressure sodium, metal halide, and high-pressure mercury. The installation of the fitting on the column can be on the post top, bracket or side entry.

6. Bulk head

Bulk head light fittings are used at the entrance of the pumping station building (located on top of the door or at the side) as well as in substation entrance doors and gates. The fitting can be suitable for indoor or outdoor installation and should be IP65 with either a high pressure sodium or incandescent lamp type).

7. Lighting design calculation:

The following formula is used to check the level of lux provided and adjust the number of fittings to be used. Professional software can be used for increased accuracy and speed of design. The following guide is given as an aid for the experienced lighting engineer and not as a learning guide for the novice engineer. The information required to populate the formulae can be found in manufacturer’s literature.

Internal Lighting (Lumen Method) Formula

Es = F x n x N x UF x MF

A

Equation 2.20.1

Es = Average illuminance (lux) of the plane

F = Initial bare lamp lumens flux (lumens)

n = Number of lamps per luminaire

N = Number of luminairies

UF = Utilisation factor

MF = Maintenance factor

A = Area (m2)

Calculation procedure

Calculate the room index (K), floor cavity index (CIf) and ceiling cavity index (CFc).




(K) = (LxW)/(L+W)hm

Equation 2.20.2

(CIf) OR (CFc) = L x W/(L+W)h

Equation 2.20.3

Where:

L = room length

W = room width

Hm = height of the luminaire plane above the

horizontal reference plane

H = depth of the cavity

Calculate the effective reflectance (REx) of the ceiling, wall and floor cavity (from tables using above calculated (CIx).

Determine the utilisation factor value (UF) using luminaire manufacturer data sheets; room index and effective reflectance (apply any correction factors).

Determine the maintenance factor (MF)

MF = LLMF x LSF x LMF x RSMF

Equation 2.20.4

Where:

LLMF = lamp lumen maintenance factor

LSF = lamp survival factor

LMF = luminaire maintenance factor

RSMF = room surface maintenance factor

Thus, the lighting design is determined as follows:

• Using the lumen method formula, calculate the number of luminairies required (N);

• Determine the suitable layout;

• Check if the (spacing / height) ratio of the layout is within the range according to UF;

• Check that if the proposed layout is does not exceeding the maximum ratio limit;

• Calculate the luminance that will be achieved by the final layout.

External and Roadway Lighting Calculation

The calculation for roadways can be carried according to BS 5489-3xlix. Caution must be taken in lamp post foundation design to ensure that the wind effect on the post is fully considered.

The flood light calculation can be carried out using the same formula applied for internal lighting calculation with slight modification.

E = N x L x BF x WLFxMF

A

Equation 2.20.4

Where:

E = Illuminance required (lux)

L = Lamp output per lumens (lm)

BF = Beam factor number of lamps per luminaire

N = Number of luminaries

WLF = waste light factor (usually considered as

0.9)

MF = maintenance factor

A = area to be lighted (m2)

Light control: The control of the lighting system can be provided by the following means to control the operation of different lighting systems within the pumping station:

• One-way light switches can be used for controlling a lighting system in an area with a single access, for example at the main access door to the station;

• Two-way light switches can be used for controlling a lighting system in an area with multiple access and egress points;

• The automatic control of external lighting systems can be achieved by two main methods:




a) Photocell controller for automatic dusk till dawn control;

b) Time clock operation for full control of when external lights are in operation.

2.21 Maintenance Access Safe access should be provided to all equipment and local control panels at all times.

Access walkways, platforms and stairs should be designed so that no dismantling is required for normal routine maintenance. Vertical access should be by staircase so that tools and equipment can be carried in and out safely. Ladder access should be restricted to infrequent visual inspection points.

Access around equipment for operation should be installed at a level where all the controls can be reached and operated easily without excessive stretching or bending and where all indicators can be seen.

Access around equipment for maintenance and repair should be installed at a level where all the maintenance points can be reached, dismantled and removed without excessive stretching or bending. Particular attention should be paid to lifting gear access and operation where heavy equipment is involved.

Access below ground to dry wells should be by staircase so that tools and equipment can be carried in and out safely.

Permanent access to wet wells and screen chambers should be provided, using stainless steel or GRP to just above TWL to allow for cleaning. The access arrangements should be designed such that an operator could be rescued from the sump with a safety harness and man-winch.

When designing access to equipment, careful thought should be given to shipping routes for removing equipment to a suitable position for further work, or for removing from the pumping station completely. Exit routes for equipment should not be the same as for personnel access unless there is an alternative escape route.

When the lifting gear has taken the weight of equipment and the equipment is released from its position, the clearance in the shipping route should be large enough for the equipment to pass through without rearrangement.

2.22 Gantry Cranes and Lifting Facilities

Permanent or temporary lifting facilities should be provided for equipment that can not be easily lifted. Consideration should be given to the weight, shape and position of the item to be lifted. As a guide lifting facilities should be provided for anything over 25kg.

For long or heavy lifts, gantry cranes should be powered in all motions. Trolley cranes should generally be power lift with manual motion, but small units should be manual on all motions.

Access must be provided to permanent lifting equipment, particularly gantry cranes, for maintenance as generally described in section 2.21.

The following types of lifting equipment are available:

• Lifting Eye and Chain Block. Suitable for single straight lifts only inside a building or dry well. Not suitable for side forces, but may be used in conjunction with other suitable lifting eyes to swing a load sideways;

• Davit, Socket and Chain Block. Suitable for most small single lifts i.e. submersible pumps up to 250kg. Above this, the davit becomes too heavy to be manhandled;

• Runway Beam, Trolley and Chain Block. Suitable when there are a number of loads in a straight line, or where a single load must moved sideways. For heavy loads or long lifts, the chain block and trolley should be electrically powered;

• Overhead Gantry Crane. Suitable for installations where there are dispersed or heavy loads that must be moved in all directions;

• Mobile Crane. Suitable for single heavy loads outdoors which must be moved in all directions i.e. large submersible pumps.




Submersible pumps should be fitted with stainless steel chains, with change-over rings every 1.0m, and the lifting equipment should be fitted with a change-over sling.

Location of lifting equipment

• Lifting equipment should be provided adjacent to all heavy items that require lifting;

• Lifting equipment should be positioned to provide a straight lift of the load and also be able to lower the load directly to a suitable setting down position;

• Where lifting through openings in floors, the lifting gear should be positioned to allow a direct single lift up through all floors without moving the lifting point or rearranging the load.

Controls for Lifting Equipment

• Overhead electric cranes and chain blocks should be provided with a low voltage pendant control suspended from a glide track, independent of the lifting block. The pendant control should extend to within 500mm of the operating floor, but not touch the floor;

• Electric chain blocks should be provided with a low voltage pendant control suspended from the block. The pendant control should extend to within 500mm of the operating floor but not touch the floor;

• Hand operating chains should extend to within 500mm of the operating floor but not touch the floor;

• Long travel drive chains should be located to avoid snagging, and allow the operator safe passage;

• With the load hook in its highest position, if a load chain touches the operating floor or any item of plant, a chain collection box should be fitted.

2.23 Ventilation, Odour Control and Air Conditioning

2.23.1 Ventilation

Ventilation of pumping stations is required to prevent the accumulation of high levels of potentially hazardous chemicals, and ensure that working conditions meet health and safety requirements. UK occupational exposure limit (OEL) concentrationsl for hydrogen sulphide and other gases associated with septic conditions are given in section 1.6 of this manual.

Typical ventilation rates for odour containment in pumping stations used in current operational practice in Doha are given in Table 2.23.1.

Table 2.23.1 – Typical Ventilation Rates for

Odour Control in Pumping

Stations

Air changes per hour

Pumping station (no man access)

One for local covers

12 for pumping stations extracted from close to the sump and process units

Pumping station working area (current practice)

20 during man access (initiated by light switch)

Dry wells (current practice)

12

Separate screen chamber

Passive ventilation through carbon filter (where there is no other route for odour escape)

Ventilation systems should be designed so that in the event of a fire being detected in any area, all the air conditioning equipment and ventilation systems are shut down. All supply and exhaust ventilation louvers should shut automatically to




compartmentalise the buildings and below ground chambers. This restricts the spread of the fire and smoke, and ensures effective use of automatic fire extinguishing systems.

Other points to consider include:

The air conditioning systems, ventilation fans and odour control equipment should be run simultaneously and ventilation fan louvers should shut, when the fan stops;

Louvers should be sized to keep the air velocity through them below 0.5m/s;

Air ducts should be designed to ensure the velocity through them does exceed 10m/s in occupied areas;

Materials should be selected to limit the corrosion effects of hydrogen sulphide (H2S).

Ventilation of Pump Rooms and Dry Wells

Air supply should be provided by either two or three duty fans and one standby fan, depending on the size of the pump room.

Exhaust air should be removed by either two or three duty fans and one standby fan, depending on the size of the pump room.

The exhaust fans should have approximately 5% less flow capacity than the air supply fans to keep the building at a slight positive air pressure. This is to avoid drawing unfiltered dust laden air into the pump room which can drastically shorten the equipment life.

Pump rooms and dry wells should typically have 12 air changes an hour for normal operation, increasing to 16 air changes an hour during man entry. The cable basement should be ventilated as part of the pump room ventilation system.

Ventilation of Wet Areas - Pump Sumps &

Screen Chambers

Wet areas should normally be ventilated by air extraction only, with a natural air supply to keep the wet area under slightly negative pressure and avoid releasing odours to the atmosphere.

Exhaust air should be removed by duty/standby fans, the number and configuration depending on

the size of the wet areas. Each fan should have a two-speed motor.

During man entry, the additional air supply should be provided by the fans running at high speed.

The fans should be sized so that with all fans running at high speed, the required air changes per hour for man entry are achieved.

Ventilation rates should be designed to ensure a maximum of 3ppm of H2S in the wet areas. The system should be designed to achieve this with only one fan operating.

Wet areas should typically have 12 air changes an hour for normal operation, increasing to 20 air changes an hour during man entry.

2.23.2 Odour Control

Air vented from pumping stations will in most cases require odour treatment. In most cases, a two bed (duty/standby) system using carbon regenerated using alkali (caustic soda or potash) is preferred. At larger pumping stations consideration may be given to pre-treatment of strong sources using catalytic iron filters.

Further details of requirements are given in Volume 5 Section 1.5. Reference should also be made to Section 1.6 of this Volume

Typical conditions to be considered in the design of the odour control unit are given in the table below.

Table 2.23.2 – Conditions to be Considered in

Odour Control Unit Design

Sewage temperature 25 – 35oC

Ambient temperature 0–50oC

Relative humidity Up to 100%

Temperature of air vented from the sewerage system to an Odour Control Unit

Up to 30oC

Radiating surfaces temperature

85oC maximum

Hydrogen sulphide from below covers

250ppm




Hydrogen sulphide with workplace air

10ppm

2.23.3 Air Conditioning

The required air conditioning systems and ventilation capacities are shown in the tables below.

Table 2.23.3 - Air Conditioning (AC) Systems

Location Air Condition system

Electric Switch Gear Dual Split AC unit system

Control Room Split AC unit system

Table 2.23.4 - Ventilation Capacities

Location Ventilation (l/s) per person

Ventilation (l/s) per sq.m.

Approximate air changes per hour. *

Electric Switchgear Room

- 0.8 1

Control Room

10 1.3 2

Kitchen and Toilet

- 10 8

Note: Figures extracted from BS 5720, Table 1.

*Depending on the dimensions of the rooms.

The designer shall assess the potential for corrosion of A/C units, particularly from H2S, and ensure that they are appropriately designed and located.

Air Conditioning of Electrical Switch Gear

Rooms

Electrical switchgear rooms should be completely isolated from the remainder of the building for the following reasons:

• The thermal loads are higher than elsewhere in the building;

• In the event of a fire being detected the air conditioning should be switched off to allow the fire suppression equipment to operate effectively.

Two split AC units working independently (mechanically and electrically) of each other should be used to air condition the room, with air diffusers discharging horizontally towards the panels. Return air should be sucked back by the split unit, via receiving air diffusers located at evenly placed points between the supply air diffusers, and fixed to the ceiling.

Each split AC units should be rated at 50% above the required capacity (i.e. 150% total), so that should one unit fail, the other unit will provide 75% of the required air conditioning capacity.

The required thermal load should be calculated on the basis of peak conditions.

The required quantity of exhaust air should be removed from electrical switchgear rooms to atmosphere by a fan with an actuated louver.

Air inlet should be by natural supply through a filtered and actuated louver.

In the event of a fire, the electrically actuated louvers should be closed to seal electrical switchgear rooms during the use of any fire extinguishing system.

Air Conditioning of Control Rooms, Kitchens

and Toilets

A single split AC unit should be provided for air conditioning the control room. No air conditioning should be provided for the kitchen or toilet.

The kitchen and toilet areas should be air conditioned by exhausting part of the control room air through them.

Exhaust air in the kitchen and toilet areas should be discharged outside the building. The fans should be run continuously for the following reasons:

• To provide the required air changes for the control room and kitchen;

• To keep the toilet and kitchen area ventilated.




Air louvers should be fitted in the bottom of kitchen and toilet doors.

2.24 Structural Design General Design Requirements

Unless local design standards dictate otherwise, in general, he design of concrete structures shall be in accordance with BS 8110-1 “Structural Use of Concrete”li and BS8007 “Design of Concrete Structures for Retaining Aqueous Liquids”lii. Likewise, the design of steel structures shall be in accordance with BS5950-1 “Structural Use of Steelwork in Buildings”. Local standards shall govern if any conflict arises. All structures shall be designed based on a ‘limit-states’ philosophy.

Unless required otherwise, all structures shall be designed for a minimum service life of 60 years.

The designer shall prepare calculations for each design package, including as a minimum the following information:

• Description of the structure and design methodology adopted;

• All assumptions made for design (i.e. geotechnical parameters, loadings, etc);

• Standards, guidelines and specifications used for design;

• Input and output from software where appropriate.

2.24.1 Substructures

2.24.1.1 Thermal Crack Control Requirements

Calculation of the reinforcement requirements for control of early-age thermal cracking shall be in accordance with BS 8007lii.

For the calculation of the likely maximum crack spacing and the reinforcement ratio the following formula shall be used:

( )21

max

maxTTR

S+

=α

ϖ

Equation 2.24.1

Equation 2.24.2

Were: ωmax = allowable crack width (0.2mm maximum) Smax = likely crack spacing (mm) R = restraint factor (0<R<0.5; to be taken as

0.5 for most structures) α = co-efficient of thermal expansion (varies

between 10x10-6/oC – 12x10-6/oC) T1 = fall in temperature between the

hydration peak and the ambient (oC) T2 = ambient placing temperature (oC) ρbar = reinforcement ratio (ρmin = 0.0035) φbar = reinforcement diameter (mm) Where the section thickness exceeds 500m, only the outermost 250m of each face shall be used in calculating reinforcement areas; however, the design temperature T1 shall still be based on the entire element thickness.

h<500mmh/2

For h < 500mm assume each reinforcement face controls h/2 depth of concrete

h ≥ 500mm250mm

250mm

For h ≥ 500mm assume each reinforcement face controls the outer 250mm depth of concrete. Ignore any central core beyond these surface zones.

Given that thermal crack control requirements determine the minimum limit of reinforcement, particular care should be given to the adopted values of T1 and T2. Factors including local site conditions, concrete mix design, formwork type, seasonal variations in ambient temperature, distance from plant to site, etc shall all be taken into account.

Considering the relatively high ambient temperatures that may be encountered in the Qatar region, consideration shall be given to limiting the concrete placing temperature T2 to a value ranging between 15oC and 30oC. Designers are referred to

max2

67.0

S

bar

bar

φρ =




CIRIA Report No’s 91liii and 135liv for further information on this subject.

Ground Investigation & Flotation

The designer shall have, at a minimum, an understanding of the basic ground conditions likely to be encountered on site, either from historical data or a desk-top study. Preferably, the designer shall obtain a Ground Investigative Report (GIR) from suitably competent geotechnical engineers giving more precise values and ground conditions. Data to be considered includes ground level (GL), ground water level (GWL), soil types, classification and properties, allowable bearing capacities and a soil chemical analysis.

Depending on the GWL and GL conditions, buoyancy (or flotation) of the structure may govern the section thickness. Flotation of all structures shall be checked in accordance with BS 8007lii against the anticipated GWL. In considering the flotation calculations, the following methodology is recommended:

• Calculate the volume of water displaced based on external dimensions of the structure and the GWL;

• Calculate the mass of the structure taking into account construction assumptions (e.g. does the site need to be de-watered until after the roof has been placed? does the site need to be de-watered until any mass concrete benching has been placed?);

• Calculate the factor of safety to obtain 1.10 as a minimum;

• Re-size any element thicknesses as required (ensuring that structural requirements are still maintained).

A factor of safety of 1.10 shall be achieved for both temporary and permanent conditions. For the flotation calculations the following concrete unit weights are recommended:

Minimum Maximum

In-situ RC 22.5kN/m3 23.5kN/m3

Unreinforced 21.6kN/m3 22.5kN/m3

2.24.1.2 Structural Analysis

Loading

All liquid retaining structures are to be designed for both the full and empty conditions, with the load combinations arranged to give the most severe combination likely to happen.

Both serviceability (SLS) and ultimate (ULS) load conditions shall be considered. The following load factors shall be adopted (unless local design codes specify more onerous load factors) as per Table 2.24.1.

Table 2.24.1 – Serviceability (SL) and Ultimate

(ULS) Load Factors

Load SLS Factor ULS

Self Weight 1.0 1.4 Dead Loading 1.0 1.4

Retained Liquids 1.0 1.4

Retained Soils 1.0 1.4 Live Loads (incl. surcharges)

1.0 1.6

In general the walls and base shall be checked against the following load combination (where appropriate):

• Internal hydrostatic pressure only (water-tightness test before backfilling);

• External soil pressure only (backfilled soil but no water);

• Hydrostatic uplift on base;

• Base ‘soft-spot’ capacity;

• Hydrostatic + soil pressure + uplift (normal working conditions);

• Roof loading.

Where required, the structure shall be designed for an appropriate wheel/vehicle live load. Vehicle live loads shall be in accordance with local standards and engineering judgement (where local standards do not cater to vehicle loads then loading shall be in accordance with BS 5400-2lv and BS6399-1lvi). A minimum live load of 5kN/m2 shall be adopted regardless of code requirements.




Elements shall be analysed in accordance with BS 8007lii, BS 8110-1li and established engineering principles. Depending on the slab arrangement (i.e. degree of restraint, span ratio’s, etc), bases and walls may be considered as either one-way or two-way spanning.

Where appropriate seismic loading shall be considered in accordance with local design codes.

Base Slabs

Base slabs designed as one-way spanning shall be designed for flexure in accordance with engineering principles and the following formulae:

156.02

≤=cu

ULS

fbd

MK

Equation 2.24.3

dK

dz 95.09.0

25.05.0 ≤

−+=

Equation 2.24.4

and

zf

MA

sy

ULS

st95.0

=

Equation 2.24.5

where: MULS = design ultimate moment (kNm) b = width of section (mm - typically taken as 1

m) d = effective depth (mm) fcu = concrete strength (N/mm2) z = lever arm (mm) Ast = area of required tension reinforcement (mm2) fsy = reinforcement strength (N/mm2)

With K ≤ 0.156, compression reinforcement is not required. Designers are referred to BS8110-1li for cases where compression reinforcement is required.

Base slabs designed as one-way spanning shall be designed for shear in accordance with engineering principles and the following formula:

( )cu

v

fdb

V8.0,N/mm5 2≤=υ

Equation 2.24.6

where: υ = design ultimate shear stress (N/mm2) V = design ultimate shear force (kN) bv = width of section (mm - typically taken as

1m) d = effective depth (mm) fcu = concrete strength (N/mm2)

Table 2.24.2 – Shear Stress and Rebar to be

provided

Shear Stress υυυυ

Form of shear

rebar to be

provided

Area of shear

rebar to be

provided

υ<0.5υc None Required -

0.5υc < υ<(υc +

0.4)

Minimum links in

areas where υ<υc Asv ≥ 0.4bsv/0.95fsyv

(υc + 0.4) < υ<5

or 0.8√fcu

Links in any

combination

Asv ≥ bsv(υ-

υc)/0.95fsyv

Shear reinforcement shall be provided based on the following:

• The critical shear stress uc shall be determined in accordance with BS 8110-1li;

• Base slabs designed as two-way spanning shall be designed for flexure in accordance with engineering principles and the following formula:

2

xsxsx nlm β= & 2

xsysy nlm β=

Equation 2.24.7

Values of βsx and βsy shall be obtained from Table 2.24.3.

• Base slabs designed as two-way spanning shall be designed for shear in accordance with engineering principles and the following formulae:

xvxvx nlβυ = & xvyvy nlβυ =

Equation 2.24.7




Values of βvx and βvy shall be obtained from Table 2.24.4.

A nominal ‘soft spot’ diameter shall be assumed in the subgrade (unless local conditions preclude this from occurring) and the base checked accordingly.




Table 2.24.3 – Base Slab Flexure Coefficients

Edge Condition

Short Span Co-efficient (ββββsx) Long Span Co-

efficient (ββββsy) for

all values of Ly/Lx

Values of Ly/Lx

1.0 1.1 1.2 1.3 1.4 1.5 1.75 ≥2.0

1. Four edges continuous 0.024 0.028 0.032 0.035 0.037 0.040 0.044 0.048 0.024

2. 1 short edge discontinuous 0.028 0.032 0.036 0.038 0.041 0.043 0.047 0.050 0.028

3. 1 long edge discontinuous 0.028 0.035 0.041 0.046 0.050 0.054 0.061 0.066 0.028

4. 2 short edges

discontinuous 0.034 0.038 0.040 0.043 0.045 0.047 0.050 0.053 0.034

5. 2 long edges discontinuous 0.034 0.046 0.056 0.065 0.072 0.078 0.091 0.100 0.034

6. 2 adjacent edges

discontinuous 0.035 0.041 0.046 0.051 0.055 0.058 0.065 0.070 0.035

7. 3 edges discontinuous

(1 long edge continuous) 0.043 0.049 0.053 0.057 0.061 0.064 0.069 0.074 0.043


(1 short edge continuous) 0.043 0.054 0.064 0.072 0.078 0.084 0.096 0.105 0.043

9. 4 edges discontinuous 0.056 0.066 0.074 0.081 0.087 0.093 0.103 0.111 0.056

Table 2.24.4 – Base Slab Shear Coefficients

Edge Condition

Short Span Co-efficient (ββββvx) Long Span Co-

efficient (ββββvy) for

all values of Ly/Lx

Values of Ly/Lx

1.0 1.1 1.2 1.3 1.4 1.5 1.75 ≥2.0

1. Four edges continuous 0.33 0.36 0.39 0.41 0.43 0.45 0.48 0.50 0.33

2. 1 short edge discontinuous 0.36 0.39 0.42 0.44 0.45 0.47 0.50 0.52 0.36

3. 1 long edge discontinuous 0.36 0.40 0.44 0.47 0.49 0.51 0.55 0.59 0.36

4. 2 short edges

discontinuous 0.40 0.43 0.45 0.47 0.48 0.49 0.52 0.54 0.26

5. 2 long edges discontinuous 0.26 0.30 0.33 0.36 0.38 0.40 0.44 0.47 0.40

6. 2 adjacent edges

discontinuous 0.40 0.44 0.47 0.50 0.52 0.54 0.57 0.60 0.40


(1 long edge continuous)

0.45 0.48 0.51 0.53 0.55 0.57 0.60 0.63 0.29


(1 short edge continuous)

0.29 0.33 0.36 0.38 0.40 0.42 0.45 0.48 0.45

9. 4 edges discontinuous 0.33 0.36 0.39 0.41 0.43 0.45 0.48 0.50 0.33




Walls

Walls may adopt vertical, horizontal or two-way spanning action. Walls may be analysed by first principles, design charts or software. Earth pressures shall be calculated using Rankine’s theory. At-rest earth pressures shall be used for structural design. The value of ko will vary according to site conditions but a minimum value of ko = 0.5 shall be adopted. Surface surcharging shall be allowed for (typical values range between 5-10kN/m2), as shall construction and permanent live loads.

φφ

sin1

sin1

+−

=akActive

Equation 2.24.9

φφ

sin1

sin1

−+

=pkPassive

Equation 2.24.10

)5.0(sin1 designforkrestAt o =−=− φ

Equation 2.24.11

Surcharge

Com

paction

Hydrostatic

Soil

kqsurch kqcomp γwHw kγsHs

¦------------------ where required -----------------¦

Where the structural arrangement calls for internal walls, these walls shall be checked for a full hydrostatic head against one side only (representing a full chamber on one side, an empty chamber on the other).

Where designed as vertical cantilevers, walls shall be checked for deflection in accordance with BS8110-1li span-depth criteria.

Roof Slabs

Roof slabs shall generally be designed in a similar fashion to base slabs, however, they should be considered as simply supported with limited fixity (and hence moment transfer) at the supports.

Particular care shall be given to roofs subject to vehicle loading.

Design Software

Slab and wall elements may also be designed using appropriate commercial software (e.g. ROBOT Millennium, STRAND 7, STAADPro, Microstran V8, etc), either as 2D, or preferably 3D, models. Appropriate spring elements shall be used to represent the soil stiffness. Designers should refer to the program user manuals for assistance with design software.

Foundations and Settlement

Where an interface between a structure (be it above ground, partially buried or completely buried) and the underlying ground exists, there is said to be soil-structure interaction. The actual behaviour of structures and soil-structure interaction is complex and leads to some simplification of assumptions in order to obtain a design.

A fundamental design concept is the selection of either a rigid structure or a flexible structure. A flexible structure will be able to tolerate a degree of differential settlement by the basic arrangement of the structure, the nature of its materials and by the inclusions of movement joints. Conversely, a rigid structure is designed to neglect any differential settlement by having sufficient strength to span across any loss of ground support. Factors to consider include the relative settlements likely to occur (i.e. immediate and long-term), any history of previous soil loading (i.e. over- consolidation) and the non-homogenous content of most soils.

The support given by the subgrade is often modelled as springs of varying stiffness (with the stiffness based on geotechnical parameters), and base slabs may occasionally be designed as beams on elastic foundations. This is a time-consuming and complicated procedure, and many design software programs are ideally suited to this task (although it should be remembered that any software output is only as good as its input).




As stated, the analysis and consideration of any soil-structure interaction is a complex affair, and in part depends on a degree of experience. Designers are strongly recommended to consult geotechnical engineers and to refer to specialist literature such as “Soil-structure interaction – The real behaviour of structures”lvii for further information on this subject.

Ground movement leading to differential settlement can cause severe cracking and leakage from liquid retaining structures, and as a general rule they should be designed as rigid structures. Where appropriate the design bearing pressure shall be calculated and checked against the allowable bearing capacity. If required, measures shall be taken to provide suitable foundations such as piling or other ground improvement techniques - consultation with suitably competent geotechnical engineers is strongly recommended. A maximum differential settlement value of 20–25mm should be adopted.

Where piled foundations are required, the design ultimate resistance of a single end-bearing pile shall be determined from the following formula:

bbss AfAfP*

_** +=

Equation 2.24.12

where:

P* = design ultimate resistance (kN) f*s = average ultimate skin resistance of

pile shaft (kN/m2) As = surface area of pile shaft (m2) f*b = net ultimate bearing resistance (kN/m2) Ab = bearing area of base (m2)

For elevated structures more traditional foundations may be required. Examples include:

• Pad (isolated) footings; • Combined footings; • Strip footings.

Simple, concentrically loaded pad (isolated) footings shall be designed in accordance with engineering principles and the following methodology:

• Determine required size of footing based on allowable bearing capacity (SLS) and adopt a suitable thickness;

• Design for flexure (ULS) taking a critical section at the face of the column, designed as a cantilever;

• Design for shear (ULS) taking a critical face located distance ‘d’ from the column face;

• Design for punching shear (ULS), adopting a shear perimeter of 4(column width + 3d);

• Adjust footing thickness as required. For eccentric column loading and other foundation types designers are referred to appropriate literature.

Structures shall be founded on a layer of suitably compacted subgrade material, a 50–100mm blinding layer, and a suitable slip membrane.

Concrete Slab

Blinding Layer & Slip Membrane

Subgrade

Movement Joints

Where effective means of avoiding differential settlement or excessive cracking can not be avoided, then consideration shall be given to the provision of movement joints at suitable locations.

The location of construction joints shall be specified by the designer and marked on drawings. Full structural continuity is assumed at construction joints, with reinforcement fully continuous across the joint. Conventional construction techniques should be followed for all construction joints (i.e. scabble concrete surface to an acceptable depth, remove all loose debris, etc).

Movement joints may consist of the following:

• Expansion Joint - No restraint to movement, can freely accommodate either contraction or expansion;

• Complete Contraction Joint - No restraint to movement, freely accommodates contraction;

• Partial Contraction Joint - Partial restraint of movement, partial contraction allowance;




• Sliding Joint - Allows two structural members to slide against each other with minimal restraint.

The use of water-stops and sealing compounds is essential for movement joints. Due care and consideration shall be given to the most appropriate product utilised.




Table 2.24.5 -Typical Allowable Bearing Pressures

Type of Ground Bearing Pressure

Type of Ground Bearing Pressure

kN/m2 tons/ft

2 kN/m

2 tons/ft

2

Clay – Soft < 75 < 0.75 Chalk - Hard Sound 600 6

- Firm 75-150 0.75-1.5 Limestone – Soft 600 6

- Stiff 150-300 1.5 – 3 Shale & Mudstone

- Very Stiff 300-600 3 – 6 - Soft 600-1000 6 - 10

Sand – Loose < 100 < 1 - Hard 2000 20

- Medium Dense 100-300 1 – 3 Sandstone – Soft 2000 20

- Compact 300+ 3+ Schist, Slate 3000 30

Gravel & Sandy Gravel Sandstone, Limestone

- Loose < 200 < 2 - Hard 4000 40

- Medium Dense 200-600 2 – 6 Igneous Rock - Sound 10000 100

- Compact 600+ 6+

Construction Material 28-day Cube Strength Max. Bearing Pressure under uniform loading

Plain Concrete N/mm2 lb/in

2 MN/m

2 Lb/in

2

Max. Bearing Pressure under Eccentric Load

= 1.25 x Uniform Pressure - 1:4:8 8.6 1250 1.7 250

- 1:3:6 11.5 1650 2.4 350

- 1:2:4 21.0 3000 5.3 760 Max. Bearing Pressure , Concentrated Load

= 1.50 x Uniform Pressure - 1:1.5:3 25.5 3750 6.5 950

- 1:1:2 30.0 4500 7.6 1140




Retaining Walls

Where required, retaining walls shall be designed in a similar fashion to the walls of liquid retaining structures. Earth pressures shall be calculated using Rankine’s theory. At-rest earth pressures shall be used for structural design. The value of ko will vary according to site conditions but a minimum value of ko = 0.5 shall be adopted. Surface surcharging shall be allowed for (typical values range between 5-10kN/m2), as shall construction and permanent live loads.

Global stability of the retaining walls shall also be considered (i.e. sliding failure, overturning failure, bearing capacity failure, etc).

Concrete

Concrete mix design shall be in accordance with BS 8500lviii or local standards, with an appropriate exposure class selected to meet the chemical environment conditions of the ground. Concrete shall have as a minimum a 28-day characteristic cube strength of 35N/mm2. A minimum cement content of 325kg/m3 and a maximum, water-cement ratio of 0.55 shall also be maintained.

Given that control of cracking from thermal effects often governs the reinforcement requirements for water retaining structures, consideration should be given to the availability and use of blended cement mixes. The inclusion of pulverised fuel ash (PFA) or ground granulated blast furnace slag (GGBS) can significantly reduce the effects of hydration temperature rise and hence reinforcement requirements. Designers are referred to CIRIA Report No 91liii (particularly Tables 5 and 6) for the use of blended concrete mixes.

It should be noted that natural conditions in the Middle East, both above and below ground, are often of an aggressive nature. The climate can significantly affect above and below ground concrete due to the high ambient temperatures accelerating chemical attack and physical degradation. The existence of soluble salts (mainly sulphate or chlorites) can be very detrimental to concrete, and the designer shall take all appropriate measures should these chemicals be detected in the soil. Factors to be considered shall include:

• Aggressive ground water;

• Contaminated aggregates; • Brackish water; • Rapid drying of concrete.

In these situations the designer shall follow the recommendations made in BS 8500-1lviii and BRE Special Digest 1lix.

Reinforcement

Reinforcement shall comply with BS 4449 or local standards. The provisions of section 7 of BS 8110-1 shall apply. High-yield reinforcement of between 400–500N/mm2 characteristic strength shall be adopted throughout.

Cover to Reinforcement

The nominal cover of concrete for all steel shall be a minimum of 40mm in accordance with BS 8007lii. This may need to be increased depending on local soil conditions.

2.24.2 Superstructures

Portal Frame Structures

Portal frame type structures are used extensively for framing of single-story buildings. They offer cost advantages over other framing systems for short to medium span structures in addition to a low structural depth, clean appearance and relatively easy maintenance of structural elements. A further benefit is the relative ease with which overhead gantry and monorail cranes can be fitted.

Portal frames are readily designed and constructed from either steel or concrete. External cladding ranges from masonry to steel sheeting to transparent plastics, and can be either structural or non-structural.

Regardless of the material adopted for construction, the same basic design methodology shall be adopted.

Load Combinations

Both serviceability (deflection and vibration) and ultimate (strength, stability and fatigue) limit state load conditions shall be considered, with the various load combinations arranged to give the most severe combination likely to happen.




Where appropriate, seismic loading shall be considered in accordance with local design codes.

The load factors shown in Table 2.24.6 shall be adopted (unless local design codes specify more onerous load factors).




Wind Loads

The calculation of wind loads will predominantly depend on local site conditions and a localised design standard. The majority of design codes used world-wide will include a simplified procedure for determining the wind forces on relatively small buildings, with limitations placed on the height, roof area and slope, and terrain factor. These simplified methods will give a quick, if somewhat conservative pressure; hence most codes also make provisions for a more detailed analysis. These detailed procedures are often tedious to perform, and lend themselves readily to spreadsheets or other software.

As the wind forces and pressures depend on local conditions the designer shall adopt any and all recommendations made in local design standards and codes. The strict use of BS 6399-2lx is not recommended, as it is tailored to British requirements, however, the design procedure as described in BS 6399-2 could be used provided local wind speeds and conditions were adopted.

Dead Loads

Dead loads comprise the self-weight of the structure and any permanently fixed loads from non-structural elements. Some common unit weights of materials are given in Table 2.24.7.

Imposed (Live) Loads

Imposed (or live) loads will be determined from the intended function of the building. For the type of buildings that could reasonably be expected to be found at water or sewerage treatment plants, the live loads will most likely be either human occupation (e.g. office facilities), various plant loadings (e.g. pump, control units, etc) or overhead gantry or monorail cranes for lifting facilities. Designers are referred to local standards or specific manufacturer data for plant loading. BS 6399-1lvi provides some recommendations for imposed loads, as listed in Table 2.24.8.

Crane Loads

The design of steel crane gantry beams for overhead cranes presents some specific problems that need to be carefully considered. The design of crane beams differs from the design of floor beams in the following ways:

• The loads are moving; • Lateral loading is usually involved; • The magnitude of loading depends on the type

of crane (i.e. either electric or hand operated); • Localised stresses occur in the web at the top

flange junction; • Lateral buckling (twisting) needs to be

considered • Fatigue assessment may be required.

As the crane operation is not a steady-state operation, there are also significant dynamic effects to be considered. This is usually done by applying dynamic load multipliers to the calculated static loads.

Designers are recommended to follow the rules set out in BS 2573-1lxi and to consult local design guides and specialist literature for the design of crane beams.

Structural Design

Structural design of simple framed buildings shall generally follow the methodology below:

• Calculate all the various loads and arrange into required combinations (paying particular attention to the wind loading combinations);

• Design the rafter elements; • Design the column elements; • Design the connections (including column base

connections); • Design the cladding; • Design the longitudinal bracing as required.

In general, structures should be considered as having pinned feet (i.e. column base plates incapable of transferring moments).

Generic Design Formulae

Simple portal frame structures also lend themselves readily to be designed using generic formula, which depend on the relative structural stiffness of the column and rafter elements. Some generic formulas are shown in Figure 2.24.1, based on pinned feet. For further generic formula (including fixed feet design) designers are referred to Reynolds “Reinforced Concrete Designers Handbook 10th Ed.”lxii




Table 2.24.6 – Load Factors for Load Combinations

Load Combination Ultimate Limit State (ULS) Serviceability Limit State (SLS)

Dead Live Wind Dead Live Wind Load Combination 1 1.4 1.6 - 1.0 1.0 - Load Combination 2 1.4 - 1.4 1.0 - 1.0 Load Combination 3 1.2 1.2 1.2 1.0 0.8 0.8 Dead Live Crane Dead Live Crane Vert. Horiz Vert. HorizCrane Combination 1 1.2 1.4 1.4 - 1.0 1.0 1.0 - Crane Combination 2 1.2 1.4 - 1.4 1.0 1.0 - 1.0 Crane Combination 3 1.2 1.4 1.4 1.4 1.0 1.0 1.0 1.0

Table 2.24.7 – Common Unit Weights of Materials

Material Unit Weight Material Unit Weight

kN/m2 kN/m3 kN/m2 kN/m3

Concrete Gypsum plasterboard 0.115

- Unreinforced 22.0 (12mm thick)

- Reinforced 24.0 Plaster render

Concrete Masonry 24.0 - Lime, 20mm tk 0.380

Bricks – Structural 22.6 - Cement, 20mm tk 0.450

- Clay 18.9 - Gypsum, 12mm tk 0.220

- Hollow clay 11.5 Polyester corrugated 0.020

Metal Cladding Sheets

- Aluminium 0.038 Thermal insulation 0.010

- Galv. Steel 0.050 (fibreglass bats)

Table 2.24.8 – Recommendations for Imposed (Live) Loads

Type of Activity Examples of Specific use UDL (kN/m2) Concentrated Load (kN)

Office and Work areas Offices for general use 2.5 2.7

Factories, workshops and similar 5.0 4.5

Catwalks - 1.0 at 1m ctrs

Balconies 4.0 1.5/m run

Warehousing and Storage areas General areas for static equipment 2.0 1.8

Plant rooms, boiler rooms, etc 7.5 4.5

(including weight of equipment)

Parking for cars, vans, etc (<2500 kg gross) 2.5 9.0




L

bb =1

h

ff =1

sI

hIk

1

21 =

h

Lk =2

33 11

2

3 1+++= kffk

( )3

1

2

4

65.01

hk

fwLHH EA

+=−=

VA = VE = 0.5wL MB = MD = -HAh

( )3

2

1111

2

8

2436

hk

bfbfwbHH EA

−−+=−=

L

wbVA

2

2

= MB = MD = -HAh

( )3

1111

4

3466

k

fbfbPbHH EA

−+−=−=

L

PbVA = MB = MD = -HAh

( )3

11

6

1265

k

fkwhH A

++= HE = HA - wh

L

whVV EA

2

2

=−= 2

2wh

hHM ED −=

MB = HAh

( )3

2

111

2

625.05.23

k

ffkwfH A

+++=

( )L

fhwfVV EA

2

2 +=−=

MB = -HAh MD = HEh HE = HA - wf

Figure 2.24.1 – Generic Formula for Portal Frames based on Pinned Feet




F = Total Load

IAB = ICD LI

hIK

AB

BC= k1=K+2

k2=6K+1 k3=2K+3 k4=3K+1

34hk

FLHH DA ==

2

FRR DA ==

0== DA MM 34k

FLhHMM ACB ===

38

3

hk

FLHH DA ==

2

FRR DA ==

0== DA MM 38

3

k

FLhHMM ACB ===

−=

3

36

8 k

KkFH A AD HFH −=

L

FhRR AD

2=−=

3

1

8

3

2 k

FhkH

FhM DB =

−=

0== DA MM

+==

3

32

8 k

KkFhhHM DC

2

FHH DA ==

L

FhRR AD =−=

0== DA MM 2

FhMM CB ==

Figure 2.24.1 – Generic Formula for Portal Frames based on Pinned Feet

L

h

A

B C

D

F

F

F

F




Foundations and Floor Slabs

The designer shall have, at a minimum, an understanding of the basic ground conditions likely to be encountered on site, either from historical data or a desk-top study. Preferably, the designer shall obtain a Ground Investigative Report (GIR) from suitably competent geotechnical engineers giving more precise values and ground conditions. Data to be considered includes ground level (GL), ground water level (GWL), soil types, classification and properties, allowable bearing capacities and a soil chemical analysis.

The analysis and consideration of any soil-structure interaction (i.e. any interface between a structure (be it above ground, partially buried or completely buried) and the underlying ground) is a complex affair, and in part depends on a degree of experience. Factors to consider include the relative settlements likely to occur (i.e. immediate and long-term), any history of previous soil loading (i.e. over-consolidation) and the non-homogenous content of most soils.

Designers are strongly recommended to consult geotechnical engineers and to refer to specialist literature such as “Soil-structure interaction – The real behaviour of structures”lvii for further information on this subject.

By their inherent nature steel portal frames with profiled sheet cladding may be classified as somewhat flexible structures, able to tolerate relatively large differential settlements between adjacent frames.

Concrete frames though, with masonry panels, are not so flexible and ground movement leading to differential settlement could cause severe cracking in the façade. There is also the strong possibility that shrinkage will occur between the frames and masonry panels, although joints at these positions can alleviate this problem.

The design bearing pressure shall be calculated and checked against the allowable bearing capacity, and if required measures shall be taken to provide suitable foundations such as piling or other ground improvement techniques - consultation with suitably competent geotechnical engineers is strongly recommended. A maximum differential settlement value of 20–25mm should be adopted.

For a lightly loaded industrial building that might reasonably be expected to be used for sewerage and water treatment plants Table 2.24.9 is a good guide to the nominal slab thickness required.

Table 2.24.9 – Nominal Slab Thickness Required

for Lightly Loaded Industrial

Buildings

Typical Application Classification

of Subgrade

Floor Slab

(mm)

Light industrial

premises with live

loading up to 5kN/m2

Poor 150

Medium / Good 125

Medium industrial

premises with live

loading between 5 and

20kN/m2

Poor 200

Medium / Good 175

Where dynamic loading (i.e. from forklifts, trucks, etc) is applicable, thicknesses will be determined from calculating flexural tensile stresses in the slab. Designers are referred to specialist literature for the design of floor slabs with dynamic loads.

Reinforcement in industrial floor slabs is located near the top surface to control crack width development. It does not increase the flexural strength of the slab. For a jointed reinforced industrial floor, reinforcement ratios of between 0.1% to 0.3% of the cross-sectional area shall normally be sufficient. This reinforcement most often takes the form of steel mesh.

Joints are required to control cracking that occurs within a slab. Three main types of joints are used for industrial floor slabs:

• Contraction Joints - Allow horizontal movement of the slab. They are provided transversely to the direction of placing, and should be spaced at maximum centres of 15m. Contraction joints may be either plain (unreinforced) or reinforced with steel dowels or shear keys, dowels being the more common method;

• Construction Joints - Transverse construction joints generally occur at unplanned locations (such as may be caused by adverse weather or equipment failure), or planned locations (such as the last concrete pour at the end of the day’s work). Longitudinal construction joints are used to form the edges of each pour;




• Isolation Joints - Isolation joints permit horizontal and vertical movement between adjacent elements (e.g. between the floor slab and column pad foundations, etc).

2.25 Site Boundary Wall/Fence

The demarcation of site boundaries is generally only required for the compound for above ground installations, such as pumping stations, storage tanks and treatment plants.

The boundary structure must provide adequate security to prevent, or at least discourage unauthorised access to the site. For this reason a boundary wall is preferable to a fence, which should only be used to provide temporary security, for example during construction or maintenance. The wall should be of solid block or concrete construction, without decorative openings.

Sewerage and drainage installations can be subject to public concern, and it is therefore important that they are compatible with their surroundings as far as possible.

Since the boundary wall is the most visible part of the installation, its general appearance needs to blend in “naturally” with the neighbourhood. The wall height, architectural features, colour and finishes should therefore match those of the surroundings, consistent with the need to provide security to the site.

The boundary wall and gate details will be subject to planning approval, along with the buildings and structures within the compound. The access gates shall be located and sized to avoid obstruction from the public.

Typical boundary wall, fence and gate details are contained in the Standard Drawings in Volume 8.

2.26 Site Facilities The extent and layout of site facilities are to a great extent controlled by the available land, and the purpose and location of the site. Site facilities should be agreed before design is undertaken, but typical requirements for urban sites would be:

• Stand-by generator plinth (or room for major installations), water tank and hydrants for washdown of vehicles and equipment, surge suppression installation, guardhouse, car ports;

• For remote locations, canteen, living accommodation and facilities for worship should be considered.

Site layouts should provide adequate space for access by operation and maintenance vehicles; with suitable paved turning areas to allow vehicles to turn and to pass each other within the compound.

Access roads and paved areas are to be provided for tankers, cranes, lorries and mobile generators. Space shall be provided for doors to buildings to open fully, and for vehicles to enter buildings for handling of equipment.

Road design and construction should be in accordance with the Qatar Highway Design Manual, with all access roads and hardstandings paved and drained. Open areas should have gravel finish to discourage weed growth.

The site layout shall accommodate the access requirements for all utilities, including the electricity supplier.

Any potential source of odour nuisance is to be located a distance of at least 15m for any habitable building.

The site drainage system shall discharge to the public system where possible, or to a SW pumping station on the site.

Typical details for site facilities are contained in Volume 8 - Standard Drawings.




3 Documentation

3.1 Reference Standards A full list of standards used in all of the manuals for design purposes is included in Volume 1 - Foreword. References used in this Volume are included at the end of the text.

3.2 House Connection Survey

When designing new sewerage systems to serve existing developments, it is necessary to establish the location of connection points from the existing buildings.

The first stage is to establish the number and locations of properties to be served by the sewerage system. This can be done by reference to mapping and aerial photographs where these are available. Using this information, the numbers of properties can be established, including those within compounds.

An external survey of the buildings within the property boundary must then be undertaken to establish the location of the discharge points from each building. Where large and complex buildings are involved, it may be necessary to undertake an internal survey to determine the facilities within the building that contribute to the sewerage system.

Individual buildings should be drained, either in parallel or in sequence, to a terminal manhole that is then connected to the main sewer. Please also refer to Vol 1 Section 3.3.1. An example house survey proforma is appended in Vol 1, Appendix 1.

3.3 Building Permit Please refer to Volume 1, Section 4.6 for a description of the procedures to be followed in this respect.




4 Health and Safety

Please refer to Volume 1 sections 4.9 and 4.10 for more detailed coverage relating to this subject. Health and Safety (H&S) design considerations for foul sewerage are not exclusive or prescriptive. In keeping with the DA policy, H&S is paramount in all aspects of infrastructure design and operation. The designer must be aware of the implications of design decisions on not only the finished product, but also on its buildability, construction stage safety, operating life and decommissioning at the end of its working life.

For this reason, it is essential that the procedures for production of a Hazard and Risk analysis are carried out, and incorporated into the pre-tender H&S plan. No design projects will be accepted as completed by DA without such steps having been taken, and provision of paperwork to demonstrate this. Considerations in design to mitigate risks will include but not be limited to:

• The designer must design out the need for entry into all confined spaces wherever possible;

• Safe access should be provided to all plant requiring maintenance;

• All above ground installations must be fenced off and made inaccessible to the general public. Security arrangements must be designed in consultation with the Operation & Maintenance (O&M) section of the DA;

• Craneage or mobile lifting facilities must be provided for all heavy equipment;

• Stairways should be equipped with handrailing and toe plates in accordance with the relevant BS;

• Tripping hazards should be avoided, likewise overhead obstructions;

• Barriers should be provided to prevent falling from height;

• All hazards should be signposted;

• Gas monitoring equipment and alarms to be designed as hard wired for all confined spaces requiring access;

• Adequate lighting to be provided wherever access is required;

• Welfare facilities should be provided to allow operatives to clean up after maintenance work;

• Manholes must be equipped with covers which are secure yet can be easily removed for maintenance purposes;

• Covers should be a minimum size to allow operatives wearing breathing apparatus. A minimum of 675mm square should be appropriate in most cases, but will depend upon the apparatus used by the O&M section of the DA;

• Flow isolation facilities shall be provided;

• Access to long tunnels to allow desilting equipment as necessary;

• Zoning classification should be established for all work carried out on existing and proposed installations.




5 Trenchless Technologies

The following is an overview of trenchless excavation techniques generally suitable for ground conditions in Doha. A brief summary of the typical purpose and diameter range appropriate for each technique is presented at the beginning of each overview. The techniques reviewed all relate to installation of new pipes. There is also extensive information available relating to sewer rehabilitation and renovation, but this relates more to the maintenance of older sewerage infrastructure. As the majority of work in Qatar relates to new build, the following section contains new build information only. Reference documents relating to this subject include the WRC Sewer Rehabilitation Manuallxiii and the Trenchless Techniques Reviewlxiv. This area of the market is under continual review and new techniques are regularly introduced.

Trenchless methods considered cover pipes ranging up to 1000mm in diameter.

The most suitable methods (microtunnelling and pipejacking) for ground conditions in Doha are presented in greater detail.

A general guide to designing structural elements is also given in the latter part of this section.

All tables and figures are presented in the end of the section.

5.1 Alternative Techniques

5.1.1 Pipe jacking (Open/Close Face)

Purpose: New Installation, Tunnelling Diameter Range: 900mm and above

Pipe jacking involves the jacking of a tunnelling shield and/or a complete length of tunnel lining into the ground from a drive shaft. High pressure hydraulic jacks are used to push the pipes through the ground behind a shield, while excavation takes place within the shield. Further lengths of pipe are added at the drive shaft and the process continues by pushing or jacking the complete string forward. It

is important to keep the string of pipes moving forward and to maintain lubrication, to ensure that the pipes stay buoyant during jacking.

A drive shaft is required, the dimensions of which vary according to the specific requirements of each situation. A thrust wall is constructed to provide a reaction to the jacking forces. The initial alignment of the pipe jack is obtained by positioning guide rails within the thrust pit on which the pipes are laid. To maintain alignment accuracy a steerable shield is used which must be frequently checked for line and level from a fixed reference. Upon completion of the drive length, the shield is recovered at the reception shaft, leaving a complete installed product pipeline.

For long lengths of pipeline, intermediate jacking stations may be necessary to allow sequential thrusting of sections of the pipeline. Drives of several hundred metres are attainable using this technique.

Spoil from the excavated face may be removed by a variety of means including auger flight, slurry pumping and on larger man-entry constructions, by skips, trucks and conveyers.

Normally the size of tunnel is of man-entry and above, i.e. greater than 1000mm. If the internal diameter is less than 1000mm and is conducted and steered by remote control, the process is generally classified as Microtunnelling.

Guidance on land requirements for shaft construction for this technique is given in Table 5.1.1.

This technique, when operating in the closed mode is generally suitable for ground conditions in Doha.

Advantages:

• Minimal surface disruption;

• Noise level and traffic disruption are minimised compared to conventional trenching;

• Compact size operation.

Limitations:

• Thorough site investigations are essential;

• The impact of varying soil properties can be significant;




• Difficult to deal with boulders occupying a significant percentage of the face area;

• Operators must be experienced and familiar with the machine and its expected performance in the expected ground conditions.




5.1.2 Microtunnelling (Closed Face)

Purpose: New Installation, Tunnelling Diameter Range: 300–2400mm

Microtunnelling is a method of installing pipes of up to 2400mm diameter. This is done using a steerable, remote controlled tunnelling machine, which is pushed horizontally into the ground from the drive shaft by a set of hydraulic jacks, in a jacking frame. When the tunnelling machine has entered the ground, a pipe is placed in the jacking frame behind the tunnelling machine and this is jacked forward, pushing the tunnelling machine ahead of it. This process continues until the tunnelling machine arrives at the reception shaft, leaving behind a length of installed pipe.

Microtunnelling systems fall into two main categories corresponding to the spoil transport method. One system uses a flight of augers running through the newly installed pipeline to transport spoil from the cutting head to the drive shaft. The spoil is then collected in a skip.

Alternatively, water or bentonite may be used to convert the soil into slurry at the cutting face. The slurry, which is water based, is then pumped to the surface along pipes within the product pipeline being jacked, where it enters a slurry processing plant. The spoil is removed and the slurry is recycled back to the cutting face. The slurry system can be used to control external groundwater by balancing the slurry pressure so that it offsets the groundwater pressure. The slurry system is generally more expensive than the auger system, and utilises more space on site.

The choice of system depends upon the soil type that is being excavated, and the distance to be tunnelled. The auger system is preferred for short drives since the removal rate is considerably faster. No slurry pumps or slurry processing plant are needed. For longer distances, especially in granular soil, weathered rocks, and where there is groundwater, the slurry system is usually more suitable.

The launch and retrieval pits will be sized according to such factors as drive diameter, access restrictions and the presence of other services.

Both systems provide face support by maintaining a positive pressure on the face through the cutting head and the soil in the collection system using an adjustable control at the head. It is imperative to know the type of ground conditions present as this will determine the type of machine to be used, the cutting head, the soil removal system and the jacking forces required.

A range of cutting heads is available according to the type of soil conditions present. The boring heads may be fitted with blades for soft soil, picks for hard soil, and soft rock and disc cutters for hard rock.

Microtunnelling machines are operated from a control cabin at the surface. Machines can drive 100m or more in soft ground for sizes of 100mm diameter upwards, from drive shafts of less than 3m diameter. These shafts can be located so that they become manholes in the finished scheme. The use of laser guidance control systems ensures a high degree of accuracy. Automatic computer monitoring is available on some systems.

Guidance on land requirements for shaft construction for this technique is given in Table 5.1.1.

Microtunnelling using slurry is generally more suitable for ground conditions in Doha than auger transported spoil.

Advantages:

• Can be less expensive than conventional trenching, especially for deep installations;

• Settlement is minimised, especially with the use of slurry machines;

• Noise level and traffic disruption are minimised compared to conventional trenching;

• Print out of line and level available, with high control and monitoring during driving.

Limitations:

• Boulders and obstructions such as timber can halt installation;

• The capital cost of equipment is high;

• Requires skilled and experienced operators.




Tables 5.1.1 - Guidance on Land Requirements, for Microtunnelling and Pipe Jacking Techniques

Nominal Pipe Diameter

(mm)

Minimum Diameter of Drive Shaft (m)

Minimum Diameter of Reception Shaft

(m)

Minimum Site Area Required

Open ground (m x m) Minimum width of site in roads (m)

250 to 500 3 3 15 x 10 5

600 to 700 3 3 20 x 10 5

800 to 1000 5 4 30 x 10 6.5

1100 to 1500 6 5.5 40 x 10 7.5




5.1.3 Directional drilling

Purpose: New Installation, Drilling Diameter Range: 300–1500mm

Directional drilling was originally developed to install pipelines under obstacles such as roads and river crossings, whereby the pipeline follows a shallow arc to avoid the obstacle. In general, the system involves large diameter steel or polyethylene pipelines being installed over long distances.

A rotating and steerable hollow drill of around 80 to 140mm diameter is launched from the surface at an angle of between 8o and 15° and is used to drill a pilot bore under the obstacle. Either a fluid jet cutter or a mud driven motor head is used, depending on ground conditions. The mud driven motor is principally used in sands, clays or soft rock with the slurry discharging from the bit lubricating the hole and removing soil cuttings. The fluid jet cutter is principally used in silts, silty clay or sands, and operates by forcing the slurry through small holes with the motive energy of the fluid jet cutting the soil.

A washover pipe of around 140mm diameter is drilled over the pilot string and follows behind the drill head. Alternate drilling then continues on the pilot string and the washover pipe until the exit point on the far side of the obstacle is reached. The pilot string is retracted and a rotating barrel reamer, attached to and pulled back by the washover pipe, enlarges the bore. Subsequent reaming continues until the required diameter is achieved. The product pipe is pre-assembled in the area of the drill exit point and usually the full pipeline length is jointed and pressure tested prior to installation. The product pipe is then attached to the reaming head via a swivel joint and pulled through the newly formed bore using the pullback capacity of the drilling rig. This can be carried out at the same time as the final back reaming operation.

A high level of accuracy is not usually required for this type of operation. A survey package fitted behind the drill head ensures that an accurate path is maintained. If necessary, the drill string can be drawn back as it approaches the target area and the bore re-drilled to improve accuracy.

This technique is also employed when a boulder or small obstacle is encountered.

The directional drilling process is a surface-launched method, therefore, it usually does not require access pits or exit pits. The rig working area should be reasonably level, firm, and suitable for movement of the rig. For maxi- and midi-HDD, an area of 100m by 60m is considered adequate. The equipment used in mini-HDD is portable, self-contained, and designed to work in congested areas.

Ground investigation is essential to ensure that the ground conditions are favourable. Directional drilling is unsuitable for use in granular soils and gravels due to the increased possibility of sidewall collapse.

Advantages:

• Installation is rapid;

• Long distances with relatively large diameter pipelines can be achieved;

• Printout of line and level available.

Limitations:

• A large area is required for the drilling rig, ancillary equipment and assembled product pipeline;

• Not suitable for gravity pipelines, with the exception of outfalls;

• The equipment has difficulty operating in granular soils;

• Accuracy of line and level cannot be maintained.

5.2 Planning and Selection of Techniques

The selection procedures presented in this manual are a general methodology that can be used to identify suitable trenchless techniques for new pipelines. However, they do not cover the cost or availability of each technique, which are controlled by local considerations that will change from time to time.




It must be stressed that the planning, feasibility and outline design stages are very closely related, and iterative in nature. Feasibility is covered under section 5.4.1, but is very much an extension of the planning process. A choice between open cut and trenchless methods will depend on environmental (vibration, noise, settlements, traffic disruption etc.), buildability (complexity of temporary works, settlement monitoring programme, advanced works etc.), commercial factors (cost vs benefit analysis), and health and safety requirements.

5.2.1 Initial Planning

Planning for the installation of pipes requires:

• Establishing system/network performance requirements;

• Establishing system design criteria;

• Topographical survey data;

• Route optimisation;

• Determination of the location of existing utilities;

• Site investigation, establishing site specific geology;

• Consideration of construction methods.

Early consideration needs to be given to the information required to procure and construct the work. This includes the following:

• Contract terms;

• Risk assessment;

• Ground Investigation;

• Statutory requirements;

• Settlement restrictions (ground, affected services and buildings);

• Noise restrictions;

• Access requirements;

• Traffic management requirements;

• Land use restriction;

• Checking the feasibility of method by consulting specialist contractor;

• Environmental constraints.

The procedure for establishing various aspects of the planning and investigation for Trenchless Techniques is set out in the flow diagrams in Figure 5.2.1.




Figure 5.2.1 – Flow Diagram for Planning and Selecting Installation Techniques for New Pipe Installation

1) SEWER DESIGN (Hydraulic)

• Establish required system performance and design criteria

• Determine type, size, depth and length of pipes to be installed

• Initial inquiries to statutory authorities

• Collect details of restrictions and requirements which will apply throughout the project

For Planning Issues Refer to Volume 1, Section

2

2) OPTIONS ASSESSMENT

• Feasibility Study

• Collate existing geotechnical data

• Availability of equipment

• Requirement for site working areas

• Availability of resources (power / water / drainage)

• Design and procure initial Ground Investigation

• Risk Assessment

• Prepare cost estimates

3) TRENCHLESS DESIGN

• Identify advantages of trenchless techniques over open cut methods (depth of service, crossing highway or other structures, site restrictions such as utility services, working hours, traffic restrictions etc.)

• Consider and select suitable trenchless techniques based on Site Investigation results (refer to Tables 5.1.1,and 5.2.1 for suitability of various trenchless techniques in recent ground conditions in Doha)

• Consider drive lengths available for selected pipe material to select suitable trenchless technique (refer Figure 5.2.2), and suitability of slurry TBM and EPB for ground condition (refer Figure 5.2.3)

• Design and procure site-specific survey, including ground Investigation to suit chosen trenchless technique as in Volume 1, Section 3

• Identify existing structures and utility services along the route of the pipe and carry out preliminary assessment of ground settlements as a result of trenchless techniques and their effects on the identified structures

• Prepare site monitoring plan (settlement monitoring points, other instrumentation such as piezometer, extensometer)

Design Process as per Volume 1, Section 4

4) CONSTRUCTION

• Decide on form of contract

• Prepare contract documents to tender

• Firm up cost estimates

Refer Volume 1, Appendix 4 - Tender Procedure Flow Charts




Table 5.2.1 - Suitability of Trenchless Techniques for Various Ground Conditions in Doha

Ground Condition Microtunnelling and Pipe

Jacking Directional Drilling Rock TBM

Soft to very soft clays, silts and organic deposits

GS

DMO

NS

Medium to very stiff clays and silts GS

GS

NS

Hard clays and highly weathered shales GS

GS

NS

Very loose to loose sands above and below the water table (Local geology: Reclaimed land)

GS

DMO

NS

Medium to dense sands above the water table (Doha geology: Reclaimed land)

GS

DMO

NS

Medium to dense sands below the water table (Doha geology: Reclaimed land)

GS

DMO

NS

Gravel and cobbles <50–100mm dia (Doha geology: Reclaimed land)

GS

DMO

NS

Soils with significant cobbles and boulders. 100–150mm dia

DMO

DMO

NS

Weathered rocks and firmly cemented soils (Doha geology: Soft weathered limestone / caprock)

GS

DMO

DMO

Slightly weathered to unweathered rocks (Doha geology: Slightly weathered to Unweathered limestone)

DMO

NS

GS

NOTE:

GS: Generally Suitable Caution is needed in the presence of identifiable groups / nests of boulders. If they represent a significant percentage of the face area it may preclude small diameter bores

DMO: Difficulty May Occur Modifications to the machine and very detailed ground investigation

needed to establish ground conditions and machine performance NS: Not Suitable Unsuitable in given ground conditions




Figure 5.2.2 - Drive Lengths for Different Trenchless Techniques and Suitable Pipe Material

DIRECTIONALDRILLING

PIPE JACKING

MICROTUNNELLING

HPPE, MDPE, STEEL

1500 m

DI, GRP, PC, PSC, RC

300 m

DI, GRP, PC, PSC, RC, VC

180 m

Product Pipe Material Key:

Abbreviation Definition

CM Cement mortar

COMP Polyester resin conforming to WIS 4-34-04

DI Ductile Iron

ER Epoxy resin

GRP Glass Reinforced Plastic

HPPE High Performance Polyethylene

MDPE Medium Density Polyethylene

PC Plain concrete

PP Polypropylene

PRC Plastic Reinforced Concrete

PSC Pre-stressed Concrete

PVC Polyvinyl Chloride

RC Reinforced concrete

STEEL Steel

VC Vitrified Clay




Figure 5.2.3 - Suitability of Slurry TBM and EPB, based on Various Grain Size Distribution Curve in Various

Loose Ground

Note: COPYRIGHT BY HERRENKNECHT AG. PRINTED WITH PERMISSION FROM HERRENKNECHT, 2003




5.2.2 Selection Criteria

Planners should consider a number of factors when deciding the most appropriate method for installation of pipes. The factors include:

• Diameter of pipeline;

• Length;

• Depth;

• Location;

• Topography;

• Ground and site conditions;

• Cost;

• Presence of other services;

• Physical obstacles (e.g. buildings);

• Traffic disruption;

• Disruption to third parties;

• Installation techniques;

• Experience of techniques;

• Safety and Risk assessment;

• Availability of services (power, water, drainage);

• Reinstatement requirements;

• Environmental considerations;

• Settlement predictions and ground monitoring, including action plan preparation.

5.2.3 Factors Affecting Choice Of Method

Trenchless techniques should be considered instead of traditional open-cut techniques in the following circumstances:

• Installation of pipelines greater than 7m depth;

• Installation of pipeline in poor ground conditions and high water table;

• Installation in congested urban areas where damage to utility services and disruption to traffic would make open-cut methods unacceptable;

• Crossing of busy highways and other infrastructures;

• Minimising the length of the pipeline route.

5.3 Geotechnical Investigations

5.3.1 Geological Strata Overview

The geology of the Doha region is described in Volume 1, Section 4.2. Generally, trenchless techniques in Doha are likely to encounter the following ground conditions:

• Superficial deposits of silty fine to coarse carbonate sand and fine to coarse crystalline limestone gravel, with occasional cobbles;

• Weathered bedrock, fractured to varying degrees comprising crystalline limestone, carbonate siltstone and carbonate mudstone;

• Reclaimed land (mainly the West Bay area) – A mixture of sand, silt and gravel overlying coastal silts and sands. Some areas using various natural and man-made rubble.

5.3.2 Groundwater Regime

Hydrogeology and groundwater levels in Doha are described in Section 4.2 of Volume 1.

The rising ground water levels in Doha should be considered at the design stage of the project. This can have long term effects on installed pipelines such as loading due to water pressure, joint sealing between pipe sections, flotation, and long-term durability.

Groundwater should also be tested for salinity in order to determine durability requirements of the pipe material. Generally, chemical analyses of the soil and water samples in Doha indicate high sulphate and water-soluble chloride contents. It is




important that dense fully compacted concrete, is used to manufacture pipes. Also, pipes should have corrosion resistant finishes. These finishes are cast into the pipes during manufacture and form an integral part of the pipe. BRE digest 250lxv recommends protection measures necessary for concrete against sulphate attack.

All pipes delivered on site should come with up-to-date Quality Certification.

Evaluation of groundwater presence and pressures during site investigation is of utmost importance in the design of, and construction of, pipes using trenchless techniques. Unforeseen groundwater can cause major problems during construction, resulting in significant delays and increased costs.

5.3.3 Soil/Rock properties

When designing and planning installations to be carried out by trenchless techniques, the planners should consult geotechnical engineers on the characteristic of the soils and/or rock likely to be encountered, together with details of the water table, ground permeability, and seasonal changes.

Site investigations, field tests, laboratory tests, reports and interpretation are described in Volume 1, Section 3. In addition to these, the following information is required for planning trenchless techniques:

• Abrasivity of rock samples;

• Historical information of reclaimed land where applicable (material used, compaction method, completion date).

A list of soil parameters required for design and construction of trenchless techniques is given in Table 5.3.1.




Table 5.3.1 - Geotechnical Parameters Required for Design

Geotechnical Parameter Symbol Application for planning

Soil and / or rock description Define types of ground Grade of rock Q, RMR Extent of ground support Percentage core recovery and core condition TCR , SCR, RQD State of weak rock or hard ground Unit total and effective weights ע ע , ’ Overburden pressure Relative Density of coarse grained soils Dr State of natural compaction of cohesionless

soft ground Moisture content W Profiling of property changes with depth Specific Gravity Gs Type of Ground Plasticity and Liquidity Indices LL, LP, PI, LI Type and strength of cohesive soft ground Particle size distribution ν Composition of soft ground Unconfined Compressive Strength qu Intact strength of hard ground Point Load Index Strength of lump lp Intact strength of hard ground lump Axial and Diametrical Point Load Index Strengths la , ld Axial and diametral intact strengths Undrained Shear Strength CU , SU Shear strength of soft ground Effective Stress Shear Strength C’ Long term cohesion of soft ground Angle of Shearing Resistance

Φ, Φ’ Long term shear strength of cohesive soft ground, short and long term Shear Strength of cohesionless soft ground

Drained Deformation Modulus E’ Long term stiffness Poisson’s Ratio Influences stiffness values Coefficient of Effective Earth Pressure Ko, Ka, Kp Ratio between horizontal and vertical

Effective stresses at rest, Active and Passive

In situ Stresses in Rock σ Magnitude of principal stresses in rock in three directions

Permeability K Characteristic ground permeability’s and variations.

PH, Sulphate and Chloride contents Ph, SO3 , CI Concrete and steel durability Chemical contamination Extent of ground contamination Abrasion Rate of cutter tool wear




5.3.4 Indicative Scope of Interpretative Reporting

The Geotechnical Factual Report (GFR) should contain all the findings of the field and laboratory work. BS 5930:1999 sets out, in general terms, the contents of a GFR. Based on the GFR, a Geotechnical Interpretative Report (GIR) is prepared.

The GIR provides an overview of the ground conditions and the likely construction methods, together with the suitability of various techniques and risk assessment. Typically, the GIR would contain the following sections:

• Outline of the proposed scheme;

• Definition of route corridor;

• Desk study and site reconnaissance findings;

• Identification of route and alignment options;

• Summary of the ground investigation work;

• Description of ground and groundwater condition;

• Interpretation of ground conditions in relation to the design and construction of the proposed scheme;

• Recommendations for design of temporary and permanent works, and further ground investigation if necessary;

• Risk assessment for various schemes/trenchless techniques.

5.4 Design

5.4.1 Feasibility Study

The information useful when planning and designing new installations includes:

• Land ownership;

• Historical maps of the area that may reveal obstacles (e.g. wells, mine shafts etc.);

• Geological map of the area;

• Aerial photography;

• Topographical survey;

• Site investigation to determine the location of existing services and potential buried obstructions;

• Contamination.

5.4.2 Pipe Design

In this section guidelines for structural design of pipes and shafts are provided with regards to construction and permanent loadings.

Frictional Resistance

During installation of pipes using trenchless techniques, frictional forces build up around the pipeline as the line of pipes is advanced behind the shield. The frictional forces arise from soil cover and surcharge loads and are affected by the quality of lubrication. The frictional forces depend on the type of soil, depth of overburden, length and diameter of the pipe(s) being jacked, the speed of excavation and most importantly, the lubrication agent injected between pipes, the quality of workmanship, and the ground properties during jacking. Empirical values for friction coefficients may vary between 0.5 and 2.5 tonnes per square meter of external circumferential area, depending on site conditions and the type of excavation. Alternatively, frictional force can be estimated from the procedure outlined in Milligan et. allxvi.

Using lubricating agents such as bentonite under pressure generally reduces frictional forces on the pipeline. If high frictional forces are expected due to factors such as ground roughness, together with high fracturing/permeability conditions, and there is a high likelihood of pressure loss. It is recommended that intermediate jacking stations be placed at regular intervals in the pipeline and/or pre-treating, undertaken in the areas of potential pressure loss.

Jacking Loads

Jacks push the pipes forwards against the ground frictional resistance (depending on the effect of lubrication), the face support pressure detailed in Milligan et. allxvi, and the force on the cutting edge of the leading pipe. A factor of safety is also used to allow for unforeseen obstacles, varying ground conditions and poor workmanship. The jacking force required is as follows:




Jacking Force = Frictional resistance + Weight of

Pipe + Face Pressure (Closed mode)

Equation 5.4.1

Pipe Design

The pipes are designed to withstand axial forces applied to the pipe during the jacking operation. As well as jacking forces, the pipes must be designed for external forces due to soil and groundwater pressures and live loads such as traffic.

Pipe Joint

Jacking force causes the maximum loading on a pipeline. The joints are designed to ensure jacking forces are transmitted over the maximum area of the pipe. The design information required is: maximum allowable concrete strength; the stiffness of packing material; and maximum allowed misalignment angle.

Where the jacking force is well distributed over the pipe end area, it would be appropriate to use a concrete strength of 0.4fcu, where fcu is the characteristic cube strength of concrete. For the highly localised stresses at the joints in the extreme conditions, a joint face stress of 0.8fcu can be used. Milligan et. allxvi, contains example calculations for determining permissible jacking force based on linear stress theories.

Pipe Lining

Pipe lining can be designed using simple compression theory. Hoop reinforcement will generally be needed in larger diameter pipes to resist bending due to ground pressures and stresses near the pipe ends due to jacking loads, or as nominal reinforcement for crack control. Structural design of the lining can be carried out using appropriate codes for the materials in question.

5.4.3 Shaft Design

Construction Method

Jacking and receiving shafts are generally vertical excavations with shoring and bracing systems. Several shoring systems are commonly used, such as sheet-pile systems with internal bracing, or precast concrete shafts.

An important factor in the design of jacking and receiving pits is groundwater control. Dewatering systems using deep wells or well points are frequently employed. However, in urban areas this could lead to consolidation settlements resulting in damage to structures and utility services in the zone of influence. Groundwater cut-off arrangements can be used if relatively impermeable soils are present below water bearing soils. Sheet piles could be driven into the impervious soils to cut off groundwater inflows or water seals could be used between caisson shaft units.

Grouting or similar methods of groundwater control are normally required when launching the pipe and advancing out of the jacking pit, or advancing into the receiving pit.

Wet caisson sinking methods are frequently used to construct shafts where dewatering or grouting methods would be difficult or uneconomical. This approach involves constructing the shaft by stacking up circular precast concrete sections while excavating inside the caisson below the groundwater level with a cutting edge. The units are bolted together vertically, complete with seals to stop water entering the shaft. After the caisson is sunk to the design elevation, a concrete slab is poured to form the base of the shaft.

Structural Design of Shaft

The base of the shaft is designed to transfer uplift and hydrostatic forces to the shaft walls. The weight of the slab and the shaft walls counteract the up-thrust forces. To some extent, the shaft also resists uplift through ground adhesion, depending on the effectiveness of the bonds at shaft external face/grout/ground. This adhesion or bond has to be assessed carefully with suitable factors of safety allowing for the quality and long-term durability of void grouting.

Shaft Base

The base can be constructed using mass, or reinforced concrete.

Mass concrete is used for small circular shafts and acts in compression by arching. The design is based on the principle of dome action to radial loading (refer to Reynolds et. allxii.




Reinforced base slabs are used in large diameter shafts. Slab design is two-directional, simply supported along the edges and of sufficient thickness to ensure that shear reinforcement is not required (refer to Reynolds et. al).

Shaft Lining

Circular caisson units are designed to withstand ground and water pressures. The circular caisson lining can be designed based on the method described for pipe lining.

5.4.4 Ground Movements

Tunnelling-induced soil settlement is estimated based on the methods proposed by Peck (1969) (refer to Pecklxvii) and Mair et al. (1993) (refer to Taylor et.al.lxviii). According to their methods, the shape of settlement profiles at the ground surface and subsurface can be characterised as a Gaussian distribution.

Existing Structure Responses

The assessment of risk to buildings and utilities should be carried out during planning stage.

Potential building damage and categorisation commonly used for new installations using trenchless techniques in urban areas is shown in Table 5.4.1. It should be noted that the assessment of soil structure interaction is a highly complex and variable issue, which cannot be covered in the scope of this manual.




Table 5.4.1 - Damage to Buildings and Suggested Actions

Risk Category

Maximum slope

Maximum settlement

Description of risk Action required

1 Less than 1/500

Less than 10mm

Negligible: superficial damage unlikely

No action except for particularly sensitive buildings where individual assessment should be made

2 1/500 to 1/200

10 mm to 50mm

Slight: possible non-structural superficial damage

Crack survey and schedule of defects. Assess particularly vulnerable buildings and pipelines individually

3 1/200 to 1/50 50mm to 75mm

Moderate: possible structural damage to buildings and rigid pipelines

Crack survey, schedule of defects and structural assessment. Predict extent of possible damage to buildings and decide whether to repair damage, control movements or demolish. Identify vulnerable services, decide whether to repair, divert or replace with more tolerant services.

4 >1/50 > 75mm High: expected structural damage to buildings, rigid pipelines and possible damage to flexible pipelines




Instrumentation and Monitoring

When tunnelling in urban areas and taking into account the complexity and environmental factors, carefully planned and executed instrumentation and monitoring is essential.

It is imperative that lines of communication are open in order to feed back the data obtained to the machine operatives. This will allow them to adjust and improve machine performance, as well as to compare the data with the predicted levels of settlement and strains obtained before the tunnelling works commence.

Typical instrumentation normally used is as follows:

• Surface markers (settlements and lateral displacements);

• Extensometers (vertical displacement profile);

• Inclinometers (horizontal displacement profile);

• Piezometers (pore water pressure profile);

• Measuring relative rotation and angular strain of buildings using electro-levels, biaxial tilt-meters, and precise levelling studs;

• Automated Total Stations can be set up and programmed to monitor targets fixed at key points, at regular intervals, and the data down loaded remotely through the use of radio relay transmission;

• Monitoring existing defects (cracks) using tell-tale indicators.

The Instrumentation & Monitoring Plan must be site specific and should include the following as a minimum:

• Location and type of instrumentation;

• Alarm, Alert and Action levels;

• Lines of responsibility and communication;

• Rapid response and emergency plan, including contact names and telephone numbers with relevant authorities.

5.5 Environmental Assessment

The use of trenchless technologies requires that several specific environmental impact issues be evaluated in detail, along with appropriate consultation with SCENR.

5.5.1 Vibration

Vibration from trenchless techniques very rarely give rise to building damage, disturbance to people through perceptible vibration, or by the generation of ground-borne noise.

The nature, duration, and number of events that occur in a specified period, and the location in which the vibration is received, all influence the public’s tolerance. The vibration dose value (VDV), described in BS 6472: 1992lxix is used to combine the effects of all perceptible vibration events that occur to establish probability that complaints will arise.

Most activities related to trenchless techniques do not give rise to vibration levels of a magnitude that would be damaging. BS 7385: Part 2: 1993 gives guidance on vibration levels that can cause damagelxx.

When required, monitoring devices can be installed to determine levels of vibration.

As a guide, key typical guidance criteria for vibration are 5mm/s peak particle velocity for construction works, and 3mm/s near schools and hospitals.

5.5.2 Noise

Noise levels from trenchless construction depend on the technique adopted. The level should be assessed in order to determine whether noise exposure is likely to reach the action levels stated below.

BS 5228-1 gives guidance on how noise arising from worksites affects site personnel and others living and working in the neighbourhood. BS 5228-2 gives guidance on legislation covering the control of noise and vibrationlxxi.

Action Levels




Where noise levels are likely to be at or above levels defined below, then action is required to reduce noise or provide noise protection.

Action levels 1 and 2 are values of “daily personal exposure to noise”, defined as LEP, d. These depend on the noise level in the working areas, and how long people spend in them during the day.

Action level 1: is a LEP, d of 85dB (A). At this level the employer has a general duty to provide ear protection.

Action level 2: is a LEP, d 90dB (A). At this level in addition to the action required above, the employer has a duty to ensure all personnel wear ear protectors at all times and mark ear protection zones with notices.

Action level 3: This is a peak action level, corresponding to of 140dB. The peak action level is most likely to be important for loud impulsive sources, such as blasting. At this level, the employer must again ensure all personnel wear ear protectors at all times and mark ear protection zones with notices.

In Qatar, SCENR has published guidance on noise ‘15 minute weighted average dB (A) at property line’ standards as follows:

Residential and Institutional – 55dB (A) (day) and 45dB (A) (night);

Commercial – 65dB (A) (day) and 55dB (A) (night);

Industrial – 75dB (A) (day) and 75dB (A) (night).

5.5.3 Dust

Generally, trenchless techniques for installation of pipes produce far less dust than traditional open excavation. Also, dust is limited to the working site and during removal of excavated materials. Sprinkling water on excavated material and covering spoil in the removal trucks can control dust on site. Also, the spoil haul route within and outside the site should be maintained in a clean condition, if necessary by spraying water.

Deposited dust limits are generally regarded to be an increase of 200mg/m2 over the baseline level. General airborne dust above 10mg/m3 is an approximate trigger level (although not an

occupational health level). Further guidance detail is provided in BS 1747lxxii.

In Qatar, SCENR has issued guidance on ambient air quality particulate matter. Over a 24-hour averaging period 99.7% of data levels should meet the standard of 150mg/m3.

5.6 Approvals – Procedures and Formats

Trenchless design will be by specialist contractor according to techniques, machinery and materials being considered.

Design will require approval by the consultant in line with the following guidance.

5.6.1 Guidance for Design Check

The consultant who is approving design and construction methods proposed by the specialist contractor shall ensure design and construction processes, safety and environmental requirements are in line with relevant sections of Volume 1. In addition, the consultant shall check the following:

• Adequate working area, and the size of drive and reception shafts are proposed as per Table 5.1.1;

• Shaft and pipes are designed in-line with section 5.4;

• The shaft lining is watertight;

• Settlement analysis and effects on utility services, highway and other structures in the zone of influence are assessed;

• Monitoring regime and settlement limits are established for above structures.

5.7 Risk Assessment Risks associated with trenchless technologies should be assessed before work starts so that the necessary preventative measures can be identified and action taken. The process of risk assessment




starts at the design stage of a project and continues during the construction phase.

Risks can be categorised as contractual, construction, operational, financial, environmental, health and safety. Contractual risks can arise from inadequate contract preparation and management. Generally, the risk increases with decreasing clarity of contract and can be dealt with through improving contract clarity and management practices. Construction risks are associated with site conditions and construction methods. These risks can be minimised by careful planning but are seldom eliminated.

The risk associated with trenchless techniques can be summarised as follows:

• Mechanical failure of machinery;

• Material failure – pipes or linings;

• Ground loss leading to high settlement or ground collapse;

• Risk of damage to utilities, road and surrounding structures;

• Unforeseen ground conditions;

• Loss of directional control.

These risks can be mitigated by the following measures:

• Choosing suitable techniques/equipment and construction material;

• Appropriate level of site investigation and interpretation of results;

• Trained operatives and choosing experienced Designers and Contractors;

• Monitoring ground, services and buildings in the settlement influence zone;

• Adequate level of supervision;

• Preparation of emergency procedures.

A typical Risk Assessment Matrix is presented in Table 5.7.1 and Risk Classification is indicated in Table 5.7.2.




Table 5.7.1 - RISK ASSESSMENT MATRIX – A typical risk assessment for trenchless techniques

Project Title: Prepared by: Date

L=Likelihood, S=Severity, RR=Risk Checked by: Page

Ref Activity / Hazard Risk Rating Design and/or Construction input to eliminate / reduce hazard

Residual Risk Rating

L x S= RR L x S= RR Risk level

Damage to utilities, roads, surrounding structures / Substantial cost for

Damage, Delay

3 3 9 Establish location of utilities before construction/ obtain Utilities Drawings/carry out Trial trenches/ Geophysical survey/ Monitoring

2 3 6 Medium

Failure of materials (e.g. pipes or lining) / Substantial cost for Damage, delay

2 3 6 Experience designers/ Comprehensive quality control/ reputable supplier

1 3 3 Low

Failure of machinery 2 3 6 Select most suitable technique/ experience contractor/reputable manufacturer of machinery

1 3 3 Low

Mix ground conditions / Ground failure, Delay, Cost

2 3 6 Select most suitable technique including trenching / experience contractor / reputable manufacturer of machinery

1 3 3 Low

Buried structures, e.g. basements, piles, Historical Mining –voids, ‘cavities’ / Impedes pipe drive, delay, cost

2 3 6 Comprehensive Desk Study to identify historical structures

Testing equipment

1 3 3 Low

Encounter major inflow of water / Ground failure, slow progress

3 3 9 Ground treatment / Dewatering / Close mode technique

2 3 6 Low

Noise, dust, pollution, vibrations / Adverse Public relations

Restricted working hours

Compensation claims

Health and Safety Claims

4 3 12 Establish necessary restrictions beforehand

Reduce by choosing appropriate method

2 2 4 Low

Contaminated Ground/Groundwater / Adverse working conditions for Workforce,

Specialist requirements for waste and Groundwater disposal

2 3 6 Comprehensive Desk Study & Intrusive Ground Investigation to identify problem areas

PPE / testing equipment

2 1 2 Low

Guidance on HARAs is given in CIRIA Report 166 (CDM Regulations – Work Sector Guidance For Designers)lxxiii




Table 5.7.2 - Risk Classification to enable each risk to be assessed in terms of probability and severity

SEVERITY / CONSEQUENCE (Hazard)

LIKELIHOOD Minor

1

Slight

2

Moderate

3

High

4

Very High

5

Extremely

Unlikely

1

1 2 3 4 5

Unlikely

2

2 4 6 8 10

Likely

3

3 6 9 12 15

Very Likely

4

4 8 12 16 20

Certain

5

5 10 15 20 25

PRIORITY OF ACTION

Score

1-5

6-10

Above 10

Rating Low Risk Medium Risk High Risk

Action PPE, Best Working Practice, Signs

Isolate, control Eliminate, Reduce by substitution




5.8 Trenchless Construction References

The following documents are helpful in design and use of Trenchless Technologies.

• British Standards Institution, 1990, BS 5930: 1981- Code of practice for site investigation, London, BSI.

• British Standards Institution, 1990, BS1377: 1990 - Methods of test for soils for civil

engineering purposes. London, BSI.

• British Standards Institution, 2001, BS 6164: 2001 - Code of practice for safety in tunnelling

in the construction industry, London, BSI.

• British Standards Institution, 1981, BS 5911-1:1981 Precast concrete pipes and fittings for drainage and sewerage. Specification for pipes

and fittings with flexible joints and manholes

(No longer current but cited in the Building

Regulations), London, BSI.

• British Standards Institution, 1997, BS 5228-2:1997 - Noise and vibration control on

construction and open sites — Part 2: Guide to

noise and vibration control legislation for

construction and demolition including road

construction and maintenance. London, BSI.

• British Standards Institution, 1990, BS 7385 -1:1990, Evaluation and measurement for

vibration in buildings. Guide for measurement

of vibrations and evaluation of their effects on

buildings, London, BSI.

• British Standards Institution, 1990, BS 7385 -1:1990, Evaluation and measurement for

vibration in buildings. Guide for measurement

of vibrations and evaluation of their effects on

buildings, London, BSI.

• 8 British Standards Institution, 1992, BS

6472:1992: Evaluation of human exposure to

vibration in buildings (1Hz to 80Hz), London, BSI.

• Building Research Establishment, Digest 250: Concrete in sulphate-bearing soils and ground

water. UK, BRE.

• Reynolds, C.E. and Steedman, J.C, 1988, Reinforced Concrete Designers Handbook. 10th ed. London, Spon Press.

• British Tunnelling Society and Association of British Insurers, 2003, Joint Code of Practice for the Risk Management of Tunnelling Projects

in the UK, UK.

SETTLEMENT AND DAMAGE TO BUILDINGS

• Burland J.B., and Wroth C.P, 1975, Settlement of Buildings and Associated Damage, Building Research Establishment Current Paper, Watford, Building Research Establishment.

• Burland J.B., 1997, Assessment of risk of damage to buildings due to tunnelling and excavation, Earthquake Geotechnical

Engineering, Ishihara (ed.), Balkema, Rotterdam, pp. 1189-1201.

• Boone S.J., 1996, Ground Movement Related Building Damage, Journal of Geotechnical Engineering, ASCE, 122(11), pp. 886-896.

• E.J. Cording, T.D. O’Rourke, and M.D.Boscardin, 1978, Ground Movements and Damage to Structures, Proc., Int. Conf. On Evaluation and Prediction of Subsidence, Florida, pp 516-537.

• Peck, R. B., 1969, Deep excavations and tunnelling in soft ground. Proc. of 7th Int. Conf. Soil Mech., Mexico, State of the Art 3, pp. 225-290.

• Taylor, R. N., and Bracegirdle, A., 1993, Subsurface settlement profiles above tunnels in clay, Geotechnique, 43(2), pp.315-320.

DESIGN

• Milligan G., Norris, P. Pipe jacking: Research results and recommendations, Pipe Jacking Association.




• B Maidl, M. Herrenknecht, L. Anheuser, Mechanised Shield Tunnelling, Ernst & Sohn Publications.

TRENCHLESS TECHNOLOGIES

• Pipe Jacking Association, 1987, A guide to pipe jacking and Microtunnelling design, Pipe Jacking Association.

• International Society for Trenchless Technology, 1992, Introduction to trenchless technology, 2nd edition, ISTT.

5.9 Trenchless Construction Glossary

The following terms used in Section 5 - Trenchless Technology, are defined below.

AUGUR BORING

A technique for forming a bore from a drive shaft to a reception shaft, by means of a rotating cutting head. Spoil is removed back to the drive shaft by helically wound auger flights rotating in a steel casing. The equipment may have limited steering capability.

AUGUR TBM

A type of tunnel boring machine in which the excavated soil is removed to the drive shaft by auger flights passing through the product pipeline pushed in behind the TBM.

CUTTER HEAD

Any tool or system of tools on a common support which excavates at the face of a bore. Usually applies to mechanical methods of excavation.

DIRECTIONAL DRILLING

A steerable system for the installation of pipes and cables in a shallow arc using a surface launched drilling rig.

DRILLING FLUID / MUD

A mixture of water and bentonite or polymer continuously pumped to the cutting head to facilitate the removal of cuttings, stabilise the borehole, cool the head and lubricate the installation of the product

pipe. In suitable ground conditions water alone may be used.

DRIVE/ENTRY SHAFT OR PIT

Excavation from which trenchless technology equipment is launched for the installation or renovation of a pipeline, conduit or cable. May incorporate a thrust wall to spread reaction loads to the ground.

EARTH PRESSURE BALANCE (EPB) MACHINE

Type of Microtunnelling or tunnelling machine in which mechanical pressure is applied to the material at the face and controlled to provide the correct counter-balance to earth pressures in order to prevent heave or subsidence.

ENTRY/EXIT ANGLE

In a horizontal directional drilling/guided boring system, the angle to the ground surface at which the drill string enters and exits in forming the pilot bore.

FLUID-ASSISTED BORING/DRILLING

A type of guided boring technique using a combination of mechanical drilling and pressurised fluid jets to provide the soil cutting action.

GROUTING

Filling of the annular space between the carrier pipe and the new product pipe. Grouting is also used to fill the space around laterals and between the new pipe and manholes. Other uses of grouting are for localised repairs of defective pipes, ground improvements prior to excavation during new installations and the filling of voids around existing carrier pipe.

INTERJACK PIPES

Pipes specially designed for use with an intermediate jacking station.

INTERMEDIATE JACKING STATION (IJS)

A fabricated steel shield incorporating hydraulic jacks designed to operate between interjack pipes to provide incremental thrust on long drives.




JACKING PIPES

Pipes designed to be installed using pipe jacking techniques.

JACKING SHIELD

A fabricated steel cylinder from within which the excavation is carried out either by hand or machine. Incorporated within the shield are facilities to allow it to be adjusted to control line and level.

JET CUTTING

A type of guided boring technique using pressurised fluid jets to provide the soil cutting action.

LAUNCH PIT

As for drive shaft but more usually associated with “launching” an impact moling tool.

LEAD PIPE

The leading pipe manufactured to fit the rear of the jacking shield and over which the trailing end of the shield is fitted.

MEASUREMENT WHILE DRILLING (MWD)

Borehole survey instrumentation that provides continuous information simultaneously with drilling operations, usually transmitting to a display at or near the drilling rig.

MICROTUNNELLING

Steerable remote control pipe jacking to install pipes of internal diameter less than that permissible for man-entry (i.e. <1000mm).

MIDI-RIG

Steerable surface-launched drilling equipment for the installation of pipes, conduits and cables. Applied to intermediate sized drilling rigs used as either a small directional drilling machine, or a large guided boring machine. Tracking of the drill string may be achieved by either a downhole survey tool or a locator.

MAN-ENTRY

Description of any tunnelling technology process, which requires an operative to enter a pipe, duct or bore. The minimum size for which this is

permissible is generally defined by national health and safety legislation (e.g. larger than 1000mm diameter in the UK).

NEW INSTALLATION

Methods by which a new pipeline is constructed.

PIPE JACKING

A technique by which the pipes are pushed through the ground behind a tunnelling shield using hydraulic jacks reacting against a thrust wall in a jacking/launch pit.

RECEPTION/EXIT/TARGET SHAFT OR PIT

Excavation into which trenchless technology equipment is driven and recovered following the installation or renovation of the product pipe, conduit or cable.

SLEEVE PIPE

A pipe installed as external protection to a product pipe.

SLURRY TBM

A type of Microtunnelling machine in which soil is turned to slurry and is used to counterbalance ground water pressure to stabilise the face, before being pumped to the surface.

SURVEY TOOLS

Downhole equipment and instruments used to determine the position of a bore in directional drilling or site investigation.

TUNNEL BORING MACHINE

A full-face circular mechanical shield machine, usually of man-entry diameter, steerable, and with a rotary cutting head. For pipe installation, it leads a string of jacked pipes. It may be controlled from within the shield or remotely.




6 References

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qt00129 vol2 final

Documents

programmable

2minimum pipe

conventional

high bay lamps

5general design

pump duty

odour control

motor protection