outline construction method statement

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APPENDIX K Dunan Hydro Electric Scheme Outline Construction Method Statement 19 th March 2013 Revision A Eugene Craig [email protected] 07886 401194 DHG Hydro Ltd 7 St James’s Gardens London W11 4RB 0131 208 0525

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Page 1: Outline Construction Method Statement

APPENDIX K

Dunan Hydro Electric Scheme

Outline Construction Method Statement

19

th March 2013

Revision A

Eugene Craig

[email protected]

07886 401194

DHG Hydro Ltd 7 St James’s Gardens

London

W11 4RB

0131 208 0525

Page 2: Outline Construction Method Statement

2

INDEX

1 INTRODUCTION ............................................................................................................................. 3

2 SITE ESTABLISHMENT/COMPOUNDS/DELIVERY AREA .......................................................... 3

3 ACCESS TRACK IMPROVEMENT/CONSTRUCTION ................................................................... 4

4 INTAKE CONSTRUCTION .............................................................................................................. 6

5 PIPELINE .......................................................................................................................................... 9

6 POWERHOUSE CONSTRUCTION ...............................................................................................11

7 OUTFALL PIPELINE AND OUTFALL CONSTRUCTION ..............................................................12

8 GRID CONNECTION .....................................................................................................................13

9 ELECTROMECHANICAL INSTALLATION AND COMMISSIONING..........................................14

10 DEMOBILISATION......................................................................................................................14

Page 3: Outline Construction Method Statement

3

1 Introduction This document describes the Outline Construction Methods. Further detailed, specific

Construction Method Statements that must be submitted by the Contractor (once

appointed)

Some specific hazards and other considerations have been identified by the Designers.

These are listed under each activity and must be included in the Risk Assessment and

Method Statement for that section of the work. For the avoidance of doubt, this must not

be assumed to be a complete list and the team undertaking the Risk Assessment should

address any other hazards which could impact the work.

2 Site Establishment/Compounds/Delivery Area

2.1 Summary of Works/Methods

The site establishment consists of the following:

• Main contractor site office – this is to be located adjacent to the powerhouse

location at approximately 243530,760611

o two portacabin offices,

o Lock-up storage container,

o Site welfare facilities.

• Client offices – to be located adjacent to the main contractor site office

• Pipeline delivery area – Pipes will be delivered to an area (grid reference

247650,757382) adjacent to the B846 approximately 3.25km east of the start of the

access track by Allt Eigheach.

• Pipeline storage area: From this temporary delivery/storage area, pipes will

transported onto site in small loads by tractor and trailer. They will be stored on site

at an area just north of the forestry, approximate grid reference 243480,760940

until required for use.

• Access tracks – the existing access track to the powerhouse location requires some

minor repairs prior to start of works. This includes repair of sections that have been

washed out and ensuring that the two proposed passing areas are safe and

accessible. From the powerhouse location to the intake the existing tracks are in

varying states of repair and some a short new section of track will also be required.

• Watercourse crossings – culverts will be installed in the two tributaries of Allt

Eigheach, to the south of the powerhouse location. A new vehicular bridge crossing

over the Allt Eigheach will be constructed, with a short stretch of new track to link it

to the existing track.

• Intake working area – a small working area near to the Intake will be prepared for:

o Intake area site office portacabin

o Intake area lock-up storage container

o Intake area welfare facilities

• Site notices

2.2 The Method Statements which have been identified for this work are:

• Site Establishment Method Statement

Page 4: Outline Construction Method Statement

4

Method Statements will be in place prior to any work commencing on this activity and shall

address, as a minimum:

• the preparation for and delivery of plant and materials to the area in question

• the establishment of the initial means of access to the site

• the type and establishment of a temporary welfare facility (until such time that the

full welfare facilities may be established)

• Public Access and Safety

Hazards which will be considered, as a minimum, in the associated Risk Assessment will

include

• the safety of road users when offloading plant

• the safety of pedestrians

• the prevention of debris being deposited on public highway

• the prevention of contact with the overhead power cables

3 Access Track Improvement/Construction

3.1 Summary of Works/Methods

The site is well served with existing access tracks but some improvements are required:

• There is a very good quality, long established track from the public road to the

powerhouse area that requires only minor improvement and the formalisation of

two passing places.

• Vehicular access across the Allt Eigheach is currently via a ford crossing. This is not

appropriate for the level of construction traffic on both practical and environmental

management grounds. A new vehicular bridge crossing is proposed.

• The track from the powerhouse to the intake follows the pipeline route. The

condition of this track is very variable and requires improvement.

The following sections give brief outline procedures for the access works.

1.1.1 Existing access track from public road to powerhouse area

This track is long established, is made of a firm gravel running surface and is in generally very

good condition. The following works are proposed:

• Minor improvement and resurfacing

• Construction of culvert crossing of Allt Gormag – this will require SEPA approval.

This is currently a ford crossing that is in a very poor state of repair.

o This work shall only take place when there is a clear week of dry weather

forecast so the levels of the water in the Allt Gormag are low

o A temporary diversion channel shall be created to dry out the existing ford

crossing area

o The existing concrete ford construction shall be removed and the materials

disposed of appropriately

o A culvert crossing shall be created using a number of 300mm dia pipes. The

crossing shall be a minimum of 4.0m wide. It shall be created a low as

possible and is designed to over-top at high river levels. The pipes shall be

concreted in place and a road surface created over.

Page 5: Outline Construction Method Statement

5

o The edges of the culvert crossing will be landscaped with large natural

boulders to minimise the visual impact of the concrete.

• Formalisation of two passing places. This is very minor work and simply involves

tidying up of two former borrow pit location.

1.1.2 Vehicular crossing of the Allt Eigheach

Vehicular access across the Allt Eigheach is currently via a ford crossing. This is not

appropriate for the level of construction traffic on both practical and environmental

management grounds. A new vehicular bridge crossing is proposed.

A steel beam construction with a wooden deck and handrail is proposed which requires

simple concrete supports at either side of the burn. The outline construction sequence is

given below:

• Temporary coffer dams are to be built around both the bridge support points

• The area inside the coffer dam is to be cleared down to a firm surface

• Any water accumulating within the coffer dammed area is to be pumped away to a

suitable settling pond to prevent contamination of the watercourse

• The bridge supports are to be shuttered and poured with concrete as required.

• The coffer dams are to be carefully removed

• The area around the bridge supports are to be landscaped with large boulder to

minimise the exposed concrete and lessen any visual impact

• The bridge and deck, which are anticipated to be delivered in a pre-assembled unit,

is installed on to the supports

• Tracks either side of the bridge are to be constructed and the area made good.

1.1.3 Access track from powerhouse to intake

Where the existing access track is on bedrock or subsoil the work required will be limited.

Here the surface will be levelled and widened as necessary to approximately 3.5m. An open

drainage channel will be installed on the upper side to prevent erosion of the track by water.

Culverts under the track will be installed as required. The track will be topped with a layer of

gravel – from either the on-site borrow pit or imported. Post-construction of the intake and

installation of the pipeline the route of the track/pipe will be finished landscaped and the

track width shall be reduced to a minimum. The track verges will be left to re-seed and

regenerate naturally.

Where the existing track is over peat or in the area where new track is required, more

extensive works will be undertaken.

• The vegetation and top layer of soil / peat will be removed as whole turves and

stacked, vegetation layer uppermost. The area removed will be sufficient to lay the

track, drainage channel and where relevant the pipe. Removal of the vegetation with

sufficient soil will protect root layers and prevent drying out. Stacks will be limited in

height to prevent compaction of soil.

• The lower layer of peat along the track length will be removed in blocks and stored

• The subsoil or bedrock will be levelled and surfaced with a gravel topping.

• An open drainage channel will be excavated along the upper side of the track to

prevent erosion from water run-off.

• Where the pipeline runs adjacent to the access track, it will be installed alongside

(further details below)

Page 6: Outline Construction Method Statement

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• Turves will also be used to cover over any exposed peat faces next to the drainage

ditch.

• A small amount of surplus peat will be generated by this construction process. This

will be re-used on site for dressing off and reinstating peat on the edges of

constructed infrastructure and for verge reinstatement on the tracks. Any additional

peat will be distributed sensitively around the site.

1.1.4 Borrow Pit

A borrow pit area has been identified at grid ref 243491, 760935. This is shown in Appendix

C.3. This area is already informally used for material to maintain the tracks. It is estimated

that 500m3 of useable material might be available from this location for use the repair of the

tracks. This has significant advantages over bringing in imported material.

The area will be borrowed to the extent that the ground is flat only, i.e. no large hole will be

made. Following removal of the material the area will be landscaped and covered in a layer

of local top soil/organic material to allow the natural regeneration of the area.

3.2 The Method Statements which have been identified for this work are:

• Access Method Statement – Access Track improvement/construction

• Borrow pit use/reinstatement

• Bridge Construction Method Statement – River Works/Bridge Construction

Method Statements will be in place prior to any work commencing on this activity and shall

address, as a minimum:

• the construction of the access tracks and maintaining public access

• the flow diversion works associated with bridge support works

• the installation of formwork for the reinforced concrete walls for bridge supports

Hazards which will be considered, as a minimum, in the associated Risk Assessment will

include

• Safe public access to track during improvement/construction

• the prevention of falls into unattended excavations

• the prevention of falls into watercourses

• the prevention of pollution of the adjacent watercourses (slippage, sediment,

cement, refuelling). See Environmental Management Plan

• the prevention of watercourses inundating the works

• COSSH assessments for fuels and cement

4 Intake Construction

4.1 Summary of Works/Methods

The construction of the intake presents the greatest risk of river contamination. However,

correct procedures and construction methods greatly reduce this risk. The SEPA guide

Page 7: Outline Construction Method Statement

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“Engineering in the water environment good practice guide: Temporary construction

methods (WAT-SG-29)” has been applied to the methods.

The following sections give brief outline procedures for the intake constructions.

1.1.5 Intake

The intake has been sited on an area where the stream bed consists mostly of exposed bed

rock, and the area around it is relatively level. All access to the intake shall be from the

southern direction only.

The river at this point is approximately 5-8m wide. The nature of the topography does not

lend itself easily to creating a full working bypass for the construction of the intake structure.

It is therefore proposed to use a “half and half” construction method where half the width of

the river is coffer-dammed off and the flow diverted to one side of the burn, thus allowing

construction of half of the intake. . The eastern side of the river will be coffered off first and

this half of the weir, including the compensation notch, will be constructed. Following this

the river is diverted to the opposite side of the river, through/over the first part of the

construction, thus allowing the remaining half of the intake to be built in the dry.

This ensures both a secure construction and also greatly reduces the risk of sediment

contamination. The construction program will be as follows:

• Prior to construction, a coffer dam will be installed to divert the burn towards the

west bank

- Sandbags will be used to form the coffer dam (a combination of individual

sand band and larger dumpy bags as appropriate)

- The coffer dam will be placed so as to maintain as much width in the river as

possible to reduce the restriction to the water flow

- If required, pumps will be used to keep the working area dry and this will be

discharged to a separate settlement pit dug several meters away from the

burn

• Intake construction (phase 1) – Compensation notch/by-pass section

- Excavation of bed rock if required – some bed rock may need to be removed

to gain sufficient depth

- Rock dowels drilled and installed as required

- The floor slab will then be cast in concrete

- Once cured the remainder of the weir will be shuttered and cast with

reinforced concrete, including being tied into the east bank

- Bank and bed scour protection (gabions and/or boulders in concrete) will be

completed

• Once fully cured, the coffer dam will be moved to divert water through the

constructed compensation notch on the east side of the burn

- The river flow will then pass through the dewatering/compensation notch in

the weir wall (V-notch plate not installed)

• Intake construction (phase 2) – sump and main chamber

- Excavation of the sump and main weir area down to bed rock – some bed

rock may need to be removed to gain sufficient depth

- Rock dowels drilled and installed as required

- The floor slabs will then be cast in concrete

Page 8: Outline Construction Method Statement

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- Once cured, the main weir wall and chamber will be shuttered and cast with

reinforced concrete

- Wing walls will be completed, tied into the banks

- Bank and bed scour protection (gabions and/or boulders in concrete) will be

completed

• Installation of fittings

- Intake Coanda screens fitted

- Compensation plate not fitted at this point (this will allow bypass for phase

2)

- Sluice gate installed and locked off

- Access hatch, safety railing and stilling wells fitted

• Re-watering

- The working area will be cleared of any loose debris before re-watering

- The compensation plate can then be installed, thus completing the weir, and

the area upstream will fill up and then flow over the screens

• Reinstate & make good

Outline construction drawings for the Intake along with a drawing showing the proposed de-

watering method is given in Appendix E.

4.2 The Method Statements which have been identified for this work are:

• Intake Method Statement – River Diversion

• Intake Method Statement – Excavation/Construction/Fittings

• Intake Method Statement – River Diversion Removal

Method Statements will be in place prior to any work commencing on this activity and shall

address, as a minimum:

• the excavation work for the intake walls and wells

• the flow diversion works associated with each particular intake

• the installation of formwork for the reinforced concrete walls and wells

• the installation of the reinforcement steel

• the mixing, placing and compaction of the concrete

• the installation of the intake screens

Hazards which will be considered, as a minimum, in the associated Risk Assessment will

include

• the prevention of falls into unattended excavations

• the prevention of falls into watercourses

• the prevention of pollution of the adjacent watercourses (slippage, sediment,

cement, refuelling). See Environmental Management Plan

• the prevention of watercourses inundating the works

• COSSH assessments for fuels and cement

Page 9: Outline Construction Method Statement

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5 Pipeline

5.1 Summary of Works/Methods

The pipeline installation shall follow these key guidelines:

• The pipeline corridor will be kept as narrow as possible at all times

• The pipeline will be fully buried

• Where possible vegetation will be stripped off the line of the pipe carefully, with soil

and roots intact, so that it can be used in the re-instatement

• During the excavation of the pipeline, trench top-soil and sub-soil material shall be

kept separate and backfilled in the same order

• Where possible imported backfill material shall be avoided and a selective backfill

method used.

• The trench shall be opened and closed as quickly as possible

• Safe exit routes/ramps will be provided from the trench if a trench is left open

overnight for wildlife

• The pipeline will not alter the drainage characteristic of the land. Clay stopper bungs

will be used as required

The whole pipeline shall be constructed using butt fusion welded HPPE material. Due to the

very restricted nature of the site with most areas off the formal access tracks being wet

grassland or peat bog, the pipes will be delivered to an area (grid reference 247650,757382)

adjacent to the B846 approximately 3.25km east of the start of the access track by Allt

Eigheach which is an existing forestry marshalling area. From this temporary

delivery/storage area, pipes will transported onto site in small loads by tractor and trailer.

They will be stored on site at an area just north of the forestry, approximate grid reference

243480,760940 until required for use.

1.1.6 General installation of butt fusion welded HPPE pipe material

HPPE is an extremely versatile pipeline material. Amongst its benefits are flexibility and a

particularly strong jointing system. The flexibility of the materials allows it to bend and

follow the contours of the ground or avoid obstacles without the need for formal bends and

anchor blocks, thus simplifying the installation.

The pipe sections (usually 12m lengths) are joined using a technique called butt fusion

welding, where the ends of the pipes are heated to melting point and then pushed together

under pressure and allowed to cool. The resulting welded joint is then essentially as strong

as the pipeline material itself. This greatly reduces the risk of pipeline failure. The limit of

this pipeline is 16 Bar pressure, making it suitable for the total pipeline length.

The general procedure is that long sections of the pipe are pre-welded together above

ground. This is either done by moving the welding machine down the pipeline, welding as it

goes, or by leaving the welding machine in one place and pulling the welded pipe sections

through the welder. When the pipeline has been prepared a long section of trench is

excavated, generally 50m-150m. During this excavation the topsoil layer will be removed

and kept separate from any subsoil material. The pipe which is now a single long section is

then lifted and dropped into the trench where it can then be backfilled. Care will be taken to

ensure that only topsoil material will be restored at ground level.

Page 10: Outline Construction Method Statement

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HPPE is a relatively strong material which if the ground conditions are good can be buried

directly without the need for imported backfill. If the ground conditions are poor, imported

backfill material might be required to prevent damage to the pipeline. The general ground

conditions are good at the site and the need for imported backfill is expected to be very

limited.

It is not expected that any pipeline trench section will remain exposed for more than 4-5

days. If a trench section is left open overnight a ramp access will be left to allow any animals

that may enter the trench to escape.

Care will be taken to ensure the pipeline does not affect the drainage nature of the land.

Where the pipe crosses under side streams or through any particularly wet areas, clay bungs

will be installed as appropriate to prevent running water scouring along the pipe.

5.2 The Method Statements which have been identified for this work are:

• Pipeline Method Statement – Polyethylene Pipe Laying and Backfilling

• Pipeline Method Statement – Polyethylene Pipe Welding

• Pipeline Method Statement – Pressure Testing

Method Statements will be in place prior to any work commencing on this activity and shall

address, as a minimum:

• the excavation of pipe trenches and pits including temporary storage of materials

• the measures to ensure trench stability

• the procedures for inspection of trenches

• the jointing of pipes

• the laying of pipes

• the bedding and backfilling of pipes

• the reinstatement of trenches

• the testing of pipework

Hazards which will be considered, as a minimum, in the associated Risk Assessment will

include

• the prevention of falls into unattended excavations

• the prevention of spoil or materials accessing the trench

• the prevention of pollution of the adjacent watercourses (sediment, refuelling) see

Environmental Management Plan

• the prevention of excessive crossings of watercourses by vehicles

• the prevention of trench collapses due to vehicles working adjacent

• the prevention of flooding of the trenches from ground or surface waters

• the avoidance the disturbance to the designated areas of archaeological interest

(see Environmental Management Plan)

• COSSH assessments for fuels and cement

Page 11: Outline Construction Method Statement

11

6 Powerhouse Construction

6.1 Summary of Works/Methods

The powerhouse is sited away from the river and does not involve any de-watering works.

The powerhouse area is adjacent to the existing access track.

The following is the outline for method for the construction of the powerhouse:

• Site preparation

- Removal and storage of vegetation and topsoil

- Excavation of foundations for pipe anchor block, turbine sump and

transformer compound, and for powerhouse wall footings

- If required, pumps will be used to keep the working area dry and this will be

discharged to a settlement pit dug several meters away from the burn.

• Construction

- Cast foundations and main floor slab

- Build superstructure and roof

- Formalise hard-standing area

• Reinstatement and landscaping

Drawings of the proposed powerhouse building are given in Appendix F

6.2 The Method Statements which have been identified for this work are:

• Powerhouse Method Statement - Slab Excavation

• Powerhouse Method Statement - Slab Construction

• Powerhouse Method Statement - Blockwork

• Powerhouse Method Statement - Roof

• Powerhouse Method Statement - Gantry

Method Statements will be in place prior to any work commencing on this activity and shall

address, as a minimum:

• the excavation of the base and foundations for the building

• the installation of formwork for the reinforced concrete slab and sump

• the installation of the reinforcement steel

• the mixing, placing and compaction of the concrete

• the construction of the blockwork building walls

• the construction of the roof

• the installation of the overhead gantry

• the installation of fittings (doors, vents, guttering etc)

Hazards which will be considered, as a minimum, in the associated Risk Assessment will

include

• the prevention of falls into unattended excavations

• the prevention of falls from height

• the prevention of pollution of the adjacent watercourses (sediment, cement,

refuelling) see Environmental Management Plan

• COSSH assessments for fuels and cement

Page 12: Outline Construction Method Statement

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• the plant (incl. valid certification) to be used for lifting

7 Outfall pipeline and Outfall Construction

7.1 Summary of Works/Methods

The outfall pipeline runs from the powerhouse building to the outfall.

The outfall itself will be constructed behind a coffer dam. The following is the outline

method for the construction:

• Prior to construction, a coffer dam will be installed to divert the burn towards the

east bank and away from the main construction area.

- Sandbags will be used to form the coffer dam (a combination of individual

sand bags and larger dumpy bags as appropriate)

- The coffer dam will be placed so as to maintain as much width in the river as

possible to reduce the restriction to the water flow

- If required, pumps will be used to keep the working area dry and this will be

discharged to a separate settlement pit dug several meters away from the

burn

- Hay bales/silt net will be placed immediately downstream of the working

area to capture any disturbed sediment

• Outfall construction

- Excavation of the outfall area down to bed rock or solid ground – some bed

rock may need to be removed to gain sufficient depth. Existing bed material

to be placed to one side for reinstatement

- The floor slab will then be cast in concrete

- Once cured the main outfall structure will be shuttered and cast with

reinforced concrete, around the pipes to the turbine sump

- Wing walls will be completed, tied into the banks

- River bed reinstated to original level

- Bank and bed scour protection (gabions and/or boulders in concrete) will be

completed

• Installation of fittings

- Outfall screens fitted

- Safety railing fitted

• Re-watering

- The coffer dam can then gradually be removed and water allowed to flow

slowly past the outfall structure

- Hay bales/Silt nets will remain in place until the water runs clean, and then

the coffer dam can be fully removed

• Reinstate and make good.

Outline construction drawings for the outfall along with a drawing showing the proposed de-

watering method are given in Appendix G

7.2 The Method Statements which have been identified for this work are:

• Outfall Method Statement – River Diversion

Page 13: Outline Construction Method Statement

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• Outfall Method Statement – Excavation/construction/fittings

• Outfall Method Statement – River Diversion Removal

• Outfall Method Statement – Pipe Laying and Backfilling

Method Statements will be in place prior to any work commencing on this activity and shall

address, as a minimum:

• the flow diversion works to accommodate the pipe and discharge chamber

• the excavation for the pipe and discharge chamber

• the jointing of pipes

• the laying of pipes

• the bedding and backfilling of pipes

• the reinstatement of trenches

• the installation of formwork for the discharge chamber

• the installation of the reinforcement steel for the discharge chamber

• the mixing, placing and compaction of the concrete for the discharge chamber

Hazards which will be considered, as a minimum, in the associated Risk Assessment will

include

• the prevention of falls into unattended excavations

• the prevention of falls into watercourses

• the prevention of pollution of the adjacent watercourses (sediment, cement,

refuelling) see Environmental Management Plan

• the prevention of watercourses inundating the works

• COSSH assessments for fuels and cement

• the prevention of spoil or materials accessing the trench

• the prevention of excessive crossings of watercourses by vehicles

• the prevention of trench collapses due to vehicles working adjacent

• the prevention of flooding of the trenches from ground or surface waters

• the avoidance the disturbance to the designated areas of archaeological interest.

See Environmental Management Plan

8 Grid Connection

8.1 Summary of Works/Methods

The grid connection is to be provided in full by SSE.

8.2 The Method Statements which have been identified for this work are:

• Grid Connection Method Statement – Overhead Line Works (SSE)

• Grid Connection Method Statement – Buried Line and Substation Works (SSE)

Method Statements will be in place prior to any work commencing on each activity.

Hazards which will be considered, as a minimum, in the associated Risk Assessment will

include

Page 14: Outline Construction Method Statement

14

• the organisation and management of multiple sub-contractors on-site

• the systems to ensure sub-contracted staff are competent and qualified

• the exclusion of unauthorised or inexperienced persons in the work areas

• the prevention of falls from height

• the prevention of electrocution

• the prevention of burns

• the prevention of fire

• lifting heavy machinery

• manual handling

9 Electromechanical Installation and commissioning

9.1 Summary of Works/Methods

This work involves the installation of the turbine, generator, control system and grid

connection/protection equipment. This is specialist work and will be carried out by specialist

contractors. All works are contained within the powerhouse building and do not present any

environmental or public hazards.

9.2 The Method Statements which have been identified for this work are:

• Electromechanical Installation Method Statement (specialist contractor)

• Control System Installation Method Statement (specialist contractor)

• System Commissioning Method Statement (specialist contractor)

Method Statements will be in place prior to any work commencing on each activity.

Hazards which will be considered, as a minimum, in the associated Risk Assessment will

include

• the organisation and management of multiple sub-contractors on-site

• the systems to ensure sub-contracted staff are competent and qualified

• the exclusion of unauthorised or inexperience persons in the work areas

• the prevention of falls from height

• lifting heavy machinery

• manual handling

• the prevention of electrocution

• the prevention of burns

• the prevention of fire

10 Demobilisation

The site will be demobilised only after there has been consultation with the landowner and

stakeholders regarding the final reinstatement and timescales/standards agreed for any

matters requiring further attention.

Areas which will be discussed shall be

Page 15: Outline Construction Method Statement

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• final condition of existing tracks

• reinstatement of hard-standings formerly used for storage, welfare, etc.

• reinstatement of fences and gates

• reinstatement of pipe route

• reinstatement of surface water control measures

• general site tidiness (litter, etc.)

• on-going safe visitor access