draft gravel guidelines

Rural Road Surfacing EXAMPLE SURFACE OPTIONS (Provisional)

DESCRIPTION One or more layers of compacted natural gravel / laterite placed directly on the existing earth. Before placing, the existing earth should be shaped and compacted with a camber (crossfall) of about 3 - 7% from the road centre line. The overall gravel thickness is typically 15 - 30cm. Finished camber 3 - 7%. Individual layer thickness up to 15cm (compacted) maximum.

GRAVEL / LATERITE (SURFACE OPTION No. 3)

ADVANTAGES DISADVANTAGES

Proven performance in tropical and sub-tropical, gravel-rich environments. Usually lower initial cost than most other surfacing options. Can be used as an intermediate surface in a planned and resourced ‘stage construction’ strategy.

Occurs in limited natural deposits of variable quality Gravel WEARS. It is essential to have a sustained maintenance programme and regular re-gravelling to replace gravel loss. High maintenance costs; particularly for re-gravelling. Dust pollution in dry weather. Traffic, climatic and longitudinal gradient (<6%) constraints on use relating to rate of gravel loss.

Note: Laterite is a particular type of natural material used for gravel road surfaces. DESIGN CONSIDERATIONS GRAVEL LOSS Material is lost from the gravel road surface due to the action of rain, traffic wear, and dry season dust (fine material) loss. Typically loss rates are 1 – 5cm of thickness per year. The rate of loss partly depends on the rainfall and traffic characteristics. Alignment gradient, surface crossfall, road width, material quality, compaction and maintenance practices can be expected to influence rates of gravel loss significantly. Gravel loss is very specific to material and location, and there is some variation between the results of trials and relationships derived in different regions and conditions. Outline predictions are provided from TRL and HDM, and summarised on page 8. There is no substitute for local experience and research, which should enable more accurate predictions to be developed. Gravel loss and dust may influence environmental considerations. . STRUCTURAL THICKNESS Thickness design is not normally used for gravel surfacing. However it is necessary to provide sufficient thickness to spread the load of the traffic so that it does not over-stress the underlying soil. TRL suggest the following structural thickness for low traffic roads for a residual foundation rut depth of 40 mm:-

24.0190

)CBR(hNlog63.0

40 −= where: N = no. of standard 80 kN axles

h = thickness of granular material (mm) CBR = subgrade CBR (%)

RE

Source: Intech Associates

F: RR SURFACE 3f – May 2002 1 ©

Draft for comment

PIARC, TRL & Intech Associates

Rural Road Surfacing EXAMPLE SURFACE OPTIONS (Provisional) SUGGESTED DESIGNS In the absence of local research or recommendations based on experience, the following are suggested:-

Recommended Initial Compacted Thickness of Gravel/Laterite Surface BASIC TRAFFIC – Traffic: Mainly non-motorised & less than 25 motor vehicles per day with few heavy vehicles

Low Rainfall – Less than 1,500mm/year and little dry season dust

High Rainfall – More than 1,500mm/year, or long dry season and considerable dry season dust

Strong Foundation – In situ soil CBR > 15 (Little traffic damage if earth road is properly shaped to drain rainwater away)

12 cm (The existing soil may be strong enough to support light traffic if the earth surface is regularly re-shaped to drain rainwater away – therefore gravel may not be necessary)

15 cm

Intermediate Foundation - In situ soil CBR = 5 to 15 17 cm 20 cm Weak Foundation - CBR < 5 (Serious wet season traffic damage, even if earth road is properly shaped to drain rainwater away)

23 cm (Also consider other types of surface)


The above overall thicknesses allow for 2cm (low rainfall) & 5cm (high rainfall) wear before re-gravelling will be carried out.

Recommended Initial Compacted Thickness of Gravel/Laterite Surface MEDIUM TRAFFIC – Traffic up to 100 motor vehicles per day including up to 20 medium (10t) goods vehicles

Low Rainfall – Less than 1,500mm/year and little dry season dust

High Rainfall – More than 1,500mm/year, or long dry season and considerable dry season dust

Strong Foundation – In situ soil CBR > 15 20 cm 25 cm

(Also consider other types of surface) Intermediate Foundation – In situ soil CBR = 5 to 15



Weak Foundation - CBR < 5 More than 30 cm (Also consider other types of surface)

More than 30 cm (Also consider other types of surface)

The above overall thicknesses allow for 5cm (low rainfall) & 10cm (high rainfall) wear before re-gravelling will be carried out. GENERAL GUIDELINES It is assumed that good quality gravel is used. Poor quality gravel will lead to higher gravel loss and reduced

surface life. A gravel surface should not be used on road sections liable to flooding. In areas of medium – high volume or intensity rainfall, gravel should not be placed on longitudinal gradients steeper

than 6%, due to erosion caused by rainwater flowing along the road surface. Gravel should be placed and compacted on a pre-shaped (cambered) and compacted earth road formation. Water

should be added if necessary to obtain good compaction (as close as possible to optimum moisture content). If earthworks compaction is not possible, the earthworks should be allowed to consolidate for at least one rain season before (minor reshaping and) covering with the surfacing gravel. The crossfall on the finished compacted gravel surface should be about 3% in dry regions. In wet regions the

finished crossfall should be 5 – 7%. Steeper crossfall will probably cause erosion by rainwater. Maximum gravel layer thickness should be 10cm after compaction with deadweight equipment, 12cm with light

vibro-equipment (e.g. pedestrian vibrating roller), and 15cm if heavy vibro-compaction equipment is used. Where the total required gravel thickness exceeds these limits, multiple layers should be constructed. For surfacing in excess of 15cm thick, the excess thickness may be substituted by an imported ‘capping layer’ of soil of compacted strength greater than CBR=15, laid on the formation before placing the surfacing gravel layer on top. The strength of the insitu soil may be checked with a Dynamic Cone Penetrometer (DCP). Subgrade soil strength measurements should be taken in the wet season for ‘worst case’ assessment. If gravel has to be hauled more than 10km, or for traffic levels higher than those shown in the tables above, detailed

initial and whole life cost (as well as socio-economic and environmental) comparisons should be made with other surface options. If timely maintenance regravelling is not carried out, the gravel surface will inevitably deteriorate to ‘earth’ standard.

REF: RR SURFACE 3f – May 2002 2 © PIARC, TRL & Intech Associates

Rural Road Surfacing EXAMPLE SURFACE OPTIONS (Provisional) EXAMPLE DESIGNS (not to scale)

BASIC TRAFFIC

Surface width 3.0 metres

Formation width 5.5 metres

Crossfall 3 – 7%

Side drain or embankment Gravel 15 cm thick At least 30 cm deep after compaction MEDIUM TRAFFIC Surface width 5.5 metres

Formation width 6.5 metres

Crossfall 3 – 7% Side drain or embankment Gravel 20 cm thick At least 30 cm deep after compaction Note that gravel material must be laid to drain across the earth formation and not be ‘boxed in’ at the shoulders. This will ensure better drainage and allow maintenance grading/reshaping without contaminating the gravel with shoulder soil.

TECHNOLOGY OPTIONS Each activity to construct a gravel surface may be carried out using various technology options; from labour, through intermediate equipment to heavy equipment, depending on local circumstances.

Activity Technology Options Quarry preparation Labour+handtools , tractor-excavator, tracked excavator, bulldozer

Gravel excavation Labour+handtools , tractor-excavator, tracked excavator, bulldozer

Gravel loading Labour+handtools , tractor-excavator, tracked excavator, wheeled/tracked loader

Gravel haulage Wheel barrow, hand cart, animal cart, light truck, tractor+trailer, medium/heavy truck

Unloading gravel Labour+handtools, tipping equipment

Spreading gravel Labour+handtools, tractor+heavy towed grader, motorgrader, wheeled/tracked loader

Watering Towed bowser, truck bowser

Compaction Pedestrian vibrating roller, medium/heavy smooth steel vibrating roller


Rural Road Surfacing EXAMPLE SURFACE OPTIONS (Provisional) SPECIFICATIONS TRL Suggested grading of gravel wearing course & Preferred plasticity characteristics for gravel surfacings

Percentage passing by mass Maximum size of particle (mm)

Sieve Size (mm)

37.5 19.0 9.5

Climate Liquid Limit not to

exceed (%)*

Plasticity Index

range (%)*

Linear Shrinkage

(%)

37.5 19.0 9.5 4.75 2.36 0.425 0.075

100 80 – 100 55 – 80 40 – 60 30 – 50 15 – 30 5 - 15

100 100

80 - 100 60 – 85 45 – 70 25 – 45 10 - 25

100 100 100

80 – 100 50 – 80 25 – 45 10 - 25

Moist tropical & wet tropical

Seasonal wet

tropical

Arid/semi-arid

35

45

55

4 – 9

6 – 20

15 - 30

2 – 5

3 – 10

8 - 15 * Higher limits may be acceptable for some laterites and concretionary gravels that have a structure that is not easily broken down by traffic. Lower limits may be appropriate for some other gravels that are easily broken down by traffic. Any variation from these limits should be based on carefully collated local experience.

The principal requirements for quality of gravel relate to grading of particle size and plasticity of the fine material binding the gravel together. The larger material must also be strong enough not to break down under the effects of traffic and weather. Material may be used ‘as-dug’ or may be blended from different sources to achieve the desirable specification. In dry climates a fairly high proportion of clay particles is

desirable to bind the surface. In wet climates a lower clay proportion is desirable to avoid slippery surfaces and excessive rutting. The above tables show some of the established specifications in use.

CSRA suggested grading of gravel wearing course (South Africa) Percentage passing by mass

Maximum size of particle (mm) Sieve Size

(mm)

37.5 26.5 19.0 13.2 37.5 26.5 19.0 13.2 4.75 2.00* 0.425 0.075*

100 85 – 100 70 – 100 60 – 85 40 – 60 25 – 45 15 – 40 7 - 30

100 100

80 – 100 60 – 85 45 – 65 30 – 50 15 – 40 7 - 30

100 100 100

75 – 100 50 – 75 35 – 55 18 – 45 7 - 30

100 100 100 100

60 – 100 45 – 70 25 – 50 7 - 30

* These sieves are the most important

CONSTRUCTION The principal concerns during construction of the gravel surface should be:-

• The formation or foundation for the gravel surfacing should be properly shaped and compacted beforehand. The road drainage system must be adequate and functioning properly.

• Gravel quality should be carefully controlled at the quarry by experienced supervisors, and an appropriate level of testing, if feasible1.

• Layer thickness control is essential, simple pegs or profile boards may be used for this purpose. Regular checks should be made by excavating through the compacted gravel surface. Initial checks should be made on number of loads delivered loose per unit length of road.

• Large (oversize) particles should be removed by hand or broken down with sledgehammers. • The laid gravel material should be at a moisture content suitable for compaction; water should be added if

necessary. • Compaction by vibrating roller will considerably improve durability of the gravel surface. It is important to ensure

that the loose gravel is spread evenly prior to compaction to ensure a uniformly dense and even surface. • Finished compacted crossfall (3 – 7%) should be checked, for example with a camber board or template, or

using strings stretched longitudinally, transversely and diagonally between the setting out pegs.


1 For some small scale or remote works it is not possible or cost-effective to arrange for testing of materials at site. In these circumstances an experienced supervisor is particularly desirable.

Rural Road Surfacing EXAMPLE SURFACE OPTIONS (Provisional) PRODUCTIVITY USING LABOUR AND LIGHT EQUIPMENT Excavation of gravel by labour: 1.6 – 2.4 m³ / worker-day (insitu), 2.0 – 3.0 m³ / worker-day (loose) Hand loading of gravel by labour: 8 – 10 m³ / worker-day (loose) Unloading and spreading gravel by labour: 12 – 16 m³ / worker-day (loose) An efficient fleet of 4 x 60 hp (45kW) tractors and 3 m³ fixed body trailers can haul about 100 – 110 m³ per day on a 5 km haul with labour carrying out all non-hauling activities, and compaction by 1 tonne twin drum rollers. Short hauls are also suitable for light truck and animal haulage. For further guidance, see references No. 5 and 11. USING HEAVY EQUIPMENT An efficient heavy plant operation involving dozer, loader, (8) trucks, motorgrader, bowser and compacter can complete approximately 450 - 500 m³ per day on a 5 km haul. The number of trucks may need to be increased for longer hauls. The use of heavy equipment is not recommended for basic access roads. Productivity depends particularly on the quality of planning and site management. CONSTRUCTION COSTS Costs will vary considerably depending on a wide range of factors such as scale of works, cross section, management quality, material haul distances, technology used, finance costs, fuel costs, equipment utilisation and support, labour costs, market conditions, payment arrangements, etc., and these should be carefully assessed for local circumstances. Guidance on costing is provided in LCS Working Paper No 3 2. Gravelling by heavy equipment usually requires considerable (and high risk) investment in expensive imported equipment. Labour and intermediate equipment approaches can be more attractive and cheaper for small local enterprises and communities, and small scale work. The principal factor influencing costs is the haulage distance as indicated in the example figure below. These costs include quarry development, loading and royalties for placed COMPACTED material. Compacted gravel volume unit costs will be approximately 30% higher than for material volume measured loose in the vehicles.

To the quarry & haulage costs have to be added those of preparation, setting out, placing, shaping, (watering) & compaction etc. These additional cost items can amount to approximately US$4.0 – 5.0/m³ of placed COMPACTED gravel. These example costs are from S. E. Asia in 2001 prices for excavation by heavy plant and haulage by trucks due to the long distances involved. Prices include overheads and profit for contractor works. They should only be used for indicative purposes. Wage rates were approximately US$1/worker day. Supervision costs would be additional and can be expected to be approximately 5 - 10% of the works costs.

Gravel quarrying and haul costs

0.0

2.0

4.0

6.0

8.0

10.0

12.0

0 10 20 30 40 50 60Haul distance (km)

US$

/cub

ic m

etre

(com

pact

ed)

REF: RR SURFACE 3f – May 2002 5 © PIARC, TRL & Intech Associates 2 LCS Working Paper No 3, Costing of Roadworks, R C Petts, 2002.

Rural Road Surfacing EXAMPLE SURFACE OPTIONS (Provisional) EXAMPLE BILL OF QUANTITIES for 1km of 3 metre wide, 15cm gravel surface for a basic access road

Item Description Unit Quantity Price (US$) per unit

Total Cost (US$)

A10 Setting up site operation, signs, safety, diversions etc.

Lump Sum - 400.00 400.00

B10 Clear worksite and repair drainage system Linear metre 1,000 0.25 250.00

B11 Setting out alignment and thickness controls Linear metre 1,000 0.07 70.00

B12 Preparation of formation or existing gravel surface

Square Metre 5,500 0.10 550.00

C10 Quarry clearing, preparation & haul road Lump Sum - 200.00 200.00

C11 Excavate, load, haul and deposit gravel at road site, maximum haul 1 km

Cubic Metre (compacted)

450 1.60 720.00

C12 Haulage extra over 1km Cubic Metre (compacted) - km

1,800 0.17 306.00

D10 Spread, water and compact gravel to 15cm final thickness

Square Metre 3,000 0.15 450.00

E10 Dismantling worksite and make good site and quarries

Lump Sum - 150.00 150.00

TOTAL (US$) 3,096.00 All rates to include royalties, fees, overheads and profit etc. Does not include earthworks, drainage and other preparatory works. MAINTENANCE The PIARC International Road Maintenance Handbook (Reference 8) provides guidance on the maintenance of gravel surfaces. Gravel surfaces wear and distort under the influence of traffic and weather. Routine or regular maintenance involves reshaping or grading of the surface to remove ruts and corrugations and draw material back into the desired camber shape and crossfall. This is important to prevent standing water which leads to pothole formation. Reshaping can be achieved by labour, by light or heavy towed graders, or by motorgraders. Possible use of large modern motorgraders should be considered carefully as their size and power can cause excessive damage to the surfacing and mixing of unsuitable material into the surface, especially on narrow roads with thin surfaces. Grading should not be carried out in conditions when the gravel is too dry to reconsolidate. Dragging can be effective in the dry season to remove corrugations and spread loose material. Any potholes should be filled with gravel material prior to grading/reshaping. The frequency of these operations on a section of road can be expected to be once or more per year. The loss of gravel from the road surface necessitates the periodic operation of regravelling. This is a high cost operation and can be expected to be required after a period of a number of years, depending on the factors discussed under DESIGN CONSIDERATIONS. MAINTENANCE COSTS Routine maintenance can be expected to cost from about US$250/network-km/year3. Periodic maintenance can be expected to cost US$400 – 2,000/network-km/year for the range of normal conditions and considerations. These costs are based on 2001 prices using labour and intermediate equipment methods, a labour wage rate on US$1/day and include normal overheads and profit for contractor works. Costs can vary considerably as discussed in the section on CONSTRUCTION COSTS. It should be noted that gravel / laterite surfaces will usually have higher maintenance needs and costs than other surface types. 3 A Tractor & Labour Based Routine Road Maintenance System, Gongera & Petts, IRF Paris, 2001.


Rural Road Surfacing EXAMPLE SURFACE OPTIONS (Provisional) Maintenance costs and arrangements should be a key consideration in selection of road surface type. Example Whole Life Cost Analysis Assumptions: US$/km Construction Cost 4,128 Surface: Gravel Rainfall regime HighMaintenance Year 1 200 Haul Distance: 5kmMaintenance Year 2 200 Subgrade CBR (%) 9Maintenance Year 3 2,064 Maintenance Year 4 200 Initial gravel thickness (cm) 20Maintenance Year 5 200 Gravel Loss (cm/year): 3Maintenance Year 6 2,064 Residual thickness allowed (cm) 14Maintenance Year 7 200 Regravelling thickness (cm) 10Maintenance Year 8 200 Paving width (m): 3.0Maintenance Year 9 2,064 Maintenance Year 10 200 Total Maintenance Cost (yrs 1-10) 7,592 Total Const. + Mtce. Cost (yrs 1-10) 11,720 NPV of Const + Mtce (yrs 1-10) 8,600 Residual Value end Year 10 4,128 NPV of Net Costs @ 10% 7,007 Construction cost to formation level, drainage and structures not included. Routine road surface maintenance by labour: US$200/km/year Does not include 'off-pavement' maintenance (drainage, vegetation, emergency)


Rural Road Surfacing EXAMPLE SURFACE OPTIONS (Provisional) USEFUL REFERENCES For detailed guidance on construction and maintenance of gravel road surfaces, refer to the following documents:- 1. R S Millard TRL, 1993, Road Building in the Tropics. 2. J Lebo & D Schelling, 2001, World Bank Technical Paper No 496, Design and Appraisal of Rural Transport Infrastructure,

Ensuring Basic Access for Rural Communities. 3. TRRL, 1988, Overseas Road Note 5, A Guide to Road Project Appraisal. 4. CSRA South Africa, 1989, TRH 14, Guidelines for Road Construction Materials, (reprint). 5. Ministry of Works Tanzania, 1998, Labour Based Roadworks Technical Manual. 6. Andersson, Beusch & Miles, 1996 Road Maintenance and Regravelling (Romar) using Labour-based Methods, Handbook

and Workbook. 7. Toole, Morosiuk, Petts & Done TRL, 2002, Management Guidelines for Unsealed Roads. 8. PIARC (World Road Association), 1994, International Road Maintenance Handbook. Available from PIARC in a number of

languages. 9. TRRL, 1987, Overseas Road Note 1, Maintenance Management for District Engineers (2nd edition) 10. TRRL, 1987, Overseas Road Note 2, Maintenance Techniques for District Engineers (2nd edition) 11. ASIST 1998, Technical Brief No 2, Productivity Norms for Labour-based Construction 12. R C Petts, 2002, LCS Working Paper No 3, Costing of Roadworks. 13. PIARC, 2001, HDM4 documentation. 14. TRRL, 1984, LR 1111 The Kenya Maintenance Study on Unpaved Roads: Research on Deterioration, Dr T E Jones. ANNEXES GRAVEL LOSS RELATIONSHIPS 1. TRL (source: LR 1111)

GLA = [(TA²/( TA² + 50)] (4.2 + 0.092TA + 3.5RF² + 0.188GR)f where

f = material factor, which for laterite = 1.3 quartzites = 1.5 volcanics = 0.96 coral = 1.5 sandstone = 1.4 calcrete = 2.0 – 4.5 GLA = Annual gravel loss (mm) TA = Annual traffic flow (‘000s) RF = Annual rainfall (metres) GR = Gradient (metres/km)

2. PIARC-World Bank HDM4 (source: HDM4 documentation)

MLA = Kgl 3.65 [3.46 + 0.246(MMP/1000)(RF) + (KT)(AADT)] where

KT = Kkt max [0, 0.022 + 0.969(HC/57300) + 0.00342(MMP/1000)(P075) -0.0092(MMP/1000)(PI) – 0.101(MMP/1000)]

and MLA = annual material loose, in mm/year KT = traffic-induced material whip-off coefficient AADT = annual average daily traffic, in veh/day MMP = mean monthly precipitation, in mm/month RF = average rise plus fall of the road, in m/km HC = average horizontal curvature of the road, in deg/km PI = plasticity index of the material, in percent Kgl = calibration factor for material loss Kkt = calibration factor for traffic-induced material whip-off coefficient P075 = amount of material passing the 0.075mm sieve, in per cent by mass


Rural Road Surfacing EXAMPLE SURFACE OPTIONS (Provisional) PHOTOGRAPHS (sources Upstream Project, Cambodia & Intech Associates)

1. Preparation of earth formation prior to 2. Excavation of gravel by labour and loading gravelling, using string & peg control of camber into trailers for tractor haulage

3. Unloading of gravel into pre-marked 4. Watering and compaction of gravel surface bays (gravel may also be hauled by truck)

5. Maintenance reshaping of gravel by labour 6. Light maintenance grading of gravel surface

7. Heavy towed grading of gravel surface 8. An unnecessarily wide gravel surface is a waste of construction and maintenance resources


draft gravel guidelines

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