Read RURAL ROAD SURFACING text version

Rural Road Surfacing



Draft for comment 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%.

Source: Intech Associates

Individual layer thickness up to 15cm (compacted) maximum.


Proven performance in tropical and subtropical, 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:log N 40 h (CBR) 0.63 = - 0.24 190

where: N = no. of standard 80 kN axles h = thickness of granular material (mm) CBR = subgrade CBR (%)

REF: RR SURFACE 3f ­ May 2002


© PIARC, TRL & Intech Associates

Rural Road Surfacing


SUGGESTED DESIGNS In the absence of local research or recommendations based on experience, the following are suggested:-

non-motorised & less than 25 motor vehicles per day with few heavy vehicles

BASIC TRAFFIC ­ Traffic: Mainly

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

Recommended Initial Compacted Thickness of Gravel/Laterite Surface Low Rainfall ­ Less than 1,500mm/year High Rainfall ­ More than and little dry season dust 1,500mm/year, or long dry season and considerable dry season dust

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 17 cm 20 cm CBR = 5 to 15 Weak Foundation - CBR < 5 (Serious wet season traffic damage, even if earth 23 cm 26 cm road is properly shaped to drain (Also consider other types of surface) (Also consider other types of surface) rainwater away) The above overall thicknesses allow for 2cm (low rainfall) & 5cm (high rainfall) wear before re-gravelling will be carried out.


to 100 motor vehicles per day including up to 20 medium (10t) goods vehicles Strong Foundation ­ In situ soil CBR > 15 Intermediate Foundation ­ In situ soil CBR = 5 to 15 Weak Foundation - CBR < 5

Recommended Initial Compacted Thickness of Gravel/Laterite Surface Low Rainfall ­ Less than 1,500mm/year High Rainfall ­ More than and little dry season dust 1,500mm/year, or long dry season and considerable dry season dust

20 cm 27 cm

(Also consider other types of surface)

(Also consider other types of surface)

25 cm 32 cm

(Also consider other types of surface)

(Also consider other types of surface) (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.

More than 30 cm

More than 30 cm


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 DESIGNS (not to scale)


Side drain or embankment At least 30 cm deep

Surface width 3.0 metres Formation width 5.5 metres Crossfall 3 ­ 7%

Gravel 15 cm thick after compaction


Side drain or embankment At least 30 cm deep

Surface width 5.5 metres Formation width 6.5 metres

Crossfall 3 ­ 7%

Gravel 20 cm thick 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.


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.


Quarry preparation Gravel excavation Gravel loading Gravel haulage Unloading gravel Spreading gravel Watering Compaction

Technology Options

Labour+handtools , tractor-excavator, tracked excavator, bulldozer Labour+handtools , tractor-excavator, tracked excavator, bulldozer Labour+handtools , tractor-excavator, tracked excavator, wheeled/tracked loader Wheel barrow, hand cart, animal cart, light truck, tractor+trailer, medium/heavy truck Labour+handtools, tipping equipment Labour+handtools, tractor+heavy towed grader, motorgrader, wheeled/tracked loader Towed bowser, truck bowser Pedestrian vibrating roller, medium/heavy smooth steel vibrating roller

REF: RR SURFACE 3f ­ May 2002


© PIARC, TRL & Intech Associates

Rural Road Surfacing



TRL Suggested grading of gravel wearing course

Sieve Size (mm) Percentage passing by mass Maximum size of particle (mm) 37.5 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 19.0 100 100 80 - 100 60 ­ 85 45 ­ 70 25 ­ 45 10 - 25 9.5 100 100 100 80 ­ 100 50 ­ 80 25 ­ 45 10 - 25 Moist tropical & wet tropical Seasonal wet tropical Arid/semi-arid &

Preferred plasticity characteristics for gravel surfacings

Climate Liquid Limit not to exceed (%)* 35 Plasticity Index range (%)* 4­9 Linear Shrinkage (%) 2­5


6 ­ 20

3 ­ 10


15 - 30

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 Percentage passing by mass Sieve Size grading of particle size and (mm) Maximum size of particle (mm) plasticity of the fine material 37.5 26.5 19.0 13.2 binding the gravel together. 100 100 100 100 37.5 The larger material must also 100 100 100 85 ­ 100 26.5 be strong enough not to break 100 100 80 ­ 100 70 ­ 100 19.0 down under the effects of 100 75 ­ 100 60 ­ 85 60 ­ 85 13.2 traffic and weather. Material 60 ­ 100 50 ­ 75 45 ­ 65 40 ­ 60 4.75 may be used `as-dug' or may 45 ­ 70 35 ­ 55 30 ­ 50 25 ­ 45 2.00* be blended from different 25 ­ 50 18 ­ 45 15 ­ 40 15 ­ 40 0.425 sources to achieve the 7 - 30 7 - 30 7 - 30 7 - 30 0.075* desirable specification. In dry * These sieves are the most important 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)


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.


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. REF: RR SURFACE 3f ­ May 2002 4 © PIARC, TRL & Intech Associates

Rural Road Surfacing



Excavation of gravel by labour: Hand loading of gravel by labour: Unloading and spreading gravel by labour: 1.6 ­ 2.4 m³ / worker-day (insitu), 2.0 ­ 3.0 m³ / worker-day (loose) 8 ­ 10 m³ / worker-day (loose) 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.


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.


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.

© PIARC, TRL & Intech Associates

Gravel quarrying and haul costs

US$/cubic metre (compacted)

12.0 10.0 8.0 6.0 4.0 2.0 0.0 0 10 20 30 40 50 60

Haul distance (km)


LCS Working Paper No 3, Costing of Roadworks, R C Petts, 2002. REF: RR SURFACE 3f ­ May 2002 5

Rural Road Surfacing


EXAMPLE BILL OF QUANTITIES for 1km of 3 metre wide, 15cm gravel surface for a basic access road

Item A10 B10 B11 B12 C10 C11 C12 D10 E10 Description Setting up site operation, signs, safety, diversions etc. Clear worksite and repair drainage system Setting out alignment and thickness controls Preparation of formation or existing gravel surface Quarry clearing, preparation & haul road Excavate, load, haul and deposit gravel at road site, maximum haul 1 km Haulage extra over 1km Spread, water and compact gravel to 15cm final thickness Dismantling worksite and make good site and quarries Unit Lump Sum Linear metre Linear metre Square Metre Lump Sum Cubic Metre (compacted) Cubic Metre (compacted) - km Square Metre Lump Sum Quantity 1,000 1,000 5,500 450 1,800 3,000 Price (US$) per unit 400.00 0.25 0.07 0.10 200.00 1.60 0.17 0.15 150.00 Total Cost (US$) 400.00 250.00 70.00 550.00 200.00 720.00 306.00 450.00 150.00


All rates to include royalties, fees, overheads and profit etc. Does not include earthworks, drainage and other preparatory works.



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.


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.


A Tractor & Labour Based Routine Road Maintenance System, Gongera & Petts, IRF Paris, 2001. 6 © PIARC, TRL & Intech Associates

REF: RR SURFACE 3f ­ May 2002

Rural Road Surfacing


Maintenance costs and arrangements should be a key consideration in selection of road surface type.

Example Whole Life Cost Analysis

Assumptions: Construction Cost Maintenance Year 1 Maintenance Year 2 Maintenance Year 3 Maintenance Year 4 Maintenance Year 5 Maintenance Year 6 Maintenance Year 7 Maintenance Year 8 Maintenance Year 9 Maintenance Year 10 Total Maintenance Cost (yrs 1-10) Total Const. + Mtce. Cost (yrs 1-10) NPV of Const + Mtce (yrs 1-10) Residual Value end Year 10 NPV of Net Costs @ 10% US$/km 4,128 200 200 2,064 200 200 2,064 200 200 2,064 200 7,592 11,720 8,600 4,128 7,007 Surface: Rainfall regime Haul Distance: Subgrade CBR (%) Initial gravel thickness (cm) Gravel Loss (cm/year): Residual thickness allowed (cm) Regravelling thickness (cm) Paving width (m): Gravel High 5km 9 20 3 14 10 3.0

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)

REF: RR SURFACE 3f ­ May 2002


© PIARC, TRL & Intech Associates

Rural Road Surfacing


USEFUL REFERENCES For detailed guidance on construction and maintenance of gravel road surfaces, refer to the following documents:1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. R S Millard TRL, 1993, Road Building in the Tropics. J Lebo & D Schelling, 2001, World Bank Technical Paper No 496, Design and Appraisal of Rural Transport Infrastructure, Ensuring Basic Access for Rural Communities. TRRL, 1988, Overseas Road Note 5, A Guide to Road Project Appraisal. CSRA South Africa, 1989, TRH 14, Guidelines for Road Construction Materials, (reprint). Ministry of Works Tanzania, 1998, Labour Based Roadworks Technical Manual. Andersson, Beusch & Miles, 1996 Road Maintenance and Regravelling (Romar) using Labour-based Methods, Handbook and Workbook. Toole, Morosiuk, Petts & Done TRL, 2002, Management Guidelines for Unsealed Roads. PIARC (World Road Association), 1994, International Road Maintenance Handbook. Available from PIARC in a number of languages. TRRL, 1987, Overseas Road Note 1, Maintenance Management for District Engineers (2nd edition) TRRL, 1987, Overseas Road Note 2, Maintenance Techniques for District Engineers (2nd edition) ASIST 1998, Technical Brief No 2, Productivity Norms for Labour-based Construction R C Petts, 2002, LCS Working Paper No 3, Costing of Roadworks. PIARC, 2001, HDM4 documentation. TRRL, 1984, LR 1111 The Kenya Maintenance Study on Unpaved Roads: Research on Deterioration, Dr T E Jones.


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 = Annual gravel loss (mm) = Annual traffic flow (`000s) = Annual rainfall (metres) = 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 KT AADT MMP RF HC PI Kgl Kkt P075 = = = = = = = = = = annual material loose, in mm/year traffic-induced material whip-off coefficient annual average daily traffic, in veh/day mean monthly precipitation, in mm/month average rise plus fall of the road, in m/km average horizontal curvature of the road, in deg/km plasticity index of the material, in percent calibration factor for material loss calibration factor for traffic-induced material whip-off coefficient amount of material passing the 0.075mm sieve, in per cent by mass

REF: RR SURFACE 3f ­ May 2002


© PIARC, TRL & Intech Associates

Rural Road Surfacing


PHOTOGRAPHS (sources Upstream Project, Cambodia & Intech Associates)

1. Preparation of earth formation prior to gravelling, using string & peg control of camber

2. Excavation of gravel by labour and loading into trailers for tractor haulage

3. Unloading of gravel into pre-marked bays (gravel may also be hauled by truck)

4. Watering and compaction of gravel surface

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

REF: RR SURFACE 3f ­ May 2002


© PIARC, TRL & Intech Associates



9 pages

Report File (DMCA)

Our content is added by our users. We aim to remove reported files within 1 working day. Please use this link to notify us:

Report this file as copyright or inappropriate


Notice: fwrite(): send of 199 bytes failed with errno=104 Connection reset by peer in /home/ on line 531