tIATRANSPORT RESEARCH LABORATORY

A field study on theunpaved roads and themaintenance strateg~ies

deterioration ofeffect of different

T E Jones, R Ro~binson and M S Snaith

OVers'eas CentreTransport Research LaboratoryCrowthorne Berkshire United Kingdom

TITLE

by

Jones,T E, R Robinson and M S Snaith (1984). A field study on the deterioration ofunpaved roads and the effect of different maintenance strategies. In: Proceedings of theEighth Re-gional Conference for Africa on Soil Mechanaics and Foundation Engineering,Harare. Zimbabwe. 1984.

Proceedings of the Eighth Regional Conference for Africa on Soil Mechanics and Foundation Engineering /Harare1/ 984

A field study on the deterioration of unpaved roads

and the effect of different maintenance strategies

T.EJONES & R.ROBINSONTransport & Rood Research Laboratory, Crowthorne, Berkshire, UK

M.S.SNA[THUniversity of Birmingham, UK

SYNOPSIS A recent study carried out by the Transport and Road Research Laboratoryhas investiqated the effect of varying maintenance inputs on the rate ofdeterioration of unpaved roads. The roads investigated had traffic levels of 50 to240 vehicles per day with wearing courses that consisted of lateritic, quartzitic,volcanic and sandstone gravels. The deteriorating surface conditions were monitoredin relation to the cumulative traffic weights and volumes, and the type of main-tenance input. The study also investigated alternative methods of maintaining theroads with motor graders, towed graders and mechanical drags. Results from theinvestigation showed increased rates of gravel loss compared with previous studies.Other factors, apart from traffic, were found to influence the performance of un-paved roads and these included the material design specifications used in construc-tion, the frequency and quality of the subseguent maintenance and the prevailingclimate.

INTRODUCTION

In recent years, unpaved roads indeveloping countries have been carry-ing vehicles travelling at higherspeeds and carrying heavier payloads.This situation has led to increasedrates of road deterioration. Thedifficulty that this has caused in manycountries has been made worse becauseof the dwindling sources of goodregravelling material.

There are three principle modes ofdeterioration for gravel roads:

Loss of fines. The loss of fines fromthe surface due to the degradation byvehicle tyres leads initially to areduction in cohesion of the surfacelayer and subsequently its disintegra-tion. This also increases the surfaceirregularity and the permeability of thegravel wearing course. Eventually itstarts the process of longitudinalrutting and, in certain soils,, precedesthe development of corrugations.

Loss of shape. This is the result ofthe combined action of traffic andclimate, and will depend on gravel typeand initial road camber. This leads tothe development of longitudinal ruttingand the loss of the water shedding

characteristics of the road. on steepalignments with poor drainage features,there will be additional loss of shapewith longitudinal gullies forming as adirect result of the water drainingalong the road.

Structural damage. This occurs whenthe maintenance or necessary strength-ening of the road has not been done intime, or when the road has been over-loaded or underdesigned. It can alsooccur as a result of damage to struc-tures such as bridges and culverts.

The loss of fines and loss of shape onunpaved roads are coimmon occurrencesand, without adequate maintenance, willreduce the effective life of the roadand increase the need for regravellingoperations. These two factors haveconsiderable influence on vehicleoperating costs which is the largestelement of the total costs of roadtransportation.

A recent study carried out by theTransport and Road Research Laboratory(TRRL) , T E Jones (1983) , investigatedthe effect of varying maintenanceinputs on the rate of deterioration ofunpaved roads. The roads investigatedhad traffic levels of 50 to 240 vehiclesper day with wearing courses that con-

293

sisted of lateritic, quartzitic,volcanic and sandstone gravels. Thedeteriorating surface conditions weremonitored in relation to the cumulativetraffic weights and volumes, and thetype of maintenance input. The studyalso investigated alternative methodsof maintaining the roads with motorgraders, towed graders and mechanicaldrags.

DESIGN OF THE STUDY

The study was carried out in Kenya incooperation with the Ministry of Trans-port and Communications (MOTC) . Anearlier study, carried out between 1971and 1974 by a joint World Bank/TRRL/MOTCproject investigated the inter-relationships between construction,maintenance and vehicle operating costsfor both paved and unpaved roads. Thisresulted in a computer model designedto aid investment decisions within theroad sector and which is known as theTRRI, Road Investment Model, S WAbaynayakaet al (1977).

The current study required the determin-ation and quantification of relation-ships between the following parameterswhich contribute to the deteriorationof unpaved roads:-

(a) Types of equipment used includedmotorised and towed graders, and.dragging implements.

(b) Performance and rate of deteriora-tion of a range of materialsgenerally used as wearing coursesfor unpaved roads.

(c) The effectiveness of grading atdifferent frequencies.

Cd) Use and effectiveness of compac-tion plant used in the maintenanceoperation in conjunction withgraders.

(e) The effect of the climate in termsof the volume and intensity ofrainfall.

(f) Depth of gravel removed by themaintenance operations.

(g) The alignment of the roads withrespect to their horizontalcurvature and vertical gradients.

Field work started by carrying out anappraisal of the condition of theexisting gravel road network in Kenya.Modes of deterioration of the roadswere then identified and methods ofmeasuring these were developed. Testsections of road were then selectedand the experimental work and monitoringof road performance commenced. Resultswere collected over a period of twoyears. These were then analysed andconclusions drawn.

Experimental test sections

The study was concentrated on threegeographical areas of Kenya where theMOTC was carrying out rehabilitationworks on large networks of unpavedroads. The use of test sections wasnecessary in order to relate thedeterioration rates of unpaved roads tothe standard, frequency and type ofmaintenance, the traffic spectrum, thenatural environment and the physicalproperties of the particular gravel.In addition, the test sections weredesigned to include variations ingeometric design standards in terms ofvertical gradient and horizontalcurvature. Horizontal curvature variedfrom less than 30 degrees/km to greaterthan 90 degrees/km and gradients variedfrom flat to greater than 3 per cent.The annual rainfall on the test sectionsvaried between 500 and 2000 mrm per year,and in each experimental area, testsections were duplicated in order thatdifferent levels of maintenance could becarried out. The levels chosen were asfollows: -

(a) High: graded every six months,

(b) Normal: graded every nine months,

(c) Nil: not qraded during thestudy.

In addition to these levels, it wasfound possible to include a small numberof test sections with maintenance fre-quencies of three and twelve months.All test sections were 300 metres inlength. In all cases, normal drainageand roadside maintenance were carriedout on a routine basis. It was expectedthat the gravel wearing course propertieswould make a significant contribution tothe rate of deterioration. Test sectionswere therefore chosen to utilise fourbasic gravel types which were asfollows: -

(a) Lateritic gravels. These areformed by the accretion of nodulesof oxide and aluminium. The testsections in the Bungoma area wereall lateritic.

(b) Quartzitic gravels. These arerounded gravels derived from thebasement rock of the area east ofMount Kenya and formed the wear ingcourse materials used on some ofthe test sections in the Meru area.

(c) volcanic gravels. These are angu-lar gravels derived from thevolcanic rocks of the Mount Kenyaarea and were employed as wearingcourse materials in other testsections in the Meru area.

(d) Sandstone g[ravels. These are finegrained gravels usually containinqquantities of quartz and feldspar

294

found near the Kenya coast. The wearingcourse materials on test sections atKaloneni were all derived from sand-stone.

Measurement of experimental variables

The principal method of measuring theroad condition involved the use of atowed bump integrator to monitorchanges in surface irregularity andoptical survey techniques to measurethe gravel lost from the road throughthe action of traffic and rainfall.Measurements were also taken of theamounts of surface loose material,depth of ruts, traffic volumes andloading, volume and intensity of rain-fall, road geometry and in-situ andlaboratory testing of the wearingcourse and subgrade materials. Themeasurement techniques used weresimilar to those used in the originalKenya study, J W Hodges et al (1975).

RESULTS OF THE ROAD DETERIORATION STUDY

Gravel loss from unpaved roads. Theloss of gravel from the wearing courseis a feature of all unpaved roads andeventually leads to permanent damageof the road structure unless remedialtreatment is carried out. To implementroutine and recurrent maintenanceoperations and to plan lonq termmeasures such as regravellinq, it isimportant to identify the rate at whichthe material is lost or eroded for awide spectrum of roads and gravel types.

The principal variables investigated inthe study were, traffic volumes andloading, rainfall volume and intensity,road alignment, gravel type and main-tenance input.

The gravel loss data from the study wasfirst plotted as a function of thecumulative traffic volume for each testsection. The data which were takenfrom measurements at approximatelythree monthly intervals showed nosignificant differences during wet ordry seasons. A selection of resultsfrom each of the four gravel typesinvestigated is shown in Figure 1.

Previous studies of gravel loss onunpaved roads by the TRRL and reportedby J W Hodges et al (1975) produced anequation for predicting the annualgravel loss for lateritic, quartzitic,volcanic and coral gravels. Theequation was of the following type:-

/2\ L.GL .f TAl (4.2+0-.092T +3.5OR +1.88VC)

.... eqnl

where GL, is the annual gravel loss inAmillimetres

TA is the annual traffic in both

Adirections measured in thou-

sands of vehicles

RL is the annual rainfallLmeasured in metres

VC is the rise and fall(gradient) expressed as apercentage

f is a constant depending ongravel type.

a

E

E

40

20P

00 5 0 20 30 40 50 60

Traffic passes a 10~3 T

Fig.1 Gravel loss against traffic passes for some volcanic and

sandstone gravel roads

Using the relevant values of annualtraffic, annual rainfall, verticalgradient and gravel constants in theequation,--the annual loss of gravelpredicted by this equation was evaluatedfor each of the current test sections.These values were plotted against theactual values obtained in the study asillustrated in Figure 2. It can be seen

35

30

E

I-

o L eisc

* nar in

0

0

5 1 0 1 5 20

Actoal trave1 los, li-n-)

25 30 35

Fig. 2 Predicted annual gravel loss against actual gravel loss for gravelroads found in this study

295

VolcaniconSadsv

U~~~~~~~~~~~~~~~~~~~~~~~

from this figure that the actual annualloss of gravel is greater than the pre-dicted annual gravel loss by about 37per cent. To bring the predicted gravelloss into agreement with the actual lossit would be necessary to increase thematerial constant f as shown below.

TABLE I

Material constants

* coral gravels were notthe current study.

Kenyan research, both in 1971-74 and inthis study, refer to test sections with3 per cent vertical gradient and 1250 mmof annual rainfall. Roads with steepergradients in wetter areas will havehigher losses, whilst the converse willbe true of roads with flatter gradientsin drier areas.

60 F-

50 k

i

0

investigated in

The differences between the gravel losspredicted by the original equations andthe gravel loss measured in this studycan be attributed to the following. Inthe original analysis, the action ofrainfall alone in producing gravel losswas calculated theoretically using theresults of agricultural studies byW H Wischmeier et al (1968) . An esti-mated value of soil erodibility wasused which gave a predicted maximumannual gravel loss of 2 mm and it wasthen assumed that there would be neg-ligible gravel loss when the trafficvolume was zero. In this study, twoexperimental lengths of unpaved roadwere constructed which were not sub-jected to traffic. Results of opticalsurveys at these sites gave values ofannual gravel loss of 4.3 mmf and 7.5 mmrespectively. These figures imply thatthe eroding effect of rainfall is higherthan that calculated by Wischmeier orby using the original TRRL, analysis.In the TRRI, 1971 study, the thicknessof wearing courses of the test sectionsvaried between 28 and 223 mm which com-pared to a range of 121-165 mm in thecurrent study. Any inherent weaknessesor strengths due to the varying thick-ness of the wearing course would alsoinfluence the resistance of the road todeformation. other contributory factorsto the increased rates of gravel lossare the increases in speeds of thetraffic resulting from general improve-ments in gravel road alignment over thenetwork as a whole.

An indication of the range of gravellosses that can occur in practice isillustrated in Figure 3 where the resultsof studies in a number of differentcountries are shown. The data from the

40

301

20

1 0

Kenyao) 1971, Coral

197 1. QOaatzitico 1971. Latefitic

.51971. Volcanfic* 1979, Lateritic* 1979. Volcanic

25-m (1 e) per 1 00 ADT

/ Ethiopia, Robi,.so

/ 7 ' 1- * C a me r. oo n . L o m a r

- M

Ivory Coast. Lombard

Ghana, Roberts

I I I

0 100 200 300 400Average dadev tralffc, ACT

500 600

Fig. 3 Gravel lass rates for different countries CT E Jones, 1 983)

Figure 3 further illustrates that annualgravel loss on unpaved roads will varybetween 10 mm and 30 mm per 100 vehiclesper day and will be dependent on climateand road alignment. This means that,annually, 70 to 210 cubic metres ofgravel will be lost from each kilometreof road per 100 vehicles per day.

These rates of gravel loss probably only.hold for the first phase of thedeterioration cycle lasting for two orthree years. They should not be con-sidered to hold over a long period oftime. As the wearing course is reducedin thickness, other developments such asthe formation of ruts will affect theloss of gravel material. However therates of loss shown in the gravel lossequation are essential requirements asan aid to the planning for regravellingin the future.

Looseness. There are two principlereasons for the presence of loosematerial on gravel roads. Firstly, itoccurs as a direct result of attritionof the road surface by the action oftraffic and rainfall. It can also bedue to the maintenance technique ofgrading material lost to the ditches andshoulders back on to the road. If there

296

Material Old NewMaterial value value

lateritic gravels 0.94 1.29

quartzitic gravels 1.10 1.51

volcanic gravels 0.70 .0.96

*coral gravels 1.50

sandstone gravels 1.38

is sufficient moisture in the materialit will be compacted by traffic, butonly in the wheelpaths. If there is.insufficient moisture, then the loosedry material will be dispersed acrossthe road by traffic and wind. Previousresearch has shown that loose materialon gravel roads influences rates of fuelconsumption for a wide range of vehicles.

The results of the looseness measure-ments for each gravel type have beenplotted as a function of cumulativetraffic and two of these are illustratedin Figure 4.

10~

8

f

z

0

o adsoe* aeie

operating costs is roughness (surfaceirregularity) . In turn, traffic is theparameter that has the most significanteffect on the rate of change of rough-ness. In the analysis, the roughnessof the sections were plotted as afunction of traffic. To obtain the bestfit of curve for the measurements, thedata were analysed using polynomialregression techniques (the results areillustrated in Figures 5(a) and 5 (b)).

8000

EE

0 6000

0

12 000 r

0 5 1 0 1 5 20 25 30Camalatine traffic pcsses 103

Fig.4 Relationship between depth of loose material and traffic passesfor sandstone and lateritic gravels

Although there was a maximum of 10 mmof loose material immediately aftergrading, the depth reduced rapidly withtraffic to a constant level. In thecase of the quartzitic gravel, thislevel was reached after approximately5000 traffic passes whilst the othergravels took approximately 6000-8000traffic passes to reach a similarasymptote. When test sections werecompacted after grading, looseness wasless than 1 mm in all cases. Therelationships between depth of loosematerial (DLM) and cumulative traffic(T) were found to be the following:

Quartzitic gravel

DLM = s.895e-0 358T *t 1.0

volcanic gravel

DLM = 6.748e 0 .183T + 1.0

Sandstone gravel

DLM = 6.925e-0 .187T + 1.0

Lateritic gravel

DLM = 6.01le 0 .334T + 0.5

....eqn 2

....eqn 3

....eqn 4

....eqn 5

SURFACE IRREGULARITY

The principal index of unpaved roaddeterioration which affects vehicle

10 000

f~8000*

o 6000

4000

7000o

R =3505 +221 T- 6.36T2

R =3545 -39.1 T +506T2

+ 0. 1 49T3

10 20 30 40 50 60 70

Traffic a 10-3

1 u E 7 t4 ,tic

- 6774/5-0

0

1~~~~~~~

* R =3384*+52.287T +0.143T2

1o *.* a- 0.01 57T3

A I I

0 1 0 20 30 40Traftic a tO 2

50 60 70

Fig.5a Relationship between roughness and traffic for sandstone andquartzitic gravels

In the majority of cases, the correla-tion between roughness and traffic isgood with regression coefficientshigher than 0.93. However, when theresults from the individual groups ofsections, built with the same graveltype, are plotted together, there areapparent differences in the deteriora-tion cycles, particularly in the casesof the quartzitic and sandstone gravels.The main difference in performance ofthe quartzitic gravels occurs at atraffic level of 15,000 vehicles wherethere are two significant increases insurface roughness on the lower traf-ficked roads. These two high readings

297

0

---~

found to perform better than the volcanicor sandstone gravels, they still deterior-ated faster than the lateritic gravelledroads.FLaetc gravel-_~ -7

~~~D A ~~~~~In the deterioration cycles, there isc D279118 ~~~~some evidence in the equations, particu-

larly in the volcanic and lateriticsections, that the rate of roughnesstends to decrease at the high levels.

R 308*2200T+32 -0033 It was observed during the study thatthere was a change of behaviour by roadusers at the higher levels of roughness,particularly on the volcanic gravelledroads which had on average the highest

0 20 40 60 803 100 120 140 level of roughness recorded. GenerallyTraffic 10

3 vehicles tended to follow a regular well

defined line of travel along a road, thepositions of which varied according to

Volcav~~raaCI0 the numbers of vehicles using the road

0 D4841b/S and the geometry of the road. ForD 482/700A example on roads carrying less than 100

* ~~~~vehicles per day most of the traffic* ~~~~~straddled the centre of the road forminq

two concentrated wheel paths. On roadswith more than 100 vehicles per day,vehicles tended to use their correct

0 3442- 2.519T +8.111 T2

0Q.C54T3 lane more often, and usually four

*0934. ~separate wheel paths developed. TheA ~~~~~~pattern of behaviour appeared also to

o ~~~~~~~be constrained by the horizontal andA

2 ~~~~~~~~vertical alignment of the road, espe-cially on roads with radii of curvaturegreater than ninety degrees per km. on

0 10 20 30 40 50 60 70 more heavily trafficked roads, theTraffic .10

3 defined wheel paths became more estab-

Reainhpbetween roughness and traffic for lateritic and lished and deteriorated to a point wherevlcatinsip rvl much of the traffic, particularly cars

volcanic gravels ~and light goods vehicles, attempted to

travel on smoother and relativelyuntrafficked parts of the road.

took place after the heaviest and mostintensive rain storm recorded duringthe study had fallen. This particularstorm had an intensity of 40 mm/hourover a period of 1½, hours and wasprobably responsible for the prematuredeterioration on this road.

The lateritic gravels deteriorated ata slower rate than all the othergravels investigated in the study.After 97,000 traffic passes a roughnessof 8000 mm/km was reached which is muchlower than the roughness levelsobtained with other gravels at lowertraffic volumes. Volcanic gravelsdeteriorated at a faster rate than anyof the other gravels, reaching a rough-ness of 8000 mim/km after only 30,000traffic passes. The three roads builtwith volcanic gravel wearing courseswere constructed to three distinctlevels of geometry, but no directcorrelation was found between theroughness of the sections and theirparticular geometric standards.Although the quartzitic gravels were

The regression equation relating rough-ness to traffic volume for each of thegravels illustrated in Figures 5(a) and5(b) are:-

Lateritic gravels2_ 3

R=3008+22.300T-0.531T -0.0033T . eon 6

Quartzitic gravels2 3

R=3384+52-281T+0.143T +0.0157T.. eon

Volcanic gravels2_ 3R=3442-2.519T+8.111T -o.e&4jI . .. eon S

Sandstone gravels2 3

R=3009+206.382T-3.688T +0.0323T .. eqn Q

where R = mean roughness in the wheeltracks measured in mm/km by abump inteqrator towed at32 km/h.

T = cumulative traffic volume inboth directions that has usedthe road since grading mea-sured in thousands of vehicles.

298

8ooc

EE

11

10a:

12000

1000C

iE 8OOC

600C

400(

200(

Fig. 5b

600C

400C

200C

Development of ruts with cumulativetraffic. In the analysis of rut depththe measurements were plotted as a func-tion of traffic and are illustrated inFigures 6 (a) and 6 (b) .

-E

ft

40 r

10

0

FLateritic1 -gravelJ1

p ~~~~RD -7.18 - 0.081T n 0.0069T 2 _0.000036T3

I I I I

0 20 40 60 80 100Criativ taffic . 10-3

120 140

60 -

80 ~

40 ~

0 1 0 20 30 40

Cumulative traffic a 103

f50 60 70 -530

i1:

20

101

0

RD = 7.09 + 0.573 T0.01 28 T + 0.00024 T

VOlCtnc gravel

.0

RD 10. 11 + 0.314T + 0.0003 1T2 +0.00002T 3

0 20 40 60 80 100 120Camalatiie traffic a 10

140

Fig.6b Relationship between rut depth and traffic for lateriticand volcanic gravel

0 1 0 20 30 40Cumulative traffic a 10

50 80 70

Fig. 6a Relationship between rut depth and traffic for quartzitic andsandstone gravels

On the lateritic gravel sections, thedevelopment of rut depths was less pro-nounced than OW-sections using othergravel types *and there was evidence ofa reduction in slope or 'flattening'of the curve similar to that obtainedwith the roughness measurements. Thevolcanic gravelled sections did not havehigh values of rut depth consideringthe high levels of roughness. This isattributed to the fact that, on theseroads, it was observed that extrawheel paths were being developed.

The regression equations which havebeen fitted to the data obtained fromthe rut depth measurements are asfollows: -

Lateritic gravels23

RD-7 .18+0.081T+0.0069T -O.000036T 3

... eqnlO0

Quartzitic gravels

RD = 7.49+0.171T+0.014T -O0.00009T3

..eqnll1

Volcanic gravels

RD=10.11+0.314T+0.00031T2+0.00002T3

... eqn 12

Sandstone gravels

RD=7.D9+0.573T-0.0128T2-f0.00024T 3

..eqn 13

where RD = Rut depth in mm under a 2metre straight edge

T = Traffic as defined earlier.

Axle load survey measurements. Axleload surveys were carried out atapproximately 4 monthly intervals oneach group of roads contained within thestudy. The surveys lasted for 5 days

299

40c

a 30

jIt

S9

It

* CfO7S/8

0 C107N/5

10,

30 1

20 1

from 6.00 am until 6.00 pm and for onenight from 6.00 pm until 6.00 am. Theprecise timing of the surveys during theyear was related as far as possible toexpected changes in the type of goodsbeing transported, such as would resultfrom the harvesting of crops. Typicalresults of the surveys are illustratedin Figure 7 which shows the axle loaddistribution for one of the experimentalroads. It can be seen that the numberof axles exceeding the legal limit,which in Kenya is 8 tonnes maximum on asingle axle, is minimal. On unpavedroads relatively small numbers of over-loaded vehicles can produce permanentdamage, T E Jones et al (1979), particu-larly if the road is in a high rainfallarea and has lost some of its shape andwater shedding characteristics.

40 fl

On the quartzitic road near Meru, therewas an annual increase of roughness of787 mm/km for a cumulative rainfall of711 mm. In the Kaloneni area, theroughness increased in the same periodby 821 mm/km for a higher rainfall of847 mm. The increase in roughness atthe two test sites plotted as a functionof cumulative rainfall is illustrated inFigure 8.

6000a0

E 4000 I-6

'2000

0 300 600 900

C umu. lative rainfall Rm,)

1200 1600

30 I-

20 I-66

0

10 1-

00 2 4 6

A.ie loads lo n s

Fig. 7 Distribution of axle loads on Kinango Roain Kaloleni area

Changes in rouqhness relatetive rainfall on untraffickParameters other than traffaffect the deterioration ragravel wearing courses. Ti-one of these is rainfall.

Kaloinni -Lusitangi Road

1 02 co m r i l a nles Fig. 8 Roughness against cumulative rainfall for non-trafficked roads

These results mean that, on untraffickedunpaved roads in areas where the annualrainfall is approximately 1 metre, theroughness of the road will increaseannually by approximately 970-1100 mm/km.

_j Rainfall records maintained on the two8 10 12 sites showed that the rainfall intensi-

ties were low, with the highest stormsrecorded during the monitoring period

dland Lusitangi Road of 10 mm per hour. Higher rainfallintensities would have influenced thechange in roughness considerably. There-fore, in areas subjected to flash storms,

!d to cumula- ie near mountains, coastal and lake:ed roads, areas, the annual increases in surface*ic will roughness of unpaved roads due to rain-ite of the fall alone could be much greater. Theie principal equations derived from the regression

analysis of the data are as follows:-

Pilot experiments were set up in twoareas to establish the effect of rain-fall on the road surface in isolationfrom other parameters. This wasachieved by constructing short lengthsof road with quartzitic and sandstonegravel wearing courses adjacent to themain test sections utilised in thestudy. These were left untraffickedexcept when measurements were takenwith the towed bump integrator. Annualrainfall figures derived from theKenyan Meteorological Department gaveaverage values for these roads of 1250mm and 630 mm. The actual rainfallrecorded at the sites during the twelvemonth period was 711 and 847 mmrespectively.

Quartzitic gravel R=3303+1. 117RR

....eqn 14

Sandstone gravel R=3550±l1.02lR R

....eqnl15

where R = roughness in mm/km measuredby a towed bump integrator

and R = rainfall in MM.R

For the purposes of predicting roughnessin terms of rainfall, it is adequate tocombine these equations to give:-

R = 3429 + 1.06 3

RR ....eqn 16

300

c uriii

-- f-r

Relative effectiveness of motorised andtowed graders on unpaved roads. InFigure 9, the performance of the towedgrader is compared with the motorgrader in terms of roughness as afunction of traffic for two of thegravel types investigated in the study.In each case, the section was gradedand then compacted, without water beingadded, using a vibrating roller. Ascan be seen from the Figure, there isvery little difference in performancebetween the sections maintained bytowed grader and those maintained bymotor grader. In each case the curvefor the regression analysis for therelevant nil maintenance section isalso illustrated for comparison.

100001Latenitic graveles Regression cure obtained for nil* Motor grade maintenance lateritic sectionsO Towed grader

8000 5

6000 I

4000

6rnoths 9 mooths 12frequency frequency frec

~oths2000

0 20 40 60 80Traffic o 103

Deterioration of roads maintained bymechanical drags. The type of equip-ment planned for use in the drag trialsconsisted of two brush drags, threecutting drags and a towed grader. Beforestarting the main investigation, it wasdecided to carry out some proving trialswith these. This was done in order todevelop dragginq techniques for eachdesign, and also to test the robustnessof each drag under road conditions. Thesites were located in two dry areas nearIsiolo and Meru and one temperate areanear Bunooma.

The plant used in the trial was asfollows: -

a) Tree broom made from an acacia tree.

b) Tyre sledge made from lorry tyres.

c) Baulk of rectangular timber(designated TRRL, A).

d) Timber framed drag with metalleading edges (designated TRRL B).

e) Metal framed drag incorporating usedgrader blades, the leading one setat 300 to the centre line (designa-ted TRRL E).

f) Towed grader.

tU~flCY Immediately prior to the dragging

100 1010 ooeration, the roughness of each testsection was measured using the towedbump integrator. At the conclusion ofthe operation, the roughness measurementwas taken again. After this, measure-ments were taken at Periods of 7, 21, 35,49 and 63 days from the day of grading.Results from the dragging trials carriedout at Isiolo are shown in Table II.

6

a:

0 tO0 20 30 40 50 60 70

Traffic n 10

Fig. 9 Comparisons of effectiveness of motor graders and towed gradersfor different frequencies of grading

It must be stressed that the towedgraders performance is related to themaintenance of an initially well con-structed road. It is not a recon-struction tool, although in the studyit was found capable of dealing withruts of up to 75 mm in depth. On somevolcanic gravelled roads with oversizematerials, the towed grader was foundto be unsatisfactory.

As can be seen from the table, there isan immediate improvement in road con-dition after the dragging operation dueto the reduction in roughness levels.However, in the case of the tree broom,the roughness quickly started revertingto its old level. Even after a periodof 7 days, the roughness had increasedby 1000 mm/km. After 21 days it hadreached 7600-9000 mm/kin, indicative of aroad in poor condition. The towedgrader achieved both the largest reduc-tion in initial roughness and the leastdeterioration with time. The performanceof the tyre sledge, TRRL "A" and TRRL 'B"drags were similar to each other, whilstthe TRRL, 'E` drag achieved results sur-passed only by the towed grader.

Table III shows that the effect of theuse of rollers was not larcge. However,with the exception of TRRL "B", rollingreduced the frequency that draggingactivities were needed. This improvementcannot be due to the small increase indry density of the wearing course. Itseffect has come from the reduction in theamounts of loose material, the crushing

301

6g

0:

TABLE II

Summary of drag trials at Isiolo

(Traffic: 108 vpd)

TABLE III

Results of drag trials with sections compacted with vibrating roller

Roughness in mm/km

Type of drag Days after dragging

Before 0 7 21 35 49 63

Tree broom 9510 6130 7162 7560 8642 10812TRRL "A" 8904 6284 6563 6324 7078 8606TRRI, "B" 8750 6068 6561 6654 7153 8666 8210Towed grader 8560 5622 5923 5911 5861 6302 7514Tyre sledge 9062 6926 6436 7084 7220 7197 9412TRRI, "E" 9112 5426 5658 5358 5921 6252 7904

of oversize material and the generalsmoothing of the dragged surface.Similar results were obtained from theother test sites at Meru and Bungoma.

The drag trials carried out on roadscontaining quartzitic, volcanic andlateritic materials with differinglevels of roughness showed that nosingle drag could provide the completeanswer to effective maintenance of lowvolume rural roads. The results, how-ever, did provide quantified informationon the relative merits of variousdesigns of drag from which the followingrecommendations are made in Table IV.

In order to be effective, drags must beused at intervals frequent enouoh toensure that they are used only to smooththe surface of the road. They must notbe used on roads which, through neglect

or infrequent maintenance, have deterior-ated to a level where heavy grading orreshaping is needed rather than recur-rent maintenance. Drags should not betowed at speeds above 10 km/h andpreferably should be towed at 5 km/h.

CONCLUSIONS

Unpaved roads lose annually between 10and 30 mm of gravel per 100 vehiclesper day. This means that 70 to 210cubic metres of gravel will be removedfrom the road surface each year. Thetotal loss of gravel on unpaved roadsin developing countries is increasingannually whilst the sources of goodroad making material are diminishing.These rates of gravel loss can be sub-stantially reduced if qreater controlis exercised in the construction of

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Roughness in mm/km

Type of drag Days after dragginq

Before 0 7 21 35 49 6 3

Tree broom 9160 6360 7820 9322 8974 9892TRRL, 'A' 8920 5968 6652 7963 8116 9175TRRI, "B" 9012 6260 6361 6949 6966 8410 8625Towed grader 8866 5861 6144 6257 6508 7010 6962Tyre sledge 9272 6610 7108 7804 8251 8712 8462TRRI, "E" 8762 5816 5864 6551 6562 6861 8123Nil 96 12 26Maintenance 96 12 26

TABLE IV

Recommended drags for varying conditions

Road condition Recommended drag

Roads with depths of loose Tyre sledge

material in excess of 20 mm Cutting drag using old

grader blades (T.RRL, E)

Towed grader

Roads with roughness levels Timber framed drag<6000 mm/km with noappreciable loose material Towed grader

Roads with rouqhness levels Cutting drag using old>6000 mm/km grader blades (TRRL, E)

Towed grader

gravel roads. More attention must bepaid to the Particle size distributionof the wearing course material, inparticular the removal at source of theoversize gravel.

The study was able to quantify separatedeterioration relationships for lateritic,quartzitic, volcanic and sandstone gravels.This was an important improvement on previousstudies of gravel roads. The study alsoshowed that there was more scooe in the main-tenance ooerat ion for the use of towedgraders and mechanical drags instead of themore expensive motor graders, and recommend-ations are made for this. On well maintainedroads, the amount of loose surface materialthat remained on the surface was found to beinsignificant, and unlikely to influencevehicle operating costs.

The effect of rainfall in isolationfrom other parameters was found toincrease the surface roughness annuallyby between 970 and 1100 mm,./km per metreof rainfall.

only a small number of vehicles travel-ling over the test sections were foundto be overloaded. Overloaded vehiclesdid not therefore significantlyinfluence the rate of deterioration.

In many countries the maintenanceactivities on unpaved roads arescheduled for the period immediately,after the end of the wet season.Maintenance organisations should alsocarry out maintenance at the beginningof the wet season to ensure that theroad has the correct profile foreffectively shedding water during therains.

ACKNOWLEDGEMENTS

The work described in this paper forms partof the research programme of the overseasUnit (Head: MrJiS Yerrell) of the Transportand Road Research Laboratory, and is con-tributed by permission of the Director.

Any views expressed are not necessarilythose of the British Government'sDepartment of Transport, Department ofthe Environment nor the OverseasDevelopment Administration.

REFERENCES

Abaynayaka, S.W., H. Hide, R Robinsonand J Rolt (1977) . `Prediction of roadconstruction and vehicle operatingcosts in developing countries," ProcInstn Civ Engrs, 62 (Part 1), 419-46.

Hodges J.W., J.Rolt and T.E. Jones(1975) . `The Kenya road transport coststudy: research on road deterioration,"TRRL, Laboratory Report 673 Crowthorne,(Transport and Road Research Laboratory).

Jones, T.E. (1983) . "The Kenya maintenancestudy on unpaved roads: research ondeterioration," Department of the Environ-ment Department of Transport, (TRRLReport in oreparation) , Crowthorne.

Jones, T.E. and MS Snaith (1979) . "Axleload distributions in developing countriesand their implications for pavementdesign and road strengthening." PTRC.Summer Meeting, University of Warwick.

Wischmeier, W.H. and J.V. Mannering(1968) . "Relation of soil propertiesto its erodibility," Soil ScienceSociety of America. Washington DC.

(C) CROWN COPYRIGHT 1984. Publishedby permission of the Controller of HerBritannic Majesty's Stationery Office.

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