Earth Roads; Their Construction and Maintenence

8/12/2019 Earth Roads; Their Construction and Maintenence

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A project of Volunteers in AsiaEarth Roads: Their Construction and Maintenanceby Jack Hindson

Published by:Intermediate Technol ogy Devel opment Group I TDG)

Available fromI ntermedi ateTechnol ogyPublications9 King StreetLondon WC2E 8HNENGLAND

Reproduced by permission.Reproduction of this microfiche document in anyformis subj ect to the same restrictions as thoseof the original document.


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,.

LARTH ROADSk Practical Guide toEurth Road Corzstructim md Muinttwancc


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JACK I-IINDSBN

EARTH ROADSA Practical Guide toEar th Road Construction and Maintenance

Edited and revised byJohn Howe and Gordon Mathwayof Intermediate Technology Transport Ltd.

I T PUBLICATIONS 1983


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Published by Intermediate TechntAogy Publications Ltd.9 King Street, London WCZE 8HN. UK

0 Jack Hindson 1983ISBN Boards 0 903031 83 3Paperback 0 c)O3031 84 1Typcsct by lnforum Ltd. PortsmcwthPrinted in Great Britain


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.

contents

EDITORS FOREWORDCHAPTER 1 INTRODUCTION1.1 Erosion on earth roads1.2 Svil wnservaliun principles and road drainage1.3 Village roads - the Oat methodCrossings1.4 Market roads - the high-level method

The high-ieve: method of construction1.5 Village and market roads: further considerationsPART I : WATER CONTROL ANI> DRAINAGE METHODSCHAPTER 2 DIVERSION BANKS2.1 Pratt ical considerationsDistance between diversion banksNatural diversions

xi11?I68

1%13

2.2

Siting of diversion banksReducing erosion by reducing amount of waterPaths bringing storm water onto the roadShape of diversion banksDiversion banks on the flatDiversion banks on hillsFree board

18

CHAPTER 3 LEAD-OFF DRAINS 213.1 Shspc of Icad-off bolster 213.2 Choosing the layout for lead-off drains 223.3 T lead-off drains 243.4 Road crossing a gentle saddhe in nearly flat country 253.5 Correct spacing of lead-off drains 25CHAPTER 4 DRIFTS, SPLASHES AND CULVERTS 274.1 Drifts and splashes 274.2 A free fall reduces depth of flooding 284.3 Widening the crossing reduces depth of flooding 28


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7.67.77.87.9

7.10

7.117.127.13

Widening the drains: no crossfallWidening the drains on crossfallsPegging of lead-off drainsGeneralSplashes and culverts

RegrowthOutward slope of side drainConstruction of embanked roads in wet arcas awayfrom stream crossingsPilot drainsPegging road and side drainsDigging side drainsSurfacing the roadWet ground at stream crossingsConsolidationSteep slopesImproving gradients by eye __ -Rtvwval of wafer from outer side of road on steepcrossfallUpper side drains on moderately steep crossfalls

7.14 GravcllingGravelling embankmentsSand/clay mixturesCHAPTER 8 C~NSTRCICTI ON OF VILL AGE ROADS8.1 Pegging and stumping8M Levrlling the road surface8.3 Div crsion banksPeggingDigging8.4 Wet areasCtiAllF.1,. 6~ CONSTKU(:iI~,N OF SPL ASHES, DRrkTS AhDg , \rER-pj9.1 SplashesShape

1m proved splashStandard splashRun-outs from splashesBuilding a gabionWarning marksSplashes to be sited on straightsSplashes which cross gullies etc.


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Splashes diagonally across roadSplashes in flat country9.2 Drifts 84L,aying rock paving at stream crossingsUtilising a rock bar

Safeguarding bridges9.3 Construction of culverts 85ExampleSurvey pegsHeight pegsBuilding the embankmentPosition of culvert pipesProvision for flood waterinadequate soil cover above pipeNotes on the construction of culvertsThrowing soil with hoe or shovelExcavation below embankmentSinkagcDesignSpring of water under the road

PART 4 MAINTENANCECHAPTER IO MAI NTENANCE OF MARKET RQ)ADS 9810.1 Prevention of flow along the carriageway10.2 Depth of side drains10.3 Genera1 maintenance operationsRemoving tree growth etc.Repairing pot-holesResurfacing the road b: widening- the side drainsTraffic using road during resurfacmLend-off drainsMaintenance of splashesRoad-work during rainsCtIAtTER It MAINTE NANCE OF VILLAGE ROADS1 1.1 Diversion banks11.2 Clearing the road11.3 Filling in the tracks11.4 Hoeing an earth road1 1.5 CrossingsCHAPTER I2 REHABIL ITATION OF ROADS12.1 Market roadsRemoval of storm water

48i 00100

103103103103104LO5106106


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Raising the roadRelative costs of stumping a new road line comparedwith iiiing up an eroded roadInitial work to be undertaken on inferior roadsReconstruction ot narrow embankmentsBank of sand etc. along edge of side drainEroded side drains

12.2 Village roads 112APPENDIX I SURVEYING TECHNIQUES AND EQUIPMENT 1131. Note on gradients2. Pacing3. Road tracers4. Dumpy and Surveyors levels5. Checking and adjusting the road tracer6. Checking and adjusting the Surveyors level7. Boning rods8. Survey pegs9. Height pegsIO. Prcscrving m;arks below ground IwA~\ tPtiNDlX 2 GLOSSARY 121


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itm? OEWOPOne of the most glaring differences between the develop4 anddeveloping countries is in the densities of their respective roadnctwc,rks. Indeed the density or quality of a countrys road system isoften used as a measure of its state of development. That roads arean essential ingredient of the development process is indisputable,but it is generaiiy believed that the limited resources of mostdcvcloping countries and the cost of modern road construction hasprcvcntcd rapid expansion or improvement of their systems.

Yet in the ehriy stages of devclopr;lcnt it is doubtful if modernhigh-cost ro:tds arc necessary: thcrc is abundant evidence to showt hut t kc cxistsncc of n nicans of conlmiliticntion is mart importantth;ul its quuiity. Thcrc is, in fact, a very considerable iiterattrrc onlow-cost road construct ion and there have been many conkrcnccsaround the world devoted sniciy to thu subject. However, it isarguakic whcthcr the roads thus described are truly low-cost. Mostof the Je:xriptions given of construction methods presume theknowicdgc and skills of a graduate civil engineer and the use ofcomplex equipment.Ihc result is a technology largely incomprehensible to the lay-man, and a road that is not lo+cost. One of the authors intentionsin this book is to fiii the need for a simplified description of roadplunning and construction at the most cicmc~ntary ievcl.Howcvcr, the ar~thors prime purpose is much more radical. It istc~ ~p :Cn how cur7le roads could hc ;onstrur:itid ;9nd maintained in away that wor.~idprevent their progrcssivc ;tnd 5&n rapid dtistruc-(ion by erosion due to the uncontrolled flow of water. The emphasisthroughout is on the proper control of water reaching the road, andcm basic principles of soil conservation - it is this aspect whichmakes the book unique. The approach is that of a soil conservation-ist to the building of an earth structure: a simple road. The focus isquite dclibcrntciy on roads designed to carry a few dozen vehicles aday at most. Fully engineered gravel or bitumen surfaced roads arenot considered.


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EDITORS FOREWORD xiThe text covers two different basic designs of earth road called,for convenience, village and market roads. These are not pre-cisely defined operationally, but the essential difference is the levelof usage, from a few vehicles per day or week (village) to perhapsfifty vehicles per day (market).Some of the descriptions given are unorthodox, particularly theuse of diversion banks, which are a major means of controllingerosion but, as the author exp ains, the orthodox approach to roadconstruction produces either a very expensive structure that can bebuilt only by skilled engineers, or one that rapidly deteriorates louselessness. The methods described in this book deserve respectbecause they are not based on untried theories, but are the directresult of building and maintaining earth roads for a period of morethan twenty years in the northernmost parts of Zambia. They are

based on the accumulated expcricnce of extensive trial and error,and, despite their apparent simplicity, they can be seen to derivefrom sound scientific principles.Emphasis is placed oi-i bhc achievement of law ~~~~~t~~~~ti~nastsby adopting a low-speed natural ?ii men t for I he ra~ad, usilabour-intensive building methods here possible, and mithe cost of earth-moving by use of material along the line of fhcroad. The use of gravel, wheelbarrows to haul cart h and borrow-pitsas a source of material are not recommended except to meetextreme local conditions.The use of diversion banks, drifts and splashes also has the effectof p(Gtively controlling the speed of vehicles. This is arguably more;: ktiw than t :c.canwntianal design speed approach, which doesnot prevent those vehicles capable of travelling fast from doing so,and thus damaging the road. The resultsnt reduction in roaddamage is an essential part of the success of these methods.Implicit in the authors appr.WA* to Zoa9-cost art h road constr:lc-tion is the recognition th,t, in ZiLiiiilin ir. C:rn-vcriPil:in2i motnrizcdtransport, such roads may be used by a range of much simplervehicles including wheelbarrows and handcarts, animal-drawncarts and pedaLdriven devices. These simple vehicles can meetrnafl:j rural transport needs and are often more appropriate to thefinancial resources of the rural population than motorizedvehicles.The task of the editors has been simply to update some of the figures


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xii EDITORS FOREWORDand generalize the colloquial terms in the original manuscript. Also,to achieve a publication that could be afforded by many of itsintended readers it has been necessary somewhat to shorten andreorganize the text, though this has not re:sulted in the omission ofany important material.

The text is divided into four major parts. The first deals with thedrainage principles and techniques which form the basis of the roaddesign and construction methods proposed. Part two covers theplanning of the road, wit.11he emphasis ori route selection, survey-ing and marking. Thc third section describes construction methods,and the final part deals with the subsequent maintenance opera-tions. Appendix 1 describes the operation and use of some simplepieces of surveying equipment referred to in the main text. To assistthe reader a glossary of road construction terms is included asAppendix 2.Metric units arc used thrc,ughout. For the most part the 5.1.system has hxm adopted but in certain cxxs the centjmctre (cm)has ~XXII sed to laciiitat~ understanding by likely rrscrsofthe book.


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CHAPTER 1

1. Erosion an Earth RoadsErosion is the wearing away of soil by water and. usually to a muchlesser extent, by wind. It varies enormously from one country toanotkr arid even from one dislrict h.1awiikr . 111ome areas thCiC isno erosion at all but in the majority of cases erosion is likely to be amajor problem, causing considerable damage to agriculture and toroads.The most usual cause of erosion is rain. Instead of the rain fallinggently during many manths of the year it is concentrated into only afew months. During these wet ri oor~ths much of t l i c rain comes inshort, sharp storms which may be very intense. These very heavystorms are the major cause of erosion and arc responsible for thewidesprcnd damage which so often occurs on earth roads.Erosion on the Carrii~geway (where the traffic runs) or in the sidedrain of the road results from too much water being allowed toaccumulate there. Most roads have a slight radient (that is, theyS- W uphill or downhilt slightly) so, if much water does collect onthem it will begin to flow. As the volume of water increases so doesits speed, causing the amount of erosion to increase to a muchgreater extent. This can be prcvcnted by diverting aIf the water intothe bush at intervals so that no excessive build-up of water isallowed t,o occur.Figure 1 a) shows, in cross-section, a typical earth road that isbeing eroded along the carriageway, as a result of too much stormwater being allowed to flow along the wheel tracks. Figure l(b)shows another earth road which is also being severely eroded, thistime in the side drains. This is because these drains have to take fartoo great a flow of storm water.Obviously these roads will have onl y a short life if nothing is doneto divert the flow of water. After a few years they will have erodedso much that they will not be worth repairing and new ones will haveto be made.


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2 INTRODUCTION

bush wheel tracks

former ground levelFigure ital. Erosion n wheel tracks.

V former ground levelFigure l(b). Erosion n side drains.Particular attention will be paid throughout the book to theproblem of erosion. The intention is IO explain how damage can beprevented by the adoption of certain simple soil conservation prae-

fices. T le ninl throicgl ro ut is the ej@im smtrol nrrd cl i .Jpnsal ofrm -off umer.

2. Soil Conservation Principles and Raad DrainageThe problem is to provide a permanent earth road which is designedto serve present and furure requirements. It must be done ascheaply as possible; therefore every operation must be fully justi-fied.Any earth road will fail, sooner or later, if avoidable erasim dueto water flow is allowed to continue unabated. Each season the levelof the road will sink a little due to removal of soil; the more it sinksthe more difficult it will be to drain the water off the road. Thismeans that erosion damage although imperceptible at first, is liableto increase rapidly in later years. Avoidable erosion due to lack ofwater control may easily be prevented, as will be described in thisbo&.However, earth roads are also subject ts wawkiable erosion.This is the loss of soil from the road surface, either in the form ofdust which has been pulverized by the passing of traffic, or as mudsplashed out of the tracks by passing vehicles during heavy rain.These losses are difficult to prevent, but are nearly always insignifi-


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INTRODID~lC3N 3cant compared with the loss of soil by avoidable erosion. They canalso be easily replaced by using simple maintenance procedures (seeChapter l(3).Solutions to the problems of avoidable earth road erosion can beobtained by the application of the general principles of soil conscr-vation, particularly 2s ?hey are applied by farmers. For example,various gradients may bc used to make water flow through ar&iAeland. However, in practice a gradient steeper than 1 in 2SV is neverused in ordinary soils for fear of eausmg erosion irr thr: bed of thefurrow during a storm. Whcr~ water does have to flow, it is run outon to permanent grass where it finds its way st,raight down the hill, ata gradient clften far stccpcr than 1 in 250. Normally no s:rosion willoccur in this grassed disposal area if the grass is preserved and notdestroyed by cultivation or overgrazing and if the volume of waterdischarged during a heavy storm is not excessive.Same disposal areas may have ~a be excavated and levelled off tothe rsquircd shape, and perhaps grassed over too, before the run-ON water can bc turned on to them. (Grass will row nnturslBy onnearly all topsoils within a year or two if Ieft undisturbed. This grassgrowth is not s 7own in must of t ic figures. in ordlcr to improve:clarity.)There arc three shapes of waterway suitable for this type of work.(Figures 2(a), (b) and (c) ). In practice there is little differencebctwccn these three cross-sections. In each of them the flow isspread out over a wide area in the form of a thin sheet of water. Thismoves only slowly through the mat of grass foliage, so the grass has agood chance to grow. This is thr: met hod that is used to conductstorm water straight down earth slopes steeper than 1 in 250.Compare the above waterways with a deep, steep-&led drain (Fig-ure 3) carrying the same flow of water down the same slope. Hcrcthe water wili flow much deeper, and hence much faster, because itcannot spread out in a shallow layer over a wide area. The grass willbe unable to survive in this drain, and soil erosion will soon set in,starting a gully. This type of drain would be safe only on very flatland where the water stands or flows very slowly. When an experi-enced conservationist has to make provision for water to dischargestraight down a hill he looks for, or xcavates, wide shallow water-ways like those in Figure 2; the wider the better. He would never

See Appendix I ftlr nn ~xplnnclriunf rllis erm.


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former bed of

(al. & t .- corr ect.

(19). Dish,,?d cnrrixt,

Figure 2. Wide, shal io w wa en443 s.contemplate a cross-section like that of the deep narrow drainshown in Figure 3 because it would not

Al l these basic cormwat im principles apply also n he contd anddisposal o f rwi-0 jf water 011 arth road s-ptms.

The earth roads that we are concerned with can be divided intotwo separate types, each with its own distinct method of construc-tion and maintenance, which will be described in detail later. One ofthese types will be called a village road and the other a marketroad.3. ViElage Roads - the Wat MethodA village road is the simplest, cheapest road or track; it may runfrom one small village to another or to a farm, a small settlement, a


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INTRODUeTION

Figure 3. Deep, mm w drain - inco rrectschool or a dispensary. It may even run from one field trj ano:her ona farm for use, for example, by animal-drawn carts. Pr11 hesl.: villageroads should be made flat with no side drain and no camber. Onthese village roads only one pair of wheel tracks wi r be formed bythe passage of traffic; if these deepen they should be filled in. Anysurface run-off water moving down t Pe slope throu h the bush willbe able to pass acK;ss the road at any point, just aa if there were I V)road there at at . Erosion is adequately controlled by miiking diver-sion banks (set Cha ~ter 2) where required along al village roacll;,Diversion banks are gentle ~unps across the road whit -4 divert awater into the surrounding bush. These banks will, of CCNWX. Wdown motorized traffic, but the most important requirenlcnt w ievillage road, bot l in hilly and in flat country, is for s ow, steadyspeeds in any weather and at any season oi the year. Dixrsi,-.nhanks help PO nsure this. The carriageway itself is merely cleared (IIsmall bushes and trees including their roots (a process which will bercferrred to subsequently as stumping) and levt:l cd so thar water canpass across it in any direction.CrossingsIn Figure 4 t le small watercourses A, B, C and B show how watermight trickle across such a road when it rains. On the left of thediagram a larger watercourse, or stream, S, crosses over the road.As the crossings A, B, C and D would be wet only when it rains,and as there is only infrequent traffic on a village road, thcre wouldbe no trouble from mud at these crossings, particul; i?y if the roadwere grassed. If, however, mud did form in one of t rc tracks wherethe water crosses, the addition of a few stones or a load or two ofgravel would put it right. Often, throwing on more soil to fill in anydepression is sutficient.


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6 INTRODUCTION

gravel small watercoursesmall watercourses

Figure 4. Water cros sing a vi l lage road.

On the left of the diagram the stream S would definitely needattention. A lot more stones, etc. would have to be spread on thebed of this crossing. Storm water might tend to wash this away, inwhich case it would have to be stabilized by laying a length ofrock-filled gabion (see Appendix 2) alafig the lower side as shown.(This subject is dealt with fully in Chapter 4.)4. Market Roads .- the High4evel MethodMarket roads carry more traffic than village roads. This type of roadwould run to a market, a food-buying depot, a rural developmentscheme or other important rural centre where traffic might amountto ten or twenty vc;hicies a day. In the dry season heavy lorries mightuse the road.On the flat stretches of a market road, where the slope is less thanI in 250, a bare earth drain (as in Figure 3) could be depended onnot to erode, but as soon as the gradient of the road steepens to, say,f in 150 or steeper, then the flow must pass through grass and notover bare earth; this means that a wide flat drain becomes neces-sary. This method should therefore be adopted whatever thegradient.The .High-Level Method o Construct ionSince they carry more vehicles than village roads, market roadsmust be cambered to allow water to run off into side drains. Toachieve this the high-level method of construction should be used.The reasons for this are illustrated by means of Figures 5 (a), (b) and(c). In each of these examples the carriageway is 3Ocm above the


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INTRODUZTION

I centre lineI

- - - ID __ ,I_ .,.I_ -

former ground level

(a). Road bui l t above gro und level-- incorrect.

-11:-_.--- - /.II=-

_ .. ---_.I,_.-.-__~--,_15ctn 30cm 15cm

(151. The high -levelmeths d- cor rect.

-----_

--.

(c l . Road bui l t at groun d evel- incorrect.

Figure 5. Market road constru ctmn methods.


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8 INTRQDU@1IONlevel of any shallow water .vhich may be standing alongside theroad, i.e. in the side drains in Figure 5 (b) and (c), and on the groundin Figure 5 (a). This 30cm freeboard is necessary to enable theroad to dry out a few centimetres below the surface. The land is flatin each case.Figure S(a) shows a road built completely above ground level byimporting soil from a borrow site. This would be very expensivebecause of the cost of transporting the soil.Figure 5(b) shows the high-level method. Soi for the road is dugand thrown from the side drain until the camber is high enough. Adrain 15cm deep (the depth of one hoe-cut, approximatciy) and acompacted road 15cm high would give the required 30cm free-board. Since the drain and the camber are formed in one operationthe cost of construction is minimized.Figure 5(c) shows what would happen if the road were built atground level, Soil has to bc excavated to form the camber and sidedrains and dumped usciessiy in the bush. Because the side drains arewell below ground level, it is difficult or impossible to run waterfrom them into the bush.

5, Village and Market Roads: Further CsnsideratiansA market road will be drier than a village road because rain failingon the carriageway of the high-level road will, in theory, be shed offthe sloping camber into the side drains. On a fiat village road, on theother hand, there will be a tendency for rain water to stand inpuddles instead of running off.Again, the carriageway of a high-level road is about 3Qcm abovethe icvei of the bed of its side drains, so the soil in the road will tendto drain dry nearly down to the level of the bed of these drains. Thecarriageway will therefore remain a lot drier than it would do in thecase of a flat village road which has no side drains at ail.Infrequent light traffic will do little damage to wet spots on aGilage road but, if traffic increases, these wet spots will soon turnillto mud; if the wet weather continues, the road may becomeimpassable.Graveiiing the surface reduces the risk of mud but transportinggravel is very expensive; instead it is far cheaper (and more satisfac-tory) to reduce the risk by raising the level of the road and camber-ing it, i.e. to rnake a high-level market road in place of a flat villageroad. Up to a point the higher it is raised (widening it at the same


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INTRODUCTION 9NTRODUCTION 9time to keep a good camber) the drier it will be. If mud still tends toime to keep a good camber) the drier it will be. If mud still tends toform in places, due to the clay soil. then gravelling may have to beorm in places, due to the clay soil. then gravelling may have to beresorted to.esorted to.A market road will serve the needs of a district until it developsmarket road will serve the needs of a district until it developsand the earth road begins to break up under the increased traffic. Itnd the earth road begins to break up under the increased traffic. Itwill then have to be properly graveher- , perhaps throughout itsill then have to be properly graveller-r, perhaps throughout itslength; reconstruction may even be necessary. T~zvsv W q~ensiveength; reconstruction may even be necessary. T~zvsv re qoensiveopm tism In wlv i r2g expert kt i tz~ckige nrd are rmt deal t wi th ii1 hispm tisvrs In wlv i r2g expert kt i tz~ckige nrd are rmt dkal t wi th i i1 hisbook. Village and market roads are the two types which will beook. Village and market roads are the two types which will beconsidered in this book. IS is clear that the key ts earth roadonsidered in this book. IS is clear that the key ts earth roadperformance is the proper control of water flow. The followingerformance is the proper control of water flow. The followingchapters will show how this can be done.hapters will show how this can be done.


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s


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CHAPTER2Diversion Banks

Diversion banks are fundamental to the proper performance ofvillage roads. Also, many of the principles upon which they arebased apply to the drainage of cambered market roads and to thesiting of lead-off drains (see Chapter 3).One sometimes sees a bush road which goes over the remains ofan old anthill or passes near the foot of one. This is shown in Figure6. The road to the left, and upstream, of the anthill may be erodingseverely due to excessive storm wate:r being trapped in it. When thisflow meets the anthill it is unable to pass over it so the water has toforce its way through the grass on the lower side of the road.Depending on the slope of the ground this flow of water may, ormay not, return to the road further on. In Figure 6 the natural slopewould take the water slightly away from the road. The section ofroad immediately below the anthill will therefore be completely

slope .

Figure 6. A natural diversion b ank.protected from any erosion due to run-off down the road. The watermay tend to form ponds where the flow meets the rise of the anthilland mud may form at that point, but this is a very minor considera-tion compared with the fact that all flow has been diverted off theroad. The presence of these long bumps in the road, which forces all


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DIVERSION BANKS 13the water off the road, affords an important, though localized,measure of protection from erosion.Diversion banks are identical, in principle, to these natural ant-hills which divert the water, the only difference being that theformer are made artifically, as and where required.1. Practical ConsiderationsThe bank forms a block in the road which diverts the water butallows the traffic to pass easily and safely, so long as it is nottravelling at excessive speed. Associated with the diversion bank is adiversion drain which feeds the water into the bush. Soil for thebank is dug out of the diversion drain and thrown onto the road(Figure 7).

25m-.-.- -,-, - --~_.-_-.--- )r

summit of bankdiversion drain - ---WY- bank

slope \-ai

run outFigure 7. Diversion bank and drain.Distame B etween Divers ion BanksThis can vary enormously. If storm water flows only very slowlyonto the road from adjacent land, the diversions might be as muchas 250m apart where the ground is flat. However, in steep countrywhere a lot of run-off water reaches the road, they could in excep-tional cases be as little as 30m apart.Natural Divers imsThere wil: be many of these along most road lines. Some will re-semble diversion banks, e.g. anthills and other bumps. Other diver-sions are streams and minor watercourses due to undulating ground.


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14 WATER CONTROL AND DRAINAGE METHODSFigure 8 shows a road crossing two streams, A and B, a kilometreapart; there is a general slope down to the right. The ground rises alittle in the middle, making it impossible for any of the flood inStream A to pass on down the road into stream B. In other words,

the rise in the road (that is, the watershed) forms an efficient naturaldiversion bank a kilometrc long and perhaps 15m high.general slope of road ._

Figure 8. A na eal diversio n bank.

In escarpment country a Wild running along iI hillside will crossfrequent gullies, etc. Each deep wutcrcoursc will divert the full flow(as in Figure 8) and IN?flow can pass on i:long the road arid cluscerosion, as it can in flat country where effccfivc natural diversionpoints (e.g. streams, gullies. etc.) are few and far between.Siting o Divers im BarrksConsider the example shown in Figure 9. It shows a village roadcrossing one stream and two small watercourses. All three can berelied o~i to divert a high flood without letting any water pass onalong the road. Diversion banks are sited at IN, IX and IX?. Thesesites are chosen because the two long slopes appear likely to erode

stream watercourse wa tercsurse

Figure 9. Si t ing of diversion banks.


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i

DIVERSION BANKS 15whereas the other four slopes do not. One diversion bank shouldsuffice on the Dl slope, but two banks are considered necessary onthe longer slope D2-D3.if a concentrated flow of water reaches the road, as in a gully,from the bush on the upper side, try to take the flow straight acrossthe road, as in Figure 1O(a), instead of letting it run down the road tothe next diversion point, as shown in Figure IO(b).

(El)WllY

\ diversionI

bank--k r:.

run-out\ R

(b)s3w\ diversion

\ road erodesLC, -C.--w - .,:Q~:::iYrun-out

\

F/gum 10. Gully cros sing road. (81. Correc t. I&). inco rrect.

There must always he some falEon the bed of the diversion drainso that it conducts the w;;;ter away. For a short drain 5m long thereshould be a fall of at least 8cm (about 1 in 60 slope); a long drainshould drop at least 8cm every 1Om (1 in 125) although a steeperslope is preferable. Where possible a site should be chosen wherethe water can get away easily so that only a little work is required tocomplete the diversion drain and bank. lncvitably some sites aremore difficult and a long drain and a big bank may be necessary. Theslope of the drain must bc continued across the road (i.e. near pointR in Figure 7) so that the water flows to the diversion drain and doesnot stand on the road.

In fiat country try to site diversion banks just above any slightdrops (slopes) in the road as the necessary 8-l 6cm fall for thediversion drain will be found in only a few metres.By diverting the flow off the road, the diversion bank protects thelength of road below it. There is no point in making a diversion bankright at the bottom of a slope since there is no road there for it toprotect. A big, strong diversion bank may be necessary, however, toprotect a bridge or a drift if the road above is eroding badly.There should always be a diversion bank immediately above acutting (this is where tr road has to be cut down some feet into the


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16 WATER CONTROL AND DRAINAGE METHODSsoil, as at the approach to a stream crossing where the stream bank ishigh and steep). This prevents any water running into the cuttingfrom the road above it (Figure I 1).

deep gullyeep gully*ad in long cuttingd in long cutting

Figwe I 7. Diversion banks in cut ting s.

Erosion in long cuttings can be troublesome because it is nor-mally impossible to divert the flow out of the cutting. In some caststhis may bc overcome if, in the first instance, the road is sited tocross a gully halfway down the cutting, as shown in Figure 1 1. Adiversion bank may then be made, discharging into the gully.Often a natural undulation may be nearly big enough to make anefficient diversion bank (Figure 12). A little inexpensive topping upof the bank is all that is required. Giversion banks should be sitedlike this when feasible.

natural bump fill

Figure 72. Diversion bank made from natural bump .There may be occasions on a village road where there is excessivesurface run-off water from the bush which would cause prolongedwetting of a long stretch of road. In such a situation the constructionof a catchwater drain (see Chapter 5, Section 2) is merited, as inFigure 13. Allow for this drain and choose the site for the bankaccordingly.


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DIVERSION BANKSflow

\ / \ \ 1,--___) catchwater drain--- ---.

Figure 73. Catch wa er drclin diverting excessive run -off water.Rocks are frequently found on hilly village roads. Labsur canoften be saved if the diversion bank can be sited to cover the rock. If,instead, the drain comes where the rock is, this will have to bc dugout or broken down.

Reduci~tg Erosion by Reclt~hg . lm-wn t of WaterIf storm water is diverted off the road at sufficiently close intervalsthere will be no erosion, and if there is a suitable place where thewater can bc diverted easily it should bc done. The aim must bc toprevent any dangerous build-up of storm water.If the flow is well controlled, then a steep gradient on the divor-sioli drains does not matter; they can bc: sited to discharge straightdown the slope (Figure 7) or in whichever direction it is mostconvenient for the water to get away.Paths Brir lging Stem Water otw the RoadA very goad way of reducing the amount of water on the road is toreduce the amount of storm water reaching it from the bush above.Much of this water arrives down footpaths, cattle and cart-tracksetc., coming from the land above the road. IF small diversion banksarc made along all these tracks, the water is diverted into the bushand hardly any will reach the road; what does reach the road willarrive there long after the storm has passed. In some cases it may beworth starting the top diversion bank 400m or more from the roadand inserting a number of banks below it, as in Eigure 14.It is a waste of time trying to divert water off a path if the path issited along the bed of a slight valley., i.e. along the watercourse itself.(The same is true of any road.) If water is diverted off the path intothe bush it will find its way back again onto the path because that isits natural watercourse.


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18 WATER CONTROL AND DRAINAGE METHODS

road

ant hill diversion bankst- _, .__,,,~_,. - __I 07 -* _/__ ,_ -ii?-,,c. -j ,,--a

Figure 74. Diversion banks on fuatpath s.

In such cases the path (or road) should bc resited to the side, upthe hill a little above flood Iwcl, so thaw cross-drainage will then bcpossible. Always allow water to follow its natural watercoursewherever this is feasible.2. Shape of Diversion BanksCompared with diversion banks in flat country, those on hills haveto be shorter and sharper, as explained below.

.Dirwsim Rmk s 017 he FlatTypical measurements for one of the larger banks arc shown inFigure 1S(s). The overall length of the bank is about 2Sm along theroad (see Figure 7). h*ieasured along the centre line of the road, thebankitself is about I2m long. The height to the top of the bank is30cm (i.e. there is 30cm freeboard.). The road therefore slopes up at1 in 20, then down again at 1 in 20 as it goes over the bank. Thisbank could probably be taken at 40 km/h in a car, without inconven-ience. The overall length of a diversion bank might be as short as15m. Since the height of 30cm must be maintained such a bankwould have a steeper gradient and would have to be traversed at aslower speed, say 25 km/h.


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20 WATER CONTROL AND DRAINAGE METHODSbe relied upon to divert the water, but will obviously mean slowerspeeds for traffic. Remember that the slope on the downhill side of adiversion bank is steeper than the road itself, e.g. in Figure I S(c) theslope of the road is 1 in 10 but the slopes on t hc clivcrsion bank andon the excavated drain, if any, are steeper, nearly 1 in 6. This makesit particularly important to avoid too steep a gradient along anysection of the road.FreeboardAs shown in Figure 15(c), the water channel is excavated a little sothat the necessary freeboard (about 3Ocm) may be obtained withoutmaking too big (i.e. too expensive) a bank. This can be done on hillsbecause it is easy to find run-outs, but in flat country it may bedifficult to find any run-out at all if the water channel is deepened.In flat country, therefore, all the freeboard must be obtained byraising the bank (about 3Ocm) and not by deepening the waterchannel at all. The principle must be that of a diversion bank ratherthan of a diversion drain.in effect the bank dams the water flowing along the road anddiverts it down the diversion drain.


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CHAPTERLead-Off

As the name implies, a lead-off drain is one that leads, or diverts,water from the side drain of a road on to the surrounding land fordispersal. Some people may think that it is sufficient if the water isremoved from the side drain only when a minor watercourse isreached. This would be all right if these points occured at suffi-ciently close intervals, but this seldom happens. Generally thenatural drainage lines are much too far apart and this makes artifi-ciai lead-offs, etc. essential. Lead-off drains are therefore used onmarket roads, to prevent too great a build-up of water in the sidedrain.It is important to have efficient lead-off drains wherever tllcv arerequired. One is shown, in plan, in Figure 16. The bolster block; theside drain and diverts the flow down P IC lead-off drain. If thelead-off drain is to work efficiently it must have a gradient of about Iin 125, or steeper, with a clear run-out at the end. The soil excavatedfrom the lead-off drain should be moved towards the road to formthe bo ster.1. Shape of LcaMMf BolsterIn Figure 16, the broken line outlines roughly the base of thebolster. The base widens where it crosses the side drain; it mav benearly Gm wide at that point so it will be quite a sizeable earthwork.The top of the IeHd-off bank and the top of the lead-off bolstershould be at the same level throughout, as far as the end point Pwhere it ties in to the road.

Every lead-off bolster should be fully stable and permanent.Cattle treading on it should not harm it much and, if a mistake hasbeen made and a little storm water does flow over the top. it shoulddo little damage. This is illustrated in Figures 17(a), (b) and (c). Ineach case the flood level of the water in the lead-off drain isindicated. Both (a) and (b) are stable but (c) is very unstablebecause of the short, steep slopes used in its construction.


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22 WATER CONTROL AND DRAINAGE METHODSrun-out/f-

3.6n-1I cmagewaywerqe-m-m-&-v-- - ~-__------- . .-- ,-a_--- /.--,--k-v-=----/ ----s-d/ ~~+H-=-a

-------++ siele drain /=- -A,,* /--/--~~,__il -_-.F. ____/_ --_I ?.____l__rrII__

8

Figure 16. Plan of ead-off d rain.2. Choosing the Layout for Lead-Off DrainsEach lead-off bolster blocks the side drain at an angle of nbaut30 degrees, much the same as a diversion bank blocking a villageroad. In order to get the water well away from the road the lead-offdrains are extended nt a gradient of 1 in 125 for a sufficient distanceto prevent the run-out flow reaching the road again. In Figure 18(a)there is no crossfall on the road so the lead-off drains are the sameboth sides. In Figure 18(b), where there is a crossfall, lead-offs fromthe upper side are longer and more expensive than in (a). This isnecessary so that the run-off water from the upper lead-off drain,which will tend to run back towards the road, is caught in the nextlead-off drain, rather than in the side drain. Lead-offs from the


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LEAD-OFF DRAINSEAD-OFF DRAINS

lead-off drainead-off drain bolster bankolster bankwater max. heightater max. height

?-. ..,- _._-,_ l--_--*~,-_- .~-. ..,- _._-,_ l--_--*~,-_- .~4m or morem or more

23

(a). Correct.

I- .__-.__.- --.-I- -- . _ _ Qm ~. . . .__.___.-._.---J

161. correct

4p--w,w 3ppl --7w-J/c). Incorrect.

f igure 77. L ead-off drains in flood .lower side are very short and inexpensive so more are inserted. Thisis why the side drain can be reduced in size, as shown.Note that village road diversion banks always discharge to thelower side whereas lead-off drains on market roads sometimesdischarge to the upper side (Figure 18(b) ).


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slope-_I-- -,-(a). No cro ssf all.

lb). With cross fal l .

Figure 18, daysu t sf 1~~~~~~3. T9 LeaQ4MTBrainsNormally kad-off drains arc at an an#z of about 30 degrees to theside drain (Figure 18). But where a watc~shed crosses a dip (called asaddle) between two hills water will cot&t in both sklic drains at thispoint. Therefore a T lead-off drain I .x tskm off from each side atright angles. as shown in Figure 19. These drains do not have to beexactly opposite each other. Water approaches a ii*ead-off drainfrom both sides instead of from one side Io~~ly. s in the case of anormal lead-off drain.The beds of these T lead-offs drains arc given a gradient of 1 in125, or steeper. When excavating the drain aCI oil is carried to theroad, and spread so as to raise the road along the lowest section,


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slope

w lead-off drains

25

sbp@ side dreins

Figure 79. TIead-off drains on saddle.keeping it level. Ihis means the T lead-off drain need not be sodeep and l ong.4. Road Crossing A Gentle Saddle in Nearly Hat CountryDetailed survey (with a surveyors level) on such a road may showthat a loq lead-off drain could remove standing water from the sidedrain. A better solution to the problem might, however, kc to raisethe whole length of road across the saddle, in the form of a lowembankment, with no lead-off drain at all if the country is very flat.The road lint must follow the exact watershed. This is a safermethod than relying on one long lead-off drain, at a very slightgradient, to remove the water, especially if the area is likely to becultivated. Such a drain might easily get blocked at one point.

i 5. Correct Spacing of had=Off BrainsIt is not possible to state the correct spacing, nor can it be deter-mined by a formula, if run-off from the bush is liable to reach the


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26 WATER CONTROL AND DRAINAGE METHODSroad at several places. Depending on this run-off the requiredspacing might vary from 20m to 200m or more. The overseer shoulddecide by experience, noting carefully any evidence of the amountof storm run-off water reaching the road.What really matters is the volume of water flowing in the drain:the distance between lead-offs is incidental.


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Culverts1. Drifts and SplashesA drift is where a road crosses a stream or river bed with the waterflowing over the road. Splashes are very similar but on a muchsmaller scale, applying only to local run-off water which has to crossthe road. Normally, they will be sited in slight natural depressions(watercourses) but if there are none of these and the ground is flatthen they will have to be made artificially, for example. by making adip in the high-level road.The basic requirements for both drifts and splashes are verysimilar. In order not to inconvcnienee traffic fhe water must notflnw deeply over the crossing. Furthermore, ahc crossing must benegotiable at reasonable spectl by traffic when thcrc is little or nowater flowing across it.A wide crossing with gentle slopes is thcreforc essential fromboth these points of view. A steep-sided narrow watercourse wouldmake a very dangerous crossing for vehicles travelling at speed inthe dry season. Moreover storm water in the rainy season wouldflow deep and fast, and this would inconvenisnce traffic and mightprcvcnt a drift being crossed.In the interests of economy, gravel and stones must be dependedon, in nearly all cases, for surfacing the crossing, ccancretc being tooexpensive. If there is a fast flow of storm water it will not affectconcrete but it will saon scour out and wash away a gravel or stonesurface. The speed of the water must therefore be rccie.ced so thatthis does not happen. Grass often helps to bind the surface butit, too, will suffer if the rate of fIow of the flood water is exces-sive;In some streams which flood deeply the water will move onlyslowly; this is because the gradient on the stream bed below thecrossing is slight. These deep, slowly moving floods do no damage atall to earthworks but the depth of the water will close the road totraffic (Figure 20(a) ); this must be avoided if possible.


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28 WATER CONTROL AND DRAINAGE METHODS2. A Free Fall Reduces Depth of FloodingIn any watercourse, big or small, with only a slight gradient, arelatively large flow of water tends to bank up (i.e. flood deeply)because it cannot get away quickly (Figure 20 (a) ). If. however,there is a raised roadway with a clear fall along its lower edge(Figure 20(b) ), the water crosses freely over the fall and does notbank up; it remains shallow and vehicles are able to pass.A quick get-away, i.e. rapid water flow, just below the crossinghas much the same etfect as raising the roadway; water will not tendto bank back deeply on the crossing. Whenever feasible, therefore,crossings should bc sited at such spots in order to reduce the amountof embanking required. Rapid water results, of course, from the bedof the stream being on a steep gradient.

bed of streamFigu re 201al. Slight gradient, deep flood.

roadway raisedFigure 20(b). Free fafl, shallow floo d.

3. Widening the Crossing Reduces Depth of FloodiagIf there is a free fall in both cases, the depth of flood in a widercrossing (Figure 2 1 b) ) is much less than that in a narrow one(Figure 2 I (a) ). The narrow crossing is more liable to hold up trafficand is also more likely to erode, owing to the greater speed of flow.The two principles just discussed apply equally to but h splashesand drifts. l3y applying them intelligently many difficulties, e.g.deep fh)oding in narrow streams, can often be readily overcome.


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DRIFTS, SPLASHES AND CULVERTS 29stream

Figu re 2 I(a). Narrow truss ing, deep flood.

raised roadway

Figu re 2 7/b). Wide crossing, shal low flood .

4. Stone-Pilled Wire GabionsA gabion consists of a heap of stones wrapped up and held in placeby wire netting. It is often placed along the lower side of a drift orsplash to support the gravel and stone filling and prevent it beingwashed away by flood water. This type of gabion is so heavy thatonly in exceptional circumstances would it be moved by a flood. It ischeap and easy to construct. One of its chief merits is that, if forsome reason subsidence does occur underneath it, it will settle andblock the cavity and no damage will be done. A concrete or masonrywall, on the other hand, would crack and perhaps collapse undersimilar circumstances.As gabions are so inexpensive, drifts and splashes can be made aswide as practicable, in order to spread the flood. A flat stone apronmay be necessary in orJer to prevent undercutting on the down-stream side (Figure 221,5. CulvertsCulverts are pipes that pass under a road to take storm water flow,rather than allowing it to flow over the road. They are normallysituated where natura watercourses cross the road. However,culverts are sometimes placed at intervals along a road that runsslightly downhill across a long even slope to transfer storm water tothe lower side of the roSd. Culverts are very expensive and are not


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gabion flat stone apron

Figure 22. Use of gabio n.warranted on village roads, only on market roads. The diversionbank alone is therefore depended upon to divert the storm watertlow on a village road.6. Types of CulvertThere arc three main types of culvert that may be used:

1) Prc-cast concrete pipes.2) Masonry construction.3) Reinforced concrctc construction.The emphasis in this chapter is on concrete pipe culverts. However,if the necessary construction skills are available, and the costs areattractive, the other types may be used. There is, in fact, a fourthmethod of constructing culverts using wooden logs- these are cheapbut do not have a very long life.7, Flooding of CulvertsWhen any culvert is installed it must always be assumed that, sooneror later, it will overtlow. It is often far too expensive to build it bigenough to cope with the worst possible storm, and a culvert is alsoliable to become blocked with debris. Provision should therefore bemade for emergency spilling of excess storm water over the road.Blocked Culvert PipesSome culverts are at the bottom of a slope, where the road crosses anatural watercourse. There may be other culverts hig:ler up theslope, where there is a long, gently sloping stretch of road with acrossfall but with no natural watercourse (Figure 23(a) ). If theformer type (culvert C in the Figure) becomes blocked the waterwill spill straight across the road to the lower side because the water


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DRIFTS, SPLASHES AND CULVERTS 31stream in valley

slope-----lbculvert A culvert B

Figur e 23(a). Culverts on slopes.

slope

bank diversion

run-outFigur e 23181. Use of bank div ersion.

is confined to its natural valley and cannot go elsewhere. This typeof blockage may cause a washout across the road, which is inconven-ient Ijut does no further dama e and is easily mended.On the other hand a blocked culvert further up the slope mayresult in very extensive damage. If culvert B blocks upI water willflow down the stretch of road from I3 to C which may becomecompletely eroded. If culvert A blocks up, the road from A t;, B islikely to erode, and this in turn is likely to block up cuIvert B, so thatBC also gets washed out. The damage done is far greater than thatcaused by any blockage at C. Except, perhaps, where there is astrong crossfall, hundreds of tonnes of soil may be lost on these longslopes and this will all have to be replaced.This damage is easily avoided if a bank diversion, as in Figure23(b), is constructed just downstream of the culvert pipe. Make use


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32 WATER CONTROL AND DRAINAGE METHODSof any natural rise, so as to reduce the amount of fiII required, bysiting the culvert just on the upstream side of the bump.Exampk of Sap Road Dri f i With CrhertFigure 24 shows a stream crossing (similar to culvert C in Figure23(a) ) which is a combination of three rows of culvert pipes and aroad drift held by a stone gabion, with emergency spillways. Thehump of soil above the pipes in Figure 24 is necessary in order toprotect the pipes from heavy traffic.

gabisn hump

i1

--- ,-. . .- --, .-__.- ,ce. - - --* - -- I~ -T I I=--j emergencyspillway

(earth)road drift - *, L=: i emergency j

(gravel andstone) 3-pipe 1

spillway (ii culvert in l ~ea~~~stream bed 0

Figure 24. Side view of z&e rslod ri f t with ~~fv~~~

The three rows of pipes arc laid in the stream bed; this avoidshaving to excavate a run-out drain. They are 3&m, or more, belowthe Ievcl of the drift (i.e. the top of the gabion). If, during a sharpstorm, the. stream tIow exceeds the combined capacity of theseculverts and of the drift, the surplus flood can spill out over thegrassed earth road on either side, as indicated.The culvert pipes must not be too much below the Ievel of thedrift. One emergency spillwq might be about I cmhigher than thedrift and the other perhaps 15cm hi her still, often the two spill-ways can bc on the same side making one long spillway slopingslightly, perhaps, from one end to the other. The lowest sectioncould be made into the drift. It all depends on the convenience ofthe site. The longer the spillway can be made, the better.The two ends oft he gabion zhould rise slightly, say 1Scm in Sm, inorder to preserve Ste slight slope up required at either end of thedrift (Figure 24). The Iength of the gabion for the drift will dependon the site and on the size of the stream when in fIood. It is notunusual for the gabion to be 20-30m or more in length, with emcr-gency spillways in addition, as an insurance.


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DRIFTS, SPLASHES AND CULVERTS 33High FloodirrgIf the behaviour of the stream when in full flood is not well known,consult local villagers. Ask them how high the flood has been knownto rise, then imagine this flood (maybe 1OOm wide) flowing over theproposed road crossing. A flood covering the whole valley shoulddo no damage to the earthworks, once they have grassed over, if thedesign is sound as in Figure 24.There may, however, be a gully in the stream bed just down-stream of the crossing and this could prove dangerous because floodwater will drop down into it, which will tend to erode the: side of thegully back towards the road.The remedy for this is shown in Figure 25. Direction banks arcmade to prevent the flood flow from falling into the gully just belowthe crossing. For the same reason it is advisable to make these banksat every stream crossing where there is a culvert with a deep run-outdrain. Gabions, anchored into the sides as shown, may be necessaryin order to mend the gully if it is eroding due to hcnvy fIooding.

-a=---v---

- ---

w--.@+ -gabions

Figur e 25. Use of direct ion b anks.

Crosshg a Strawa Bed Subject ts Sewrt, Floodi~~gIf the site is selected with care the flood may cause no inconvenienceat all. The first essential is to be able to spread the flood over asufficicnlly wide area of spillway, which will not erode or which caneasily be protected from erosion. If the flood is not spread out, andthe traffic is not to be held up, an expensive bridge must be built.


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34 WATER CONTROL AND DRAINAGE METHODSII , gabion---------------- cut-------spillway ------- ~~---------a

------Atraining wall

II Em-I- -*-- r_^-----eiA -- ...mlli Ucal I I wall c--,__c\soillway gabions ct ,-__ x_4-- -- - ------- _

dam basinexc v tedg bion

Figu re 26/a), Comp fax crossing for severe f~~~di~~~

dam wall

Figure 26(b). Side view o f crossing along road.

The flood can be spread by dividing it in half as shown in Figure26(a), (b). First the whole stream bed is searched until the widestpossible crossing point is found. The stream bed is dammed, leavingmaximum space for a spillway either side. A drift is constructedwhere the road crosses each of the spillways. Between them it runsup and over the dam wall. Direction banks (Figure 26(a) j preventthe flood returning directly into the stream bed.


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DRIFTS, SPLASHES AND CULVERTS 35The spillways will need some cutting and filling in order to give awide, horizontai cross-section for some distance (Figure 26(b) ).Gullies, etc, in the spillway could be protected with gabions, orblocked, as shown (Figure 26(a) ). Training walls of heavy, looserocks may be useful in order to encourage flood water to flow in thedirection required.The road runs over the top of the dam wall; if the spillways areadequate the wall need only be about lm high.Obviously this layout is only feasible if the valley is a suitableshape. The spillways should not riced much cutting. and only a verylittle filling, to make them horizontal (i.e. horizontal at right anglesto the line of flow). Atry Iiiid-in places shauld be consolidated.The danger point, regarding possible erosion, is most likely to bewhere the flood drops down to the stream again; gully erosion isliable to eat back from this point to the spillway crest (in this case,the road). &event this, whcrc necessary, with gabions (Figure26(a) ). It pays to discharge the water back into the stream at two orthree points, instead of ut just onto point, in order to reduce theerosion Lability.From the foregoing observations it can bc said that big floods instreams, and even in som rivers, can bc negotiated satisfactorily(using only gabions and grass protection, without concrete work) ifthe floor of the valley is wide and flat enough to allow the flood tospread sufficiently. I n narrow valleys the water cannot spread out.

The flood will then run deep and fast, and, in consequence, any roadcrossing will have to be constructed in concrete which is very expen-sive; it is an engineers job and is not considered in this book.8. Rctn4ht Drains from Large PipesA 6Ocn1culvert pipe ~~ccds 6Ocm of road soil on top of it to prevent itbreaking. The hcd of its run-out drain must therefore be 1.2m belowthe level of the road.Where there is only a slight crossfall, if the road is not wellembanked, a deep long run-out drain will be required (Figure27(a) ). These deep drains are expensive to dig and to maintain andthey will give continual trouble in future years. Discharge from asimilar pipe at a higher level on a well embanked road (Figure27(b) ) is much more satisfactory, and much cheaper, because thepipe is nearly resting on the surt;lce of the ground.The depth at which the culvert discharges cm be reduced in tw,o


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I 36 WATER CONTROL AND DRAINAGE METHODSnarrow valleyroad surface

pipe level

(a). Pipe below g roun d level , deep drai rxI embanked road

(6). P@e at grou nd /eve% shaffow drain.

Figur e 2 7. Run-su t drains fro m farge p@es.

road surtace

\ 6Ocm IIrn

. . . .-- C-- -I---A( >Ocm\ pipe culvertsu---I--x_ - _---_ --- _adischa


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DRIFTS. SPLASHES AND CULVERTS 37ways. One way is to place a length of stiff metal, e.g. the tailboard ofan old lorry, above the pipe as in Figure 28 type B. This protects thepipe from heavy traffic so that the soil cover can be safely reduced.The other method is to make a flat, reinforced concrete box culvert,type C in Figure 28, which only needs a thin covering of soil above it.Construction is, howevor, an experts job.


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CHAPTER 5recautions gainst Silting

While it is easy to control erosion on earth roads by inexpensiveconservation practices, control of silting is somctimcs extremelydifficult.1. Types of SiltingSill LIepos ited itt Stil l WaterSilt-laden water flowing into a dam will, in time, fill it with silt up tothe level of the spillway. In the same way a side drain will silt up tothe lcv~l of the hcd of its IGKI-off drain (Figure 29)-

bed of side drain

Figure 29. Cross-section th rou gh a s&d side drain.

Si l t Lkposited by Rmrl ing Wa rrIf silt-laden water tlows slowly through thick grass some of the siltwill be deposited, even on a slope. The grass will grow up throughthe deposit and more silt will be accumulated. This can happen as agradual process in a grassed side drain far above the level of itslead-off drain.


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PRECAUTIONS AGAINST SILTING 39Sand Deposi ted by Uncontro l led Floods of Local Run Off WaterWhen flood water gets completely out of hand and a series oflead-off drain bolsters are washed away during a storm, the upperreaches of the road will erode badly. At the bottom of the slope, if itis fairly flat, long sheets of sand will be deposited in the side drainsand perhaps over the whole road too, obscuring it entirely. Most ofthe coarser particles (sand and gravel) carried down by the flood aredeposited and the finer particles (silt and clay) are removed insuspension in the water.2. RemediesReduct ion of Sil t Load in Run-Off Water Reaching the RoadImproved conservation agriculture and forestry above the roaddoes a lot to reduce the amount of run-off water, and the amount ofsilt, reaching the road in small channels, etc. Qften an easy way toreduce the amount of silt, and the amount of run-off water, reachingthe road is to make diversion banks on all villa roads, paths, cattletracks, etc. throughout the entire catchment in question. A very bigimprovement is thus obtained at little cost.Si lthg Due to Surface Wash Off the RoadIf the surface of the road and side drain, etc., is washing badly it willcause silting in the drains further down. Surface wash can be pre-vented almost entirely by allowing an earth surface to grass over.This is why, in the high-level method, grass growth is encouragedeverywhere except on the carriagway itself, if it is to be gradedmechanically. On village roads the carriageway too is allowed tograss over, since maintenance is normally by hand.Catch water DrainsWhen a gully, or small watercourse, meets the road, no damage iscaused if the flow of water is taken straight over, or under, the road,as in Figure 10(a). Even if the water carries a heavy load of silt, it isall washed straight across the road and away. No silting is caused inthe side drains. The damage occurs when the water is allowed toflow along the road, as in Figure 10(b). The road erodes, or silts up,or both.The principle should therefore be to prevent numerous smallgullies, etc. from discharging silt-laden water into the side drain


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1clope1

(al. .catchwaterdrain

(6).\ slope

\

minor\ully */

Figur e 30. Ca ehwa ter drains.


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PRECAUTIONS AGAINST SILTING 41(where the silt would be deposited) and, instead, to collect thiswater before it reaches the side drain and then to discharge it acrossthe road at given points. It is collected by means of catchwaterdrains (Figures 30(a) and (b) ). Figure 30(c) shows a similar catch-water drain protecting a flat village road. As they are completelyseparate from the road, any erosion along the bed of the catchwatcrdrains will not endanger the road itself.A large flow of storm water rcquircs a large catchwater drain totake it away but the gradient must not be too steep. A gradient of 1in 250 should be all right but a stccpcr gradient may be allowed ingravels, etc. If only small flows are being considered, the drain canbe constructed with a slope of 1 in 125. The steep drain in Figure30(b) would probably be much steeper than 1 in 125 but it might besatisfactory in erosion resistant soils. If it carries a large flow ofstorm water, it will need one or more lead-off drains to divert waterinto the gully.LmdQtJ DrainsIf much silt is carried in suspcnsiun in the water which tlows in thelead-off drains, it will tend to deposit along the flatter stretches ofthese drains. Grass will grow through this dcpasit and mares silt willthen be held up. Soon the lead-off will cease to flow; the storm waterwill by-pass the intake bolster and will flow on down the road.Frequent maintenance is necessary to remove the successivedeposits of silt if the lead-off is to be kep\ wcrking. It is generallymuch cheaper to divert the silt, by means of cat&water drains, etc.before it rcnchcs the side drain.


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PART 2


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CHAPTER 6Surveying

1. Selecting the Road LineFirst make sure that all interested parties agree on the piaccs to belinked by the road, and the standard required, i.e. market or villageroad.GradientsIn the case of village roads it is as well to assume that they may beconverted to market roads as the country develops. Steep gradientsmust therefore bc avoided. In later years loaded lorries will be likelyto use the road rluring the rainy season, and while they will not havedifficulty ascending gradients of 1 in 18. a hill of 1 in 12 might causewheel spin in the wet. On most soils an earth slope of 1 in 8 woulQprove impossible for a loaded lorry to climb in wet weather unlessthe surface was gravelled; but that is expensive. Ot s muc11 etter,therefore, to increase the length of the road somewhat if. by sodoing, steep gradients can be avoided.To upgrade a good village road to a market road is not anexpensive operation (except on steep crossfalls) but to reduce a longgradient from, say, 1 in 7 to 1 in 10 means makingacompletely newroad over that stretch; this isobviously very expensive. Always planroad lines, therefore, so that they will not have to be shifted in futureyears.Wil ters lwd Road LirwsCross drainage on a market road is either under the road (bridge orculvert pipes) or over the road (spl ash or drift); both are expensive.If a true watershed road line can be followed none of this crossdrainage will be necessary; lead-off drains on both sides will con-duct water away into ;he bush and this is cheaper than conducting itacross the road.Consider Figure 3 1. A road is required to run from point X tocross the stream. The best place for a drift is found at Y but the


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SURVEYING 35Y stream

Figu re 3 1. Watershed road line.country bctwccn X and Y is unknown. To find the watershed routebetween these two points start from the stream at Y and workuphi l l ; this will be route E. If a start is made. instead. from X thewatershed shoulders A, B, C, D and F might all be explored in errorbefore the correct route E is found.

Roads cannot always follow watershed lines. Consider Figure32(a): this watershed consists of a rock ridge, impossible for anyroad, A road from C to D will therefore have to pass below thewatershed, with a crosstall all the way. In bigure 32(b) the truewatcrshcd route is undesirably long so a shorter line is chosen; thisroute involves a stream crossing and stretches with cross drainage.

, I I I/ %T;:g;

(al.

\ . . ?i

lb). watershedFigure 32. Roads no t on watershed.

The cost per kilometre of these roads will inevitably be greaterthan that for similar roads along watershed routes.Straight Roads Lksi rabl~~Roads which twist and turn without good reason are not only anuisance, they arc more expensive to maintain and may even bedangerous. On sharp corners, high grass and tree growth on the


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46 LOCATING THE ROADinside of a curve has to be cut down in order to reduce the risk ofaccidents due to poor visibility. Long gentle curves should thereforerepiace sharp corners wherever feasible.Figure 33(a) shows another typical problem. To avoid the twostreams the road must pass throught points A and B; this is awatershed line. The two points are reasonably near, but not toonear, to the sources of the two streams, 1 and 2. Point C is thenearest point to the hill where the road can pass. Point D is the bestsite on the big stream for a drift where the road can cross. The roadline ABCD is therefore satisfactory. The curves are gentle and theroad cannot be shortened at all. Before deciding finally on point C,the possibility of a better road line round the north side of the hill(opposite to C) should be fully investigated. Another possibleimprovement might be to bend one of the straight lines AB, BC orCD slightly, if required, to keep exactly on the watershed; themerits of doing this would have to be considered.In open country it is easy to peg a straight line by eye between twopoints A and B, etc. but in thick bush country an overseer is neededWIIOcan use a compass and plot a traverse, as described in thefollowing section.2. Traversing the Road LineThe overseer will need an oil-swung prismatic compass, a 50m tapeor a cyclometer wheel, ruled or squared paper, a board to write on,a protractor, rules, pencil and flags.The following example is given to illustrate the method. Theoverseer starts off his survey from point A in Figure 33(a) to crossthe big stream D to the east some 9km away through unknowncountry. He is told of the hill near C so he begins his traverse bytaking a reading of, say, 03 degrees on his compass (Figure 33(b) ),hoping to pass just to the south of the hill. He walks on this bearingfor 3.15km, at which point he finds he is approaching stream No. 2,so he stops. Me decides to change direction and walk round the topof the stream (the source) so that he can get a watershed road line.He therefore sets off again, this time on a bearing of 11 degrees,walking upstream. The overseer stops at 0.7km and changes direc-tion again to keep near the stream (No. 2); he then walks at 24degrees on the new line and stops after 0.45km on this bearing. HeA cyclometer is a standard bicycle wheel fitted with a simple device for measuringdistances.


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SURVEYING 47

North

(a). Choosing the road fine.

1 MagneticJ-or th Bresolved 3,58km O.& kmV. -.M. . . . . .at 255V.,A

3.15kmat93Ostreamno. 2

lb). Plottin g the traverse.compassline

-------9Ax BXerror line

(cl. Pegging the fine straight.

Figur e 33. &we ying the road l ine.


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48 LOCATING THE ROADconsiders he has reached point B in Figure 33(a) which is where thefinal road must make a slight change in direction round the top ofstream No. 2. This is point B in Figure 33(b). The correct road lineshould then be a straight lint between B and A.Plot t ing the Trmww urd Rm~lving ItThe overseer plots his traverse carefully on paper - from A, 3. I5kmat 93 degrees; 0.7km at 1 I degrees; 0.45km at 24 degrees ==B(Figure 33(b) ). He then resolves the line BA which gives a bearingof 255 degrees (from B) and distance 3.SXkm. He then sctsoff fromB on a bearing reading 255 degrees. We marks the line as he goeswith flags and stops at 3.5Skm; thas should be the point A. No doubtthere will be some error, in which case he measures the distance andthe bearing of this error line to reach A. If the crrar is big he mayhave to plot his traverse agai:l, this time from B to A, then resolveAB, measure the bearing and walk the new line AB on the newbearing.In the above cxamplc it is assumed that the final line AB is free ofobstructions and is suitable for a road. If it is not, the best route mustbe found by rcpcating the proccdurcs auttined.Cbr~~pkting the TrawrseIf the traverse ended at B then the line bA would be resolvedimmediately. However, if in the present traverse (Figure 33(a) )there is still some distance to go beyond B, the line AB is left tilllater. The ovcryccr carries on from B traversing to C and on to ILHcrc hc plots all his travcrscs resolving the lines AB, CB, DC.IHc must make quite sure that the most suitable spot has beenchosen for the drift over the stream at I. This might mean searchingupstream and downstream for perhaps a kilamctre or more. If a hadsite is selected there may be major problems in future years. Somehours, or even days, spent investigating the relative merits of thedifferent possible sites is time well spent.The overseer walks back along the resolved bearing DC, onelabuurer marking +Jw ine behind him as he goes. If the whole lint issuitable for a ro& and if it passes through, or near, point C heapproves it and continues on from C to B on the new resolvedbearing. If this too is suitable and passes near B he continues to A(his starting point) from B, again on the new resolved bearing whichin this case is 25s degrees (Figure 33(b) ).


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SURVEYING 49Marking the Compass Lir teThis is the line trodden by the overseer reading the compass. Thelabourer following him marks the exact line with periodiccuts with ahoe and he also slashes trees adjacent to the line so that it can befound again without difficulty.Pegging the Line StraightThe compass line will not be perfectly straight but will wanderslightly. The line can be made straight using tall pegs with flags asmarkers.Consider the compass line BA in Figure 33(c) which is about3.5km long. Do not start pegging the line from one end. as in thecase of the error line in Figure 33(c). Here a point Bx, exactly on thecompass line, has been selected and a line from B through Bx hasbeen projected to Ax; very large errors are likely to result framdoing this,Instead, start nearly halfway between A and B, (FiPut in ii tall peg A,, with a flag, cxact y 011 hecompass line, in one oft hc hoc marks showing where the overseer trod Go a short distancetowards B and put in a similar peg B, also with a flag. Cut justenough trees alongthis line AIB, to cnablc one flag to be seen fromthe other one. Move both flags outwards, to A, and B, (which arealso on the exact compass line) and go on cutting in between so thatthey cm be seen the one from the other. Move them outwards againto A:, and B:, and continue cutting in between to clear just enoughfor the lint of the two flags to be seen.This method coc~lel e continued until the whole line AB, iscomplctcd; alternatively, if the complctc line of sight is obstructedby undulations in the ground, the line A$, could be prajected bothways to reach up to A and B. In this way a perfectly straight road lineis obtained.Aecordi r lg Okstr i tc t ior ls Mw Compiz~s LirwIn cases where the line may have to be shifted slightly, it is helpful totake note, in passing, of all rocks, gullies, huts and other obstruc-tions near the lint on both sides. By inserting this information on aplan, an improved line avoiding all obstructions can often beseloctcd without a further exploratory survey.


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CHAPTER 7Construction of

1. PeggingThe road line is first surveyed and peg ed. Tall straight pegs, about1 m high, arc inserted along this lin at about ) S-2()m inter\sq)s.This will form the centrc tine of the road.Next cut a straight 2-l m stick. Lay it down at each peg, at rightangles (90 degrees) to the line, and hammer in a side peg at theother end of the stick (Figure 34). Repeat on the opposite side oft heccntrc line. To get the right angle, stand at the pegon the centre line,look at the stick then took at the line, both ways; repeat two or threetimes, moving the stick as required. Take a new ri 111 ngle for thesecond peg and do not just project the two former pegs. The twooutside lines of pegs now show the edges of the +2m road (Figure35(a) ). The side pegs should be strong and about I m high. Theyshould be hammered in hard because they may have to remain inplace for many months.2. Dimensions of RoadThis 4-2m road (pegged with the 2~lm stick) is the minimum formarket roads. If preferred, a 2-4m stick, giving a +8m road, may beused. If the ground is liable to be at all clamp, the 2-4m stick nrruf bcused. Use a 2-7111 stick if the ground is liable to be wet and a 3*0mstick where it gets very wet. Where the road has to go through

Figure 34. Inserting side pegs.


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CONSTRUCTION OF MARKET ROADS 53swampy ground with deep, soft mud and standing water, use a 303mstick (giving a 6-6m road, 1302m overall including drains). For animportant market road passing through wet swamp a 3.6m stickmight be used.The width of the road will therefore vary as the drainage problemvaries. The width of the carr iagwaJ7, ?m rmu y is the same for al leurr*h roa& namely 14m either side of the centro line, total 3-6m.This remains exactly the same whether the road is 4.2m or &hmwide. It is the widthof the verge which varies, in thiscasc from 3Ocmto Mm.3. Stumping and ClearingEvery tree stump, big or small, alive or dead, must bc dugout dcqlyby the roots. Encourage labourers to dig big holes so that the sideroots can be cut further from the stump; they will then be less likelyto shoot up again.

Cpt hakun1 11 IIIWreyed ine)

Ibranch to cut off 1

tall pegstrpng side

drain \ 1clrainI 1 stumwd 6-2m n1__1- -~ _..i~_____~-~-_~~ .-...m--

Figure 3518). Clearing, stump ing and m arker drains.

On market roads stump to 1m outside the road limit on both sides(Figure 35(b) ). The two lines of pegs will then be clearly seen. Ifthere is one big tree just outside, or on, the line it might be allowedto remain, in order to keep down costs.Some trees have stumps that can be burnt far down into theground; this is the easiest way to get rid of them. If available,working cattle, or a tractor can be used to drag away the small


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54 CONSTRUCTION METHCX9S

---.---J marker

Figur e 35(b). Extending marker drains.stumped trees without further cutting up. Bigger trees will have tobe cut so tnat the pieces can be pulled away one by one. Dump thesein the bush well away from the road line. If branches of treesoverhang the road at all they must be cut off (Figure 35(a) ).4. Rocks in RoadDig out rocks and remove them, if feasible. If not, break them downto ground level, or lower, with a heavy sled e hammer. If ncccssary,make a hot fire on top of them for 2 or 3 hours, then remove the fireand ashes and immediately throw on plenty of cold water. The rockshould crack and can then be broken easily with the heavy hammer.

Grass is probably going to grow on the verges of the road conceal-ing any low obstruction, such as a rock, from drivers. This is poten-tially dangerous, and if it is not feasible to remove the rock or breakit down to ground level, it should be permanently marked, e.g. by awhite-painted iron stake, fixed so as to show above the grass.After all trees, rucks, etc. have been removed, fill in all the holes,throwing off remaining sticks, and spreading all bumps. Hammer inall pegs again, keeping the centre line. A small 4-wheel drive vehicleshould now be able to travel along either half of the road forinspection purposes.5. Marker DrainsThe two lines of side pegs, which show the road limits, must now bemarked (Figures 34 and 35). One man can do this. He ties a string orrope between two of the side pegs and pulls it to lie straight.Standing on the string in order to hold it in place, he marks the line


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CONSTRlJClTION OF MARKET ROADS 55of the string on the ground by scraping the line with a sharpenedstick and having reached the peg he removes the string. Then, facingalong the road, he takes a shallow cut (about 8cm deep) with hishoe, along the outside of the line made with the string, as shown inFigure 35(b). The marker drain is no wider than the width of hishoe, say 20em. Swinging his hoe he throws the grass sods to the lowspots in the road. He should move backwards, throwing the soilsideways. Trim the inside edge of the marker drain and leave itclean; this edge will be required in future. Leave the outside edgerough.Both marker drains are dug. even on a steep crossfall. With themarker drains completed it is easy to see if anyminor alterations inthe road line are needed. Not much work is lost if, at thisearly stage,it is decided to shift the line a little; in this case new marker drainsare dug.6, Widening the Drains:On hand-made roads the side drains, measured from the side peshould be as wide as, or wider than, the len 11 fthc stick used W~CIIpugging, e.g. a 4*2m road (pegged with a - 1n-1 tick) should llavcside drains 291m or wider. Figure 36(a) shows the final shape of theroad.The first main digging operation (No. 1) consists of extendingboth marker drains outwards, di ing 6Ocm wide and 15cm deepand throwing the soil to the road (Figures 35 and 36).Use a stick 8Ocm lang for checking the width measured from thepeg line (Lilcm for marker drain plus 6&m for No.1 drain). Theforeman must see that each labourer carries his own measuring stick8Ocii1ong.This No. 1 operation should fill up all the places in the road, thusproviding a reasonable running surface. Any high spots must belevelled without delay. Stumping was originally done to about lmoutside the road limit pegs so No.1 excavation is all within thisstumped area (Figure 35).The centre line of pegs should now be taken out so that traffic canpass over the whole road and compact it. More depressions willform, especially where the stumping holes were. No.2 operation isthen done (Figure 36(b) ). It is exactly the same as No.1 task,namely 60cm wide, 15cm deep, now measuring from the side pegline with a stick 1.4m long.


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56 CONSTRUCTION METHODS

or more or more/rjgilr ~36##4$). ;ir.d nLmr n -4.. &.:I.. &.~.~.-..$.*tro .&oIUI aiup bi WI u rrryrr revelllQIno. 3 no. 1

Figu re 36(b). Widening the side drains.

Some trees may occur in this strip. All soil and grass is thrown tothe road and levelled; small bushes, etc. are discarded, but biggertrees are left in place in the drain so long as thq have not beenloosened by the excavation. No.2 operation should provide a slightc:im bcr to the road.No.3 operation (Figure XI(b) ) should be done some monthslater when further incqualities in the road surface become visible.Dig as for No. 1 and 2 but use a 2m stick, again measuring from theside peg line. As usual the labourer spreads the soil thrown to t hcroad; at the same time any bumps must be cut down in order to get agood level running surface over the whole road. A lot of trees maybe left standing in No.2 and 3 drains but they will not impede hoeand shovel work as it is easy to work between them.No.4 operation could be done the following year (use a stick 2.6mlong for measuring) by which time the road surface should be wellcompacted.Each of the four operations is in the nature of a re-surfacing whichfills in all the low spots. To put all this soil on in one operation is


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CONSTRUCTION OF MARKET ROADS 57bound to leave a very uneven surface after traffic has consolidatedit. There is nothing to prevent an operation No.5, and even a No.6,being done if the road has to be enlarged at all.

7. Widening the Drains on CrossfallsNormally the side drain will be about 15cm deep (Figure 35(b) ).On steep crossfalls the side drain starts at this depth (upper sideonly) and increases across the width of the drain into the hill. Wherethere is a lower side drain (i.e. on slight crossfalls) the beds of thetwo drains should be level with each other, i.e. horizontal, as shownin Figure 37(a).It requires skill to select this level correctly. Consider Figure37(a). Excavation of the upper and lower side drains togetherprovides enough soil to build the road up sufficiently, as shown. Ifone, or both, side drains are made a little shallower than in theFigure, then thcrc will be lit,tlc if anycamber on the road (Figure37(b) ). If one or both side drains are made decpcr than those inFigure 37(a) the road will be built up too much and labour will havebeen wasted. It will also be more difficult to remove water from thisdeep side drain.Where the crossfall is a little steeper than it is in Figures 37(a) (b)and (c) no lower side drain will be required (Figure 37(d) ), andonly the marker drain is dug in the first instance.The level oft he bedof the upper side drain is indicated in Figure 37(d). On steepercrossfalls the edge of the upper verge could be cut down (lowered)by about 8cm in order to reduce the amount of fill required for theroad.Another consideration is the height of the camber above tho lcvclof the beds of the side drains. It should be 3Ocm on tlat and gentleslopes and a littlc more, say 304cm where the road has a steepergradient.The foreman must therefore be taught to select the correct levelto which to dig the side drains. We must learn to be able to select therequired height by eye; if he is supervising many labourers he willhave no time to use his road tracer (see Appendix 1) on this work.After completion, check some levels by sighting with the road tracerfrom the bed of one drain across the road to the bed of the otherdrain. Check also the height of the crown of the road.


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CONSTRUCTION METHODS

-vslight crossfallupperside drain

e-e::-_p-I_-mlower side

horizontal drain(a). Correct.

lb). Side drair ls too shallow .

Is). Side drain tso deep.

upper side drain

(dl. Steeper c ross all, low er d rain no t required .Figu re 37. Widening the drains on crossfak.


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CONSTRUCTION OF MARKE T ROADS 598. Pegging of Lead-Ofl DrainsRefer to Figure 38. Method (a) is usually adopted, in order to savedigging holes when pegging. For method (b) instructions are givento dig 15cm deep at the two rn+~+ and in between as required, inorder to give an even gradient (1 in 125). Boning rods (see Appen-dix 1) may be used to check the levels through uneven ground.Method (b) might be used to explain to new labourers the way todig, and prevents errors due to undulations in the ground. Theresulting drain is just the same as (a).Method (c) shows how to find the run-out. The fall on the bed ofthe drains is still the same, i.e. 8cm in 1Om or 1 in 125. Method (d),;1s in (a), saves the trouble of digging the hole (on the left) whenpegging but it must be made quite clear to the labourcr that he is todig 1Scm deep at one peg and nil at the other (at the run-out). Theseprinciples apply to the pegging of all drains.9. Generalsphs hc~s an l Ci.rl t rtsThcsc should bc construct4 whcrc required as soon as the diggingof the initial side drains (operation No. 1) is finished. Occasionally adiversion bank may bc required on a market road. This too shouldbe built at an early date.Rcgro cvhA lot of tree regrowth mayoccur during the first year or two afterclearing; this should bc dug out deeply and thrown away. Just to cutoff the tops of the shoots will not stop regrowth,Oli lwrrt i Slope 0 Sit ic lhai

Documents

Earth Roads; Their Construction and Maintenence