Indian Roads CongressSpecial Publication 42

GUIDELINES ONROAD DRAINAGE

New Delhi 1994

Indian Roads CongressSpecial Publication 42

GUIDELINES ONROAD DRAINAGE

(CC CON~J~hoN GROUPP~8.No ~79M~~J~1prj.c~1~M~L~ROAD

.~t.. ::1:~~&~~MM~MS-COO 069

Published byThe Indian Roads Congress

Copies can be had fromThe Secretary, Indian Roads (To tigress,Jamnagar House, Shahjahan Road,New Deihi-ilOOll

NEW DELI-lI 1994 Price Rs. 60/-(Plus packing & postage charges)

* ~ ~

- ~

V ~/ (The Rights bfP~f4i~4onand li-ans!inin, Jre reserved,)C.

Published in September. 1994

Edited and Published h~Shri ftP. Gupta. Seeretan. Indian Roads CongressPrinted at Sagar Printers & Puhlisher*~Nes.. i)ethi (l(X)O copies)

MEMBERS OF ThE HIGHWAYS SPECIFICATIONSAND STANDARDS COMMITTEE

(As on 30-10-1990)1. RP. Sikka

(Convenor)2. P.K. Dutta

~4ember-Secretary)3. S.S.K. Bhagat

4. P~Rama Chandran5. Dr. S. Raghava Chari

6. kN. Chaudhuri7. N.B. Desai

8. Dr. M.P. Dhir

9. J.K. Dugad

10. Lt. Gen. MS. Gosain

11. Dr. A.K. Gupta12. DX. Gupta

13. D.P. Gupta

14. S.5. Das Gupta

15. Dr. L.R. K.adiyali

16. Dr. 1K. Kamboj17. V.P. K.amdar

18. MX. Khan

19. Ninan Koshi

20. P.K. Lauria

21. S.P. Majumdar22. NV. Meranj23. TX. Natarajan

AddI. Director General (Roads), Ministry ofSurface Transport

Chief Engineer (Roads~. Ministry of SurfaceTransport

Chief Engineer (Civil), NDMCChief Engineer (R&B), Govt. of KeralaHead, Transportation Engineering. Regional

Engineering College, WarangalChief Engineer (Retd), Assam P.W.D.Director, Gujarat Engineering Research InstituteDirector (Engg. Co-ordination), Council of Scien-rifle & Industrial ResearchChief Engineer (Mech.) (Retd.), MOSTDirector General Border Roads (Retd.)Professor & Co-ordinator, University of Roorkee

Chief Engineer (HQ), UP., P.W.D.Chief Engineer (Planning), MOSTSenior Bitumen Manager. Indian Oil CorporationLtd., Bombay259, Mandakini Enclave, New DelhiScientist SD, Ministry of Environment & Forest

Secretary to the Govt of Gujarat (Retd.), R & BEngineer-in-Chief (B&R), Andhra PradeshAdd!. Director General (Bridges). Ministry of Sur-face Transport

Secretary to the Govt. of Rajasthan P.W.D..Director, R&B Research Institute, West BengalPrincipal Secretary (Retd.), Govt. of Maharashtra.Director (ReId.), CRRI

28. G. Raman

29. A. Sankaran

30. Dr. AC. Sarna

31. RK. Saxena

32. N. Sen

33. M.N. Singh

34. Prof. C.G. Swaminatban

35. MM. Swaroop

36. The Chief Engineer37. The Chief Project Manager

(Roads)38. The Director

39. The Engineer-in-Chief

40. The President

41. The Director General

Engineer-in-Chief, M.P., P.W.D

Engineer-in-Chief~um-Secretaiyto theGovt of Orissa

Deputy Director, CRRI

Director & Chief Engineer,Maharashtra Engineering Research InstituteDy. Director General, Bureau of IndianStandards

Chief Engineer (Retd.), C.P.W.D.General Manager (T&T), RITESChief Engineer (Roads) (Retd.), MOSTChief Engineer (Retd.), MOSTGeneral Manager (Technical),Indian Road Construction Corporation Ltd.

Badri, 50, LA. Puram, MadrasSecretary to the Govt of Rajasthan (Retd.). PWDConcrete Association of India, Bombay

Rail India Technical & Economic Services Ltd.

Highways Research Station, Madras

Haryana P.W.D., B&RIndianRoads Congress (V.P. Kamdar).

(Ex-oflicio)(RoadDevelopment) &AddI. Secretary to theGovt.of India (iLK. Sarin) (Ex-officio)Indian Roads Congress (D.P. Gupta)

24. G.S. PaInitkar

25. MM. Patnaik

26. YR. Phull

27. G.P. Relegaonkar

42. The Secretary

43. MB. Jayawant

44. 0. Muthachen

45. AT. Patel

(Ex-officio)

Com~spondingMembers

Synthetic Asphalts, 103, Pooja Mahul Road,Chambur, Bombay

Dir. Gen. (Works) (Retd), CPWDChairman & Managing Director. Appollo EarthMovers Pvt. Lid, Ahmedabad

CONTENTS

Page

I. INTRODUCTION 1

2. SCOPE 4

3. GENERAL CRITERIA 4

4. ROAD GEOMETRICS 6

5. SHOULDER DRAINAGE 8

6. MEDIAN DRAINAGE 11

7. DRAINAGE OF HIGH EMBANKMENT 11

S. DRAINAGE AT CULVERTS AND BRIDGES 12

9. OPEN DRAINS 15

10. HYDROLOGIC DESIGN 17

II. HYDRAULIC DESIGN 27

12, SUB-SURFACE. DRAINS 33

13. INTERNAL DRAINAGE OF PAVEMENT 35STRUCTURE

GUIDELINES ON ROAD DRAINAGE1. INTRODUCTION

1.1. Adequate drainage is a primary requirement for maintainingthe structural soundness and functional efficiencyof a road. Pavementstructure including subgrade must be protected from any ingress ofwater, otherwise over a period of time it may weaken the subgrade bysaturating it and cause distress in the pavement structure. That is whyrapid dispersal of water from pavement and subgrade is a basic con-sideration in road design. Also, quick drainage takes away the waterfrom pavements surface and reduces chances of skidding of vehicles.Because of inadequate surface drainage, the structural stability ofpavement is undermined by

(I) weakening of pavement structure and subgrade through infiltration of waterfrom the top, and

(ii) erosion of shoulders, verges and embankment slopes caused by water runningoff the pavement.

1.2. The role of proper drainage to ensure longevityof pavement hasbeen emphasised in IRC:37-1984 ~Guide1inesfor the Design of Flex-ible Pavements. Among the measures mentioned therein to guardagainst poorly drained conditions are maintenance of transverse sec-tions in good shape to reasonable cross fall so as to facilitate quickrun-off of surface water and provision of appropriate surface and sub-surface drains, where necessary. Some other measures, such as, exten-sion of granular sub-base over the entire formation width, provision ofdrainage layer, adequate height of formation level above HFL/groundlevel etc. are also mentioned. Infiltration of water under the pavementthrough adjoining earth shoulders (verges) is also a major cause ofweakening of the pavement. Road design must take this intoaccount.

1.3. Despite measures for quick drainage of pavement surface aswell as provision of a fairly watertight surface, water enters from topand travels through various pavement layers and gets accumulated at

2the interface of sub-base/base course and subgrade specially in aboxed type pavement section causing considerable functional pro-bLems. While in new road construction, this aspect could be taken careof by providing adrainage layer at this level, in the existingboxed typepavement construction, this is an acute problem and special measuresoeed to be thought of and taken as per actual site requirements fordraining out the locked water.

1.4. A clear idea about internal drainage of a pavement structureincluding permeability reversal conditions obtaining where animpervious/less pervious course is overlaid by a pervious/more per-v~ouscourse, for example, a stabilized soil layer overlaid by waterbound macadam, is essential because many pavement structuresmalfunction on account of inadequate drainage provisions.Mechanism of failure on account of inadequate drainage facilities in apavement system should be understood and suitable remedialmeasures taken against it to ensure desired performance during theservice life of the pavement.

.5. Considering the importance ofdrainage, the Drainage Commit-tee of IRC in one of its meetings decided that separate guidelinescovering specific requirements for different situations such as rural(plain and rolling), hilly and urban sections of roads and airfieldpavements should be prepared. These guidelines on road drainage arethe first such guidelines on this subject in this country. They areapplicable in non-urban (rural) road sections in plain and rollingterrain.

1.6. initial draft of these guidelines was prepared by S/ShriRajendra Kumar Saxena, Convenor and Indu Prakash, Member-Secretary, as per the decision of the Drainage Committee at its meetingon 25,10,1988. Earlier S/Shri R.P. Sikka and J.B. Mathur had preparedtwo chapters on Deisgu of Surface Drains for the draft document onDrainage for the consideration of the Drainage Committee. Thematerial of these two chapters have been appropriately utilized in thepreparation of the initial draft of the present guidelines. Contributionwas also made by Shri RD. Mehta in preparation of the final draftwhich was discussed by the Drainage Committee (personnel givenbelow) at its meeting on 28.7.1989 and was approved subject to somemodifications. The Committee also authorised S/Shri Rajendra

3Kumar Saxena, Convenor and Indu Prakash, Member-Secretary tobring out the final draft version incorporating the approved modi-fications.

Rajendra Kumar SaxenaIndu Prakash

G.M. ShonthuK.L. BhanotS. SachdevaOP. GoelL.R. KadiyaliV.1C AroraDharmvirK. MukheijiRA. God

AX ChakrabortyP.C. MathurPP. Vakharia

The President IRC(N.Y. Merani)

ConvenorMember-Secretary

T.K. NatrajanD.S.N. AyyarN. SenR.P. Sikka.J.S. SodhiNV. PatilC. ThirunavukkarsuOP. Mathur

The D.G. (R.D.)(K.K. Sarin)

1.7. The Highways Specifications & Standards Committee dis-cussed the guidelines in their meeting held on 30.10.90 and a groupconsisting of Convenor, S/Shri R.K. Saxena & J.B. Mathur was consti-tuted to finalise the document based on the comments of members.The Member-Secretary, Highways Specifications & Standards Com-mittee has forwarded modified guidelines to IRC Sectt. on 19.5.93. Theapproval of Executive Committee on the modified draft was obtainedthrough circulation. Thereafter modified guidelines were approved byCouncil in their meeting held on 19th June. 1993 at Pondicherry, sub-ject to certain modifications to be carried out by the Convenor,Highways S & S Committee on the basis of comments of members.Accordingly, the Convenor, HS&SCommittee had forwarded modified

Members

Corresponding Members

S.P. Kadam

Representative of Engineer-in-Chiefs Branch

Es-Officio Members

The Secretary IRC(D.P. Gupta)

4guidelines on 2-2-1994 for printing as one of the publications of

IRC.

2. SCOPE

These guidelines deal with drainage of non-urban (rural section)roads running through plain and rolling areas. The aspects coveredare influence of alignment and geometrics of the road drainage ofshoulders, verges and median (central verge), internal drainage ofpavement structure, drainage of suhgrade, drainage of high embank-ment and surface and subsurface drains. Examples of estimation ofpeak run off and hydraulic design of surface drain are also given.However, it may be noted that drainage of urban roads, hill roads, air-field pavements and cross drainage structures have not been coveredunder these guidelines since separate guidelines on these subjects areproposed to be brought out later on.

3. GENERAL CRITERIA

3.1. Alignment of the road can have a vital bearing on the problemof drainage. Therefore, in case of new roads surface drainage shouldhe one of the criteria in fixing proper alignment. For~example,locations parallel to large streams and running close to them are likelyto give rise to constant trouble besides several converging tributarieswould be needed to be crossed, An ideal alignment should avoid steepand heavy cuts/fills as these situations have the potential of throwingup piquant problem of drainage and erosion control. Problems ofthese types are often prominent in rolling terrain since alternate cutsand fills, unless designed with an eye on the smooth dispersal of sur-face water, could play havoc with the natural drainage of the area andgive rise among other difficulties to subterranean flow under andacross the road. In each case where cutting is involved meticulous careis needed right at start to anticipate the strength of the drainage cours-es so that necessary design measures to avoid instability of the roadcan be taken. No doubt surface drainage is just one among many otherconsiderations in road location but it warrantscareful attention whichshould be given.

3.2. Normally in plain areas road subgrade elevation in fill sections

5is so fixed that the difference between formation level (top of sub-grade) and highest water table/high flood level is not less than 0.6 to Imetre and between formation level and ground level not less than 1metre. However, in sandy areas and deserts it will be preferable thatthe road is taken on natural ground surface or in slight cutting or fill-ing. it that is necessary to satisfy the ruling gradient of the road. Insuch a terrain, high embankment is likely to be eroded easily, whilecuts are likely to be blocked by sand storms. In cut and fill sections andhill roads where it may he difficult to satisfy the said 0.6 to 1.0 mcriteria, drains may be provided to lower down the water table.

3.3. If a consolidated view is taken, thereare three aspects of surfacedrainage design in which the road engineer is particularly interested.First of all he is concerned with fast dispersal of precipitation on theroad surfitce so as to minimise danger to moving vehicles. This isachieved by proper geometric design of the road, e.g., by crowning thecarriageway or one side cross fall, giving proper cross slope to theshoulders and verges, providing requisite longitudinal gradient etc.Second requirement is that water from road and the surrounding areashall be successfully intercepted and led away to natural outfalls. Thisis accomplished by a system of suitable surface drains, shallow ditchesby the side of the road or deep catch water drains on the hill slopes.Thirdly the engineer must build adequate cross drainage structures atriver crossings and minor streams.

3.4. Survey and investigations is a basic necessity for designing asystem fulfilling the above objectives. The work may involve

~ preparation of alignment plan, longitudinal and cross sections and contourmap:

(ii,) hydrological survey such as rainfall analysis and run off estimation:tiii) hydrographical survey and(iv) geotechnical investigation.

Recourse to remote sensing methods such as aerial photographyand satellite remote sensing can be made if necessary facilities areavailable. The factors which may have bearing on road drainage suchas rainfall, topography and natural drainage of the area. crossfall andlongitudinal profile, existing drains and internal drainage of pavementlayers etc. should be recorded.

64. ROAD GEOMETRICS

4.1. Longitudinal Gradient

4.1.1. Wide roadways increase the surface area to he drained andconsequently the quantities of rain waler that must he removed. Flatterslopes both longitudinal & transverse slow down the flow of rain walerover the roadway and decrease the draining capacity. This throwsemphasis on careful selection of grades. Generally longitudinalgradient is governed by factors like the cost of construction, type ofvehicle and transverse slope by the quality of pavement surface.Flowever, minimum gradients are governed by drainage consideration.On uncurhed pavements near level longitudinal gradients may not beobjectionable, when the pavement has sufficient crossfall/eamher todrain rain water laterally. But for better internal drainage of pavementlayers, especially of granular material, a slight longitudinal gradient ispreferable. Also, in cut sections and tnedians a slight gradient is desir-able to fitcilitate the removal of water. A minimum longitudinalgradient of 0.3 per cent is considered adequate in most conditions tosecure satisfactory drainage.

4.1.2. Due to gradients the drainage problems usually get accen-tuated at vertical curves. This happens becaus of the various lowslopes of pavement close to the level point of the curve. In some instan-ces the length of the curve may have to be adjusted to satisfy thedrainage requirements. In general. difficulties of drainage are moreacute on valley curves, especially if these are situated in cut sections.Prudence will lie in valley curve being avoided at such locations, as faras practicable.

4.2. Pavement Cross Slope/Camber

4.2.1. Pavement cross slope/camber is often a compromise betweenthe requirements of drainage and those of vehicular traffic. From con-sideration of comfort to the traffic steep cross slopes are objectionablebut from drainage stand point of view a reasonably steep cross slope/camber will he helpful in minimising ponding of water on flat grades.Flat slopes are major contributors to the condition which produces

7hydroplan ing (condition where one or more tvres of a moving vehicleare separated by a thin hIm of water) and accidents on high speedroads. And therefore, higher than minimum crossfall/camber valuecan he adopted where feasible and/or necessary. Moreover, it shouldhe borne in mind that the crossfall/camher for a particular pavementcourse should match to its draining requirement otherwise flatter onewould result in sluggish drainage conditions in that course.

4.2,2. In geometric design pavement crossfall/camher could hemade to slope either on one side or on both sides with a crown in themiddle of the road pavement. Unidirectional cross slope is to hefavoured where the roads are provided with carriageways which areseparated by a narrow median without the central drainage or the roadis in hilly section with curvilinear alignment so that it is impracticableto provide two sides crossfall/camber. though if the straight length ismore than 130 metre a crowned section could still he resorted to. Ondivided roads crossfall/camher is usually made to slope away frommedian except at super elevated sections where that would not be poss-ible. On hill roads preference generally is to drain the carriagewaywater towards the hill side particularly where the road banking is sus-ceptible to erosion SC) that the drain on the roadway could carry awaythe discharge safely to proper outfall.

4.2.3. When the road is on gradient. the water travels on a path per-pendicular to contour on the road surface and takes longer time toreach shoulder from the crown. In these cases the camber should notbe less than one half the gradient, e.g.. if gradient is I in 20, cambershould not he less than I in 40. Thus, it is seen that in the case of steepgradients on long length of the road, there is need 10 increasecamber.

4.2.4. IRC:73-l980 ~Geometric Design Standards for Rural (non-urban) Highways recommends the camber or cross slope on straightsection of roads as given in Table 1.

For a given surface type the steeper values may he adopted in theareas having high intensity of rainfall and lower values where intensit\of rainfall is low.

8Table I

(rossfatl/Camber Values for Different Road Surface Types

Surtace Type Crossfall/Camher

High type bituminous surfacingcement Concrete

or 1 .7..2.0O~)t iii (~)to I irt 50

bituminous surfacing 2.0 to 2.5~i (I in 50 to 1 in 40)Watcr bound macadam. grave] 2.5 tO 3~(1 in 40 o 1 in ..~Farth 3.0 to 4.0~(1 in 33 to I in 25)

4.2.5. The Indian practice for National Highways is 2.5 and 2.)) percent for bituminous construction for annual rainfall above and below104) cm respectively. 2 per cent for plain and reinforced cement con-crete. 2.75 and 2.5 per cent for thin premix carpet and surface dressinghor the said rainfall categories respectively. 4.() and 3.1) per cent farwater hound macadam and gravel similarly. 4 per cent for unturfedearth shoulder (verge) and S per cent for turfed earth shoulder(verge).

5. SHOULDER DRAINAGE

5.!. Quick drainage from road shoulders is generally ensured bykeeping the surface of the shoulder properly sloped and smoothed.The rain water trapped in the depression on shoulders caused by themovements of traffic penetrates into the road sub-grade and weakensit. Progressively this results in premature failure of various pavementlayers. Theretbre, proper maintenance of shoulders is very desirable.Shoulders should he shaped regularly. specially before and during themonsoons in order to avoid damage to the road pavement and its sur-face. Keeping in view the increased intensities of traffic the only effec-tive and sure method of maintaining the shoulders is to have pavedand/or hard shoulders instead of earth shoulders (verges).

5.2. A common defect in some of the road is occurrence of shoulders

9at levels higher than pavement surface. In such situations, during rainthe water on road surface does not find a free outlet and accumulateson top of it. Apart from finding its way through cracks and voids inpavement surface the pavement edge at its shoulder provides a poss-ible entry point to the water. Therefore, such defect where shoulderblocks the drainage shall be rectified.

5.3. i)rainage of pavement layers across the earth shoulders (verges)has an important hearing on the performance of the pavement. Thispoint has been stressed to some length in IRC:37-1984. Ideal treatmentparticularly when shoulders are of impervious type would be to extendthe subbase/base course with drainage material across the sideshoulders upto the side drains and give a generous cross slope to per-mit rapid flow. Alternatively, a continuous drainage layer. 75mm to104) mm, might he laid under the shoulder at the bottom level of sub-base or bottom most granular subbase layer 15 cm in thickness may heextended in the entire formation width upto the edge of the formationas shown in Fig. 1 where extension of base or subbase is too expensive.hurried drainage ditches filled with permeable material could he cutacross the shoulders to a depth of 50 mm below the suhh;t~eat 3 to Smetres intervals. Width of such trenches could he from 0.5 to 0.7 met-res. Where the road is on a gradient such shoulder drains may hearranged in-herring-hone pattern to intercept the water quickly andtheir spacing may not exceed width of pavement.

5.4. The crossfall of the shoulder should he as per IRC:73-l98()which stipulates that on earth shoulders (verges) the crossfall shouldbe at least 0.5 percent steeper than the slope of the pavement subject tominimum of 3 per cent. For paved shoulders the crossfall appropriateto the type of surface should be as per Table I. When both paved and,or hard shoulders are provided in combination the paved shouldermay he at least 0.5 per cent steeper than the cross slopes in carriagewayand hard shoulder may he at least further 0.5 per cent steeper. Earthshoulders (verges) where provided will have 4 per cent slope. Illustra-tive diagrams of paved and hard earth shoulders are shown in Fig. I.Thc width of shoulders could vary. Hard (granular/treated soil i.e.stabilized) is preferable to earth shoulders (verges) from overall con-siderations of improved pavement performance.

NP.AE D PAVED PAVED UNPAVE5

sou~noLSi

I-lrn++ISrn

TWO LAiiE CARRIA-----~~ouL~R

/

ING S4FACE GRANULAR

IAl NEW/EXiST!NG ROAD z *

SUBGRADE

WEARING SURFACE

OF VARIABLE II3 I EXISTING ROAD THICKNESS

NOTATIONS :- n. CROSSFALL ICAMBERl OF PAVEMENT

WR~M; WATER BOLNO MACADAM. WMM; WET MIX MACADAM

/ in+,, = CRoSSFALL SHALL NOT BE LESS THAN 2.5 TO 3% ON GRANULAR SHOULDER- STEAPER VALUES SHALL. BE USED FOR RAINFALL EXCEEDING IS5Cm PER YEAR.

2 WHEN FEASIBLE, HARD SHOULDERS SrlOULD BE PREFERRED.

Fig. IJypie;li cross section of paved shoulders (not to wale)

11

5.5. Superelevation creates certain problems for the shoulder slopeon horizontal curves. In such reaches, shoulderon the inner side of thecurve should have a somewhat steeper slope than the pavement.Shoulder on the outer side should he made to drain away from thepavement with low rates of superelevation and low rates of shoulderslope. With higher rates of superelevation, the outside shoulder shouldhe kept level or rounded appropriately so that part of the shoulderdrains on to the pavement and part away from the pavement.

6. MEDIAN DRAINAGE

6.1. Generally it is undesirable to drain the median (central verge)area towards the pavement surface but where the medians are narrow(less than 5 metres in width) these could be crowned for drainageacross the pavement. Very narrow medians 1.2 to 1.8 m wide areusually provided with kerbs and are necessarily paved. Medians 1.8 to5.0 metre wide are usually turfed and crowned so that the surface watercould run towards the road pavement. These medians may be with orwithout kerbs. On the other hand medians wider than 5 metre aregenerally not built with anykerb at the edge. In their case and speciallyif the carriageway is also sloping towards the median provision of acentral swale becomes a must for satisfactory drainage of median area.The swale should not be deeper thanjustnecessary to carry the run off.Usually the side slopes should not be steeper than 6:1 to reduce hazardto the out of control vehicles. For the median drain, flat prefabricatedconcrete gutter sections could be used to advantage. At intervals therain water could he removed from the median by inlets and carriedthrough a drain to an outlet channel. Inlet spacing is determined bythe design discharge, longitudinal slope, capacity of the median chan-nel and allowable velocity in the median channel.

6.2. Earth surfaced median should not be crowned or cross slopedto drain on th~ro.ad pavement because washed away soil may depositon road pavement making it slippery and accident prone.

7. DRAINAGE OF HIGH EMBANKMENT

7.1. The problem of erosion of slopes and shoulders is most severein high embankments (usually more than 8m) having steep slope in

12

longitudinal direction such as in approaches to bridges, when theembankment has been built with an erodible soil without longitudinaland cross drains and it has no vegetation worth the name or pitchingon its slopes and earth shoulders (verges) In these cases the watergains velocity and eventually when it leaves the roadway at anundefined spot it may cause serious erosion of slopes manifestingsometime in the form of deep gulleys extending right upto carriagewayand at time undermining the pavement courses. Therefore in suchcases where high embankments are on longitudinal slopes, lon-gitudinal and cross drains may be provided. The longitudinal drainsmay be at the edges of roadway. Once water is channelised in theseside drains it is led down the slopes by means of stepped outfalls orlined chutes at about 10 metre interval ultimatelydischarging into sidechannel at the bottom. Fig. 2 shows a typical drainage arrangement insuch a situation. Fig. 3 gives typical chute sections.

7.2. There are various methods such as vegetative turfing by seeding,transportation of turfs, saw dust mulching, asphalt mulching. jute andcoir netting which could he deployed to protect embankment slopesand are covered in IRC:56-1974 and are not the detail subject matter ofthese guidelines. Geogrids/g~ocells can also be used to support thegrowth of vegetation.

7.3. Longitudinaland cross drains together with treated slopes pro-vide better answer to the erosion problem of high embankment slopesthan common method of stone/brick pitching which may be costly aswell as not very effective in many situations.

8. DRAINAGE AT CULVERTS AND BRIDGES

For culverts and bridges provision of suitable cross slope/camberand pipes near the kerbs at regular intervals, covered with gratings atthe inlet points, are necessary aids for achieving efficient drainage.Drainage is especially important in the cas,e of earth-filled arch spans.as inadequate drainage would saturate the earth filling and decreasethe load hearing capacity of the structure. Special drains will also benecessary at natural low spots of piers of arch bridges to tapaccumulated water and allow it to flow out. Other general

13

C~UC

-J&

II

EtaaCUI,VMUCUC.

a

a

U

-JS~

~1

3CI

S

14

F~.C~~ThH~UL.AR_CI4JTE SECTIDI

~ H~ ClJ.~cINDEROR SAND BED

ULL,,CIItJJE IN GR0JTED .5UB~LEST~C

. + O6i~ + ..O.6i~

~ ROCK

(Iv> P.C.C. TRAPcZIDIDAL ...CIt1T~~CT1~1

(I> !51J4[TRIC VIEL~~ EM~4~NENTSIDE SLIPE CHUTE.

Fig. 3.Typica~chute sections

15

requirements are laid down in Clause 117 of IRC:5-1985 StandardSpecifications and Code of Practice for Road Bridges - Section 1.

9. OPEN DRAINS

9.1. 1)epending on their location and function open drains areknown as side drains, catch water drains, intercepting drains or gut-ters. The catch water drains and intercepting drains are not being dis-cussed in these guidelines. Open side drains are normally provided onone or both the sides of the roadway in order to intercept surface waterrun off from the carriageway and shoulders/verges. In the cut sectionsthese may be located on the roadway itself. Where the road is inembankment, side drain could be at ground level as indicated in sub-sequent para 9.5. Sometimes in the case othigh embankment these arcalso provided on the edges of roadway in order to protect theembankment.

9.2. Type of road traffic and rainfall intensity are some of the mainfactors which influence the shape, location and capacity of opendrains. Width and depth of drains should be adequate for the waterdraining into them. That is to say that drain should have sufficientcapacity to carry natural peak run-off without water overflowing theroad surface. Some of the hydraulic design aspects of the open drainsare discussed in the subsequent para 9.7.

9.3. The choice of cross section of open drains is generally limited to3 types - triangular, trapezoidal and rectangular. Each of the cross sec-tion type has its own advantages and disadvantages, for example thetriangular section may be most suitable from traffic consideration. Itsgentle slope in continuation of the road shoulder allows greater usableroad width. But this form of cross section has the disadvantage oflesser flow capacity. Rectangular section is well suited for roadsidedrains when larger discharge is required. But unless these are coveredor kept sufficiently away from the traffic, they may prove to be greatertraffic hazard. Trapezoidal section is a compromise betweentriangular and rectangular section.

9.4. Base earth surface in the drain can withstand only a limitedamount of flow without erosion problem. The problem will be severe

16

in silt and sand where permissible flow velocity is between 0.3 to 1.0 m/sec. in stiff clay the said velocity may be 1.5 m/sec. but in all the cases~hetolerable flow velocity can be increased significantly by lining thechannel. Also, by lining the drain, the side slopes can be steepened.For example, the unlined section may require 4:1 to 2:1 side slope butsections with brick lining can even be vertical. The following liningsare feasible on the drain surface:

(a) Torfing

Tuning is useful andcheap method in humid areas for preventing erosion but itrequires proper maintenance so that undesired growth of vegetation may notreduce the flow capacity of drain. The tuned surface has good resistance andflexibility and assumes the shape of drain bed without breaking or cracking.Also if it is property maintained it has unlimited Life and any minor damage tothe turf will be repaired by itself. From the consideration ofmaintenance turfingis more suitable for triangular drains having 4:1 to 3:1 slopes otherwise trim-ming the grass may be difficult This method is less suited for rectangular andtrapezoidal drains since maintenance will be ditficult.

hi Stone/Brick Masonry

It provides stronger surface capabte of taking wear and tear as compared to turfinc. The method is particularb useful wherc the drain is required to carry alarge ainLiunt of dchris or where the water velocity due to either quantum ofdischarge or slope will he high. In such cases tunuing will be easily uprooted. It tsalso useful or paving the roadside drains of rectangular section where turfingccii riot he carried out. ihe stones/bricks can be either Laid des or bedded inconcrete with joints tilled in cement mortar. In areas with annual rainfall ofuser IN) 10111 special~ if the intensity of rainfall exceeds 511 mm per hour. theiflasours should he bedded on concrete to prevent ingress of water under theroad structure and to present the stones/bricks from being pulled out or washedassar This method has the defect that cracks in the masonry cannot be prevented out can (her he etkctivelv repaired. thus certain anlount of percolation willtake place, ibis method is not suitahle in known unstable areas particularlydde taces ~~here once disturbed, it will not he possthle to repair thert.tsonr\ etiectisely.

Ic t oncrgting

The ads an tages a lid d isads an tages are the same as for stone/brickI 0151 urn.

Stone Slab L.ining

thus method is useful in traingular section drains and can be used in other seetrolls in comhination with masonrslconcreting. The technique has no spectul

advantage over masonry and concrete except that it is cheaper in certain areaswhere flat stone slabs are easily available.

(e) Boulder pitchingBoulder pitching can be used to prevent erosion.

(11 BitumInous TreatnientIts use is primarily limited to quick sealing of the surface. When used in con-junction with boulder pitching, bituminous treatment can be very handy. 11) to15 cm impregnation with bitumen cutbacks or emulsion on the sides and baseof a catch water drain is a quick method of ensuring prevention ofseepage water.

(g) Polyethykne LiningThis type of lining is very flexible and totally impervious though the lining canbe easily punched by boulder or debris, Nevertheless it is the only material thatcan be effectively used on unstable surfaces. The damage to polyethylene sheet-ing canbe reduced by laying filter material layers as cushioning to stoneboulder pitching.

9.5. The open drains if provided at ground level.should be kept suf-ficiently away from the toe of embankment. When the drain is unlined,it should be beyond 4H:IV imaginary line drawn from the edge ofshoulder as shown in IRC:lO-196l. When due to lack of space thedrains are located near the toe, they should be provided with erosionrestraint lining such as concrete, stone slab etc., so that erosion doesnot cause any instability of the embankment.

9.6. The drains should be connected to some natural water

course.

10. FIYDROLOGIC DESIGN

10.1. Hydrologic analysis is a very important step prior to the hyd-raulic design of road drainage system. Such analysis is necessary todetermine the magnitude of flow and the duration for which it wouldlast. Hydrological data required for design include drainage area map,water shed delineation, arrow indicating direction of flow, outfalls,ditches, other surface drainage facilities, ground surface conditions,rainfall and flood frequencies. Factors which affect run-off are sizeand shape of drainage area, slope of ground, load use characteristics,geology, soil types, surface infiltration and storage.

18

10.2. Highway drainage facilities range from very small roadsidechannels and culverts to large drain systems and bridges. The extentand depth of hydrological analysis required depend on the importanceand value of structures in terms of initial cost as well as its life cyclecosi. The niost important factor in selecting the design value are costand safety. The optimum design return period can be determined bysimple economic analysis. if the probability of a hydrological eventand the damge that will result, if it occurs, are both known. As thedesign return period increases the capital cost of structure increases.but the expected damage decreases because of better protection effect-ed. Fig. 4 illustrates the method of selecting the optimum returnpe riUt].

10.3. To estimate the amount of run-off requiring disposal at a giveninslani. the engineer must have information regarding rainfall inten-sities within the catch ment area and the frequency with which this pre-cipitation would bring peak run-off. However, all the methods invogue for estimating their peak run-off are based on laws of pro-bability and predict future run-off on the basis of accumulated records.Therefore, knowledge must be coupled with experience, if data are tobe correctly interpreted. One method widelyused due to its simplicityis the Rational Method. Other methods include unit hydrograph,empirical formulae and run-off from stream flow records.

10.4. The rational method is an universally accepted empirical for-mulae relating rainfall to run-off and is applicable to small catchmentareas not exceeding SO km2. The formulae is

Q = 0.028 PAiL Eqn. IWhere

Q Discharge (Peak run-off) in cum/sec.P = (.oeflicient of run-off for the catchment characteristicsA = Area of catchment in hectares

= Critical intensity of rainfall in cm per hour for the selected fre-quency and for duration equal to the time of concentration.

l0.~.Coefficient of run-off (P) for a given area is not constant but

19

RECLIRRENC~ tM1tRVAL (~(EA~S~

1 2 3 10 2~ 50 100 200

400

~300

200

0 I t lit 10.5 0.2 0.1 0.~4 0.02 0.01 0.OOS

Aivtu~t e~cc,,ckncepr~b~bit~ty(~)Dor~ct9eevent,s for v~rloLa5 rtturn p~rJ0ctS

so.

7060

50 cost40

3020

L)

0

2 5 ~o 25 50 100 200

RtCURRANCE INTERVAL (YEARc)0 RIsk cost 0 Copitet cost ATotat cost(b) llydrosconoMtc anatysys

Fig. 4. E)eterminMtion of the optimum design return period b~hydro-ehonomic analysis

Dpt!nuM ~~stgn r,turtu

peyiod (25 y.ars)1

*~r,UI5 totaL cost

20

depends on large number of factors even for a single storm. Factorsafftcling it are porosity of soil, type of ground cpver, catch ment area,slope and initial slate of wetness and duration of storm. To gel themaximum discharge. value of P as it exists at the end of the designperiod of storm is chosen. The stiggested values of P for use inRational Formulae are given below in Table 2.

Table 2Suggested Vnlues of Coefficient of Run-off

S.No. Description of Surface Coefficient ofRun-off (P)

F. Steep bare rock and watertight pavement surface (con-crete or bitumen)

0%

2. Sleep rock with some vegetative cover 0.80

3. Plateau areas with light vegetative cover 0.70

4. Bare stiff clayey soils (impervious soils) aw5.

ti.

Stiff clayey soils (impervious soils) with vegetative coverand uneven paved road surfaces

l..,oam lightly cultivated or covered and macadam orgravel roads

0.50

0.44)

7. Loam largely cultivated or turfed 0.30

5. Sandy soil, light growth. parks. gardens. lawns &mcadows

0.20

9. Sandy soil covered with heavy bush or wooded!forested areas

0.10

10.6. The primary component in designing storm ~ater drains is thedesign storm viz, rainfall value of specified duration and return period.As the extent of drainage system for roads is small,, even intense rain-fall of short durations may cause heavy outflows. Extreme values ofrainfall of various short dur.ations are, therefore, required in designingroad drainage systems.

10.7. The storm duration chosen for design purposes is equal totime of concentration and is based on the assumption that the maxi-mum discharge at any point in a drainage system occurs when the

21

entire catch menl is .contributtng w the flow. The time of concentrationfbr .any watershed is the time required tor a given drop of water fromthe most remote part of the watershed to reach the point ol exist. Theymay have two componetits: (i) entry time: and (ii) time of flow, if thedrainage point under consideration is at the entry of the (Irainage sys-1cm, then the entry time is equal to the time of concentration. If.however, the drainage point is situated elsewhere, then the time of con-centration is sum of the entry time and the time required by the rain-drop to traverse the length of the drainage system to the pointunder study.

10.8. Iime of concentration can be estimated with reasonableaccuracy by anyone familiar with the laws of hydraulics and experien-c:ed in drainage design. All that it calls for is a reconnaissa.nc.e of thewatershed to trace the flow path and estimate the velocity of water invartous sections. For urban areas, an entry time of 3 to.S minutes isnormally used, hut in the case of grassy plots it may take 10 to 20minutes for the water to flow over a distance of 30 m. Table 3 showsentry time values for typical agricultural catchmtnt areas in roilingtopography for guidance. These are n. cant to be applied to catchmentareas possessing about 0.5 m of fall per 10 m and having length abouttwo times the average width. Fig. S gives a graph for estimating time ofcon.centrat~on for catchment of different lengths. character andslope.

Table 3Concentrstion Values for Typical .%grieultural fatchment treas in Rolling Country

Size of catch-mnent area in

Hectares

Minimumconcentration

time in minutescatch

in

Size ofment area1-tectares

Minimumconcentration

time in mi n Lttes

0.4 1.4 40 171.2 3.0 80 232 3.5 120 294 4.0 IS) 355 4.0 240 4712 8.0 321) 602.0 12.0 4(K) 75

22

sdCURVES TO ESTIMATE THE

TIME OF CONCENTRAT[DN30 40 51)

BARE POORSOIL TURF

AVERAGE.TURF

SMOOTHPAVEMENT

553

501

45it

400

30

25~

200

DITCHSECTION

0

#3

U

U

z

)0-J

z

-JU>0

Fig. 5-Time of concentration in minutes

23

10.9.. Once the time of concentration has been fixed, the next stepconsists in reading the intensity of rainfall from the appropriate rain-fall map for a storm duration equal to the time of concentration andadmitted design frequency. Unfortunately, rainfall maps of India forduration less than 1 hour are not yet available. Since on highwaydrainage probiems, the time of concentration is generally of the orderof 5, 10, 15, 20, 30 or 40 minutes, it would be necessary to apply certainconversion factors to 1 hour rainfall values in order to obtain theintensity of rainfall for the desired period. The conversion factorsgiven in Tables 4 and 5 correlating the total rainfall with shorterdurations were determined for lower Gangetic Basin (comprising ofpart of Bengal and Bihar). The values for other areas might bedifferent.

Table 4

~nMinutes Rainfall as Ratio of 60 Minutes Rainfall

Duration 5 10 15 20 30 40 50 60 90 120minutes

Ratio 3.7 2.85 2.4 2,08 1.67 1.33 1.17 1 0.834 0.661

Table S

Relation Percentage of 24 hours Extreme Rainfall to Shorter Duration Extreme Rainfill

Minutes Hours

Duration IS 34) 45 1 3 6 24Percentage 16 25 31 39 55 65 100

10.10 Because of lack of data relevant to Indian conditions, judge-ment could be exercised in choosing conversion factors based on theabove information to convert 1 hour rainfall to shorter duration forrough estimation of the run off. A general equation given in IRC Spe-cial Publication No. 13, may also be used for deriving intensity for

24

shorter duration. The Eqn. isFjT+1

l=T~t+l)Where

= Intensity of rainfall within a shorter period of t hrs. within a storm

F = iotal rainfall in a storm in cm falling in duration of storm of 1 hours.

= Smaller time intenal in hrs. within the storm duration of T hours.

The one hour rainfall maps of India for return periods of 2,5,10,25and 50 years are given in Figs. 6 and 6A.

10.11. The type of highway and traffic carried are ihe principal fac-tors to be considered in determining the design frequency. In highwaysections where a drain is provided at the end of shoulders, it is moreeconomical to select a design frequency that will keep the speed ofwater on the travelled way within tolerable limits and allow removal ofwater within 2 hours of the cessation of the storm. For importantroutes like National and State Highways. we could consider adopting25 years frequency with the stipulation that for underpasses and dep-ressed roadways it may be increased to 50 years. In the case of lowercategory roads, the design frequency selected could be 10 years. Ideallythe choice of design storm should be based on cost-benefit analysis inwhich comparison could he made of the cost of constructing a high-quality drainage structure capable of handling the run-off from aninfrequent storm, with the cost of damage, which would be caused bynot doing so. If this approach is adopted it is quite possible that forroads such as n3otorways. storms of relatively rare frequency would heconsidered for design.

10.12. To highlight the different issues involved in roadsidedrainage design. typical design sections have been worked out &Tabulated atAnnexure-L The example illustrates the effect of change indesign frequency on the section of the drain and of the effect of time ofconcentration on catchment area and design section. It will he obser-ved that selection of a higher design frequency increases the drain sec-tion and hence the cost of the drainage scheme. However, the time ofconcentration and the catchment area are interdependent and arefixed for particular site conditions.

25

26

aaaEaCI-

aCCa0CC=~0

NN

a,N0a

0(U(U

27

10.13. More accurate 24 hour rainfall data for various parts of thecountry is now available from Directorate of Hydrology (smallcatchments), Central WaterCommission, New Delhi. This data can heconverted to shorter duration data using Table 5 or equation men-tioned above. Fig. 7 gives a map of India showing the Zones for whichrainfall maps are available. Conversion factors for converting to rain-fall . intensities for shorter periods in each area are also given inthis publication.

11. HYDRAULIC DESIGN

11.1. General

Once the quantity of mn-off has been determined, the stage is setfor the next step of hydraulic design of the drain. It is convenient todiscuss the design of side drains for urban and rural areasseparately.

Side drain sections in urban areas are generally restricted to righttriangular sections due to the provision of a vertical kerb at the end ofthe carriageway or the shoulder. The gutter section is normally 0.3 to Im wide having a cross slope steeper than that of the adjacent surfacing,usually 1:12 or the cross slope of the pavement might continue to thekerb. The kerb confines the storm run off to the gutter section. Theoverflow spills to the adjacent paved surface, when the gutter capacityis exceeded. At intervals the water is removed from the gutter sectionby inlets. The spacing of the inlets is determined by the design dis-charge, the carrying capacity of the gutter and the allowable spread ofwater on travelled way. A suggested assumption is that the flow shouldnot encroach on the outside lane by more than 1.8 m for a storm of 20minutes duration and one year return period. It is reasoned that stormsof shorter duration have such high intensities that vehicles must travelslowly since vision is obscured by rain pelting on the windshields. Thecapacity of a gutter depends upon its cross-section, grade and rough-ness. Similar right triangle ditches are also sometimes used on ruralhighway where a kerb is placed on the outer edge of the surfacedshoulder on a fill section when water cannot be permitted to run downthe embankment slope.

28

.r. ~

~ 0

EIG 7 ~1AP OF~ INDIASHO~ING

NAIN RIVERS SUB~ZONES ANDSTAID BOUNDARIES

C H I N A

BENGAL

SEA

Y ~. ~t

3f a.

INDIAN

DC AN

Fig. 7. Map of India showing main rivers sub-zones and state boundaries

29

In rural highways, side ditches are northally placed alongside theroadway in order to intercept surface water running off the car~riageway and shoulders. In cut sections they also serve to preventwaterrunning down the cut slopes and invading the roadway. Side ditchesare usually V-shaped or trapezoidal in cross-section. On low-costroads the V-ditch is very often favoured because it can be moreeconomically formed. If equipment is available, the same is alsoamenable to quick and economic maintenance with the help of amotor grader. V-shaped drains are very popular in India in hill st,ctions. On high type of roads, the trapezoidal section is generally ~ferred because of its greater carrying capacity. Normally, due to lack ofeconomic justification small roadside ditches are not hydraulicallydesigned. Instead the ditch side walls are simply cut to the naturalangle of repose of the soil and to a depth usually 0.3 to 0.6 m or more.In the latter respect care should always be taken to ensure that thedepth is such that sustained flow in the bottom of the ditch never risesabove the subgrade level. On important roads, however, the hydrauliccapacity of ditches should be checked to ensure that they are able tohandle the expected flows without danger either to traffic, the ernbank~~ment or the road structure. This is especially important of the ditchescarrying water from adjacent back slopes as well as from the roadway.Vehicle safety considerations usually govern the ditch side-slopes onimportant roads, preference being given to the use of relatively flatslopes, especially on the side closest to the carriageway. Capacity of aditch can better be increased by widening than by deepening the chan-nel so that velocity and erosion are also reduced.

11.2. Open CIiaud Dei~a

For uniform flow in open channels, the basic relationships areexpressed by the Mannings Formula

Q 1/n AR213 SF2and V = 1/n R213 S112where Q = discharge in cum/sec,

V mean velocity rn/sec.n = Mannings roughness coefficientR = hydraulic radius in rn which is area of flow cross section divided by

wetted pcnmctcr,S energy slope of the channel, which is roughly taken as slope of drain

bed.A = Area of the flow cross-section in m2

30

In design of roadside channels, the flow of water is assumed as sub-critical flow. The slope and velocity are kept below the critical level.Critical depth of flow ~dcin open channel is that depth at whichspecific energy is minimum. On mild slope flow is sub-critical andnormal depth of flow dn is more than critical depth. For rectangularchannel dc = (Q2/b2g)U3 where ~gis acceleration due to gravity and bis width of channel. If dndc.

Values of ~nfor various channel surfaces are given in Table 6. Thesoil classification used in the Table is the Extended CasagrandeClassification. Also shown are the maximum permissible velocityvalues for various types of ditch lining. Velocity values in excess ofthese will cause erosion in the ditches,which will not only increase themaintenance cost, but also, in the case ofside ditches mayweaken theroad structurally.

Open-channel design can be accomplished by solving the Man-nings equation numerically. As this procedure is tedious and timeconsuming. chart solutions have beendeveloped to solve the problemscommonly occurring.

Table 6

Mannings n and Maximum Permissible Velocity of Flow in Open Channels

S. Ditch Lining Mannings ii AllowableNo. velocity to

prevent eOsionmlsec.

2 (3) Natural EarthA. Without Vegetation

(i) Rock(a) Smooth & Uniform 0.035-0.040 6(b)Jagged & irregular 0.04 -0.045 4.5-5.5

(ii) Soils (Extended Casagrandeclassification)G.W. 0.022-0.024 1.8-2.1OP. 0.0230.026 2,1-2.4

0,020-0.026 1.5-2.1G.C.

31

(Contd. Table 6)

(I) (2) (3)

G.F. 0.024-0.026 1.5-2.1SW. 0.020-0.024 0,3-0.6S.P. 0.022-0.024 0.3-0.6S.C. 0.020-0.023 0.6-0.9S.F. 0.023-0.025 0.9-1.2CL and CT 0,022-0.024 0.6-0.9MI and ML 0.023-0.024 0.9-1.2OL and 01 0.022-0.024 0.6-0.9CH 0.022-0.023 0.6-0.9MH 0.023-0.024 0.9-1.5OH 0.022-0.024 0.6-0.9Pt 0.022-0.025 0.6-0.9

B. With vegetation(i) Average turf

(a)Erosion resistant soil 0.050-0.070 1.2-1.5(b) Easily eroded soil 0.030-0.050 0.9-1.2

(ii) Dense turf(a) Ero~ionresistant soil 0.070-0.090 1.0-2.4(b) Easily eroded soil 0.040-0.50 1.5-1.8(c) Clean bottom with bushes 0.050-0.080 1.2-1.5

on sides(d) Channel with tree stumps

No sprouts 0.040-0.050 1.5-2.1With sprouts 0.060-0.080 1.8-2.4

(e) Dense weeds 0.080-0.012 1.5-1.8(1) Dense Brush 0.100-0.140 1.2-1.5(g) Dense willows 0.150-0.200 2.4-2.7

2. PavedA. Concrete with all surfaces,

Good or Poor(i) Trowel finished 0.012-0.014 6(ii) Float finished 0.013-0.015 6(iii) Formed, no finish 0.014-0.016 6

B. Concrete bottom, float finished.with sides of

(i) Dressed stone in mortar 0.015-0.017 5.4-6(ii) Random stone in mortar 0.017-0.20 5.1-5.7(iii) Dressed stone or smooth concrete 0.020-0.025 4.5

rubble (Rip-rap)(iv) Rubble or random stone (Rip-rap) 0.025-0.030 4.S

(Conid, Table 6)U i2) (31 (4)

C. (I ravel bottom with sides of)i) Formed concrete 0.017-0.020 3(ji) Random stone in mortar 0.020-0.0238 2.4-3((ii) Random stone or rubble (Rip-rap) 0.023-0.033 2.4-3

U. Brick 0.014-0.017 3F Bitumen (Asphalt) 0.013-0.016 5.4-6

The Manning equation cannot be used without modification tocorn pute flow in right triangular sections as used in urban or hillyareas because the hydraulic radius does not adequately describe thedrain section particularly when the top width of water surface may bemore than 40 times the depth (d) of curb. To compute drain flow theManning equation for an increment of width is integrated across thewidth /~dand the resulting formula is:

Q = 0.315 F1 (Z~IW3

5V2n

Reciprocal of cross slopeDepth of Channel in mSpread of water in inz5/3

(1+4i4~Z2)Vt

channel section, fomiula is

(7) Ct1~3 ~I/1W).err F, (Z) = 0.63 z53

(Z2+ 1)13

Lqn. 5

Where

T=F

1 (7) =

SH8JLtIER

a

I.

PAVElENT

Triangular Channel Section

Eqn.6

33

This equation could be corrected to give depth of flow ~das

rQ.n1318 Z2 + ii~~d = 1.l892.j~J ~ z513 .1 Lqn. 7

12. SUB-SURFACE DRAINS

12.1. Two main objectives of subsurface drains are to lower level ofwater table and to intercept or drain out underground water. To beeffective they should not be less than 0.5 m below the subgrade level.Also subsurface drains should not be used for surface drainage. Theirnormal applications are as follows

The subsurface drain in cut slope as in Fig. 8(A) can carry away theunderground water which otherwise would have caused sloughing ofthe slope. Horizontal drains drilled through cut slopes may be alterna-tive in such situation.

Drainage of subgrade is an important application. Subsurfacedrains placed on each side of the road as in Fig. 8(8) can lower downthe water table under the road. It may however be noted that such adrain may not be effective if the subgrtlde consists of fine grained soilssuch as clay. In that case it may be more satisfactory to raise theroad level.

Subsurface drains may be provided in pervious subbase or basecourse in situations where it may not be practical to carry them underthe shoulder (Fig. I). The drains carry off the water which permeats tothe base or subbase through the surface. Such an application is shownin Fig. 8(C).

12,2. The subsurface drain may consist of perforated pipe or openjointed solid pipe in a trench with backfill around it or it may simplybe free draining material in the trench without any pipe. The per-forated pipes may be of metal/asbestos cement/cement concrete/PVCand unperforated pipes of vitrified clay/cement concrete/asbestoscement The top of trench is sealed by providing impervious cap sothat only subsurface water may enter the drain. In pipe drain the inter-nal diameter of pipe should not be less than 150 mm. Holes in the per-

34

~MPERvInJ~ CAP

PEH~LJRA1EIIDR

Th~Pr~,~101~ ~ GPEN JOINTED PIPE

N~OAII

~ T~TERCEF~TIDNJr REE WATER IN CUT ~LOPC

PAVERENTIMPERVIOUSCAP

~ srroREU ~dATERTABLE AFTERDRAINAGE

~ UO~ER~NG,(ATER TABLE

SHOULDER

~

~ ~

SUBORA1N BASE.~SUBBASE N

(C~ BAEE/VJBDASE DPA~AGEIN C~UT AREA

Fig. 8. Examples of typical sub~surfacedrains

35

forated pipes may be in one half of the circumference only. Size of theholes may be close to D5~size of material surrounding the pipe subjectto being minimum 3 mm and maximum 6 mm. D~stands for size oUthe sieve that allows 85 per cent of the material to pass through ~t. Thebackfill may consist of sand-gravel material or crushed. stone satisfy-ing the grading of Table 7 in case where no specific design exercisebased on filtration and permeability criteria has been carried out. Thebackfill should be free of organic material, clay balls and otherdeleterious material..

lable 7

Grading Req.ir~.entfor Filter Material Per Cent by Weight Passing the Sieve

Sieve Class I Class Il Class IIIDesignation

53mm 10045 mm 97-1(X)26.5 mm 1(X) 22.4 mm 95-1(X) 50-10011.2 mm 100 4.8-100 20-t~)5.6 mm 92-1(X) 28-54 4,322.(~mm 83-1(X) 20-35 0-101.4 mm 59-% 0-S710 pm 35-80 6-18 355 pm 14-44.) 2-9 ~l~0~tm 3-IS . 90 pm 0-S 0-4 0-3

Nose I. When the soil around the trench is fine grained (fine silt or clay or their mix-ture) then Class I grading, when coarse silt to medium sand or sandy soil then Class 11grading and when gravelly sand then Class 111 grading should be adopted.

~te2. The thickness of backfill material around the pipe should not, be less than ISOmm. Therefore considering that the minimum diameter of the pipe is ISO mm, the widthof the trench should not he less than 450 mm.

12.3.. When the suhsurfac~.~consists of only free draining material,the drain may be constructed without any pipe. The trench may befilled with material such as gravel, slag or stone aggregate free fromorganic and deleterious substances. This drain is known as aggregatedrain. Its grading may he as per Table 8.

36

Table S

Grading Requirement for Aggregate Drain

Sieve designationPer cent by weight passing the sieve -

13.2 mm 100Il. 2 mm 92-1005.6 mm 27-462.8mm

3-16

1.4 mm .

12.4. The subsurface drain can be provided with geotextile eitheralong the trench or.around the pipe or both as shown in Fig. 9, Thegeotextile acts, as both separation and filtration layer. When geotex-tile is provided, the filtration requirement in the grading is not impor-tant as far as material on both sides of it are concerned.

12.5. Outlet of pipes should be carefully positioned to avoid possibleblockage and protected with grating or screen securely fastened inplace. For a length of 500 mm from the outlet end the trench for pipemay not be provided with granular material but backfilled withexcavated soil and thoroughly compacted so as to stop water directlypercolating from backfill material around the pipe. The pipe in thissection should have no perforation.

12.6. The designing of sub-surface drain on rational basis is notsimple. It requires permeability estimation, usage of seepage principlesto estimate inflow quantity and calculation of outflow conductivity ofdrainage system. The flownets are useful in determining inflow quan-tity. Based on Darcys law:

Q KiaWhere

Q discharge in m3/sec.A Cross sectional area in m2i Hydraulic gradient

K Coefficient of permeability in rn/sec.

Some typical values of K are given in Table 9

37

-J-4wirw

~

t~Q.

-4~0D

.JZU,-.

II-.>

)(~)~M~

LiiD

E~

~IZL~.

~

~tiJOL&

J

.-40LD

DQ

~

I

Lii

I-

wLD

-JCLiiC

Lii

-JUI-.0U

w

38

Table 9

Coefficient of Permeability for Typical Soils

Type of Soil C oefficient of permeability in misec.

Impervious soil such as stiff clay < lxlO5

Semipervious soil ~uch as silty clay, I x io~to 1 x i0~sandy silt, silt

Pervious soil such as sand, gravel > I x l0~

However, it may be noted that drawing flownet to get value of hyd-raulic gradient (c) in layered section iS not an easy job.

In a simple case shown in Fig. 10 the discharge per unit length ofpipe per unit time can be calculated from dimensionless ratiosindicated therein. It may be noted from Fig. 10 that discharge is max-imum in the beginning and reduces as the flow stabilizes.

13. INTERNAL DRAINAGE OF PAVEMENT STRUCTURE

13.1. Knowledge and understanding of internal drainage of pave-ment structure including subgrade is essential for efficient functioningof the road structure as a whole. Adequate drainage of the pavementstructure should form the part of its design. Boxed type pavementshoused in earth shoulders (verges) should not be constructed at all.Sub-base/base should have self draining provisions by extendinggranular drainage layer fully over the road formation width. In ~ddi-tion care should be exercised to provide crossfall appropriate to thedraining layer to guard against any sluggish flow on~accountof inade-quate crossfall than needed for the type of material used in that layer.Road suhgrade must also he provtded with a crossfall appropriateto the draining characteristics of the material with which it is built sothat there is no accumulation of water at the top of the subgrade duetosluggish flow at that level.

13.2. System functioning of various pavement structures built with

CC

V.3>C

V

C,~

~

C,~

.C~

.~

~*C

C,~

isC

CC

.E~

,,~.~

C

V~

C.

.~

.~

~E

~

.~2

~

II11

ItIt

C.~

>C

.4aI3

r~-

c~

QC

C

~f.~

1~E

~N

39

IUCzCLii0Li.LiiU-jCUI-,.C

~,

40

pavement courses of different speciflcations should also be kept inmind uhile designing them in order to ensurethat there is no problemat interfacial drainage between thetwo pa~cmentlayers. For example.a denser pavement layer i.e. with lesser voids should not be overlaid.caseS with a pavement layerhaving more voids since it causes per-meabilit~ resersal conditions detrimental to the survival of theoserlaid course(s). In case ofexisting pavements where such a situa-tion might become unavoidable from other considerations, then theoverlaid layer having largervoids should be drainS offlaterall> other-wise interfacial drainage problems would be created which will causepremature failure of the overlaid layer itself.

41

.4n,~vur~~J

Typical Exaple of Roadside Drainage

Given Typical highway cross~sectionas shown in the figure. at New Delhi,with a continuous longitudinal gradeof I in l00.Th~soil in the region iseasily erodable soil with average turf.

ieq~a The design of side drain for various points along the highway.

3O~r.~- H~*B~ PAV.f~~~

~EcOURSr~

Typical highway cross~sectionlM.c~.rgrCakaladuna

(a) C..~d..t .(~rv.IVThe drain is carrying runoff from half the roadway width and the adjoining

agricultural land. The coefficient of runoff from the various surfaces are:

Bituminous concrete pavement 090rurfed shouId~sand drain slopes 0.3(1Agricultural land 0,44)

l~ ~O.90x7xL+O3i6xL+O4x30xLav (7L + 6L + 30L)046

Where L Length of road under consideration.

(b) T~eo( co.ce.tratlo.The remotest point in the cross-section is the end point of agricultural land; the time

required for water to reach drain from the remotest point 30/v.

Assuming v 0G6 m/se~over the agricultural land and 03/rn/sec. in the drain_ 30 i L

X + Minutes(8.33 L/18) Minutes

and L (t833) x I.

42

Time 10 15 20 30(innI ti 30 120 210 390

40

570

50

750

60

930

90

1470

120

2.010

(e) Area

Area contributing to flow at any point L mfrom start of grade of I in HX) is given by

43 x LA1 hectareI0,000

43 (t 8.33)= x 18 hectare

I0,()00

lime 10 15 20 30 40 50 60 90 120(rnts)A1 1)129 0.516 0.930 1.677 2.451 3.225 3.999 6~32I f~.M3(hectare)

(d) Rainfall Intensity iFrom rainfall maps of India, 1 hour maximum rainfall near Delhi is

given below

Frequency Rainfall in Cmsfor 1 hour

Conve(From

rsion factor2 year frequency)

2 Years 3.6 cm 15 years 5.5 cm 153

10 years 6.2 cm 17225 years 8.0 cm 2.2250 years 9.2 cm 2,56

Now conversion factors for converting 60 minutes rainfall intensity to intensity ofother durations are as below for 2 year frequency.

43

,[)urationt (mt.s)

.5 tO 15 20 30 40 50 60

Conversionfactor

3.~7 2.85 2,4 2.08 1.67 1.33 .17 1.1) 0.834 0.667

Rainfallintensityin cm

13.32 10.26 8.64 7.488 6.012 4.788 4.212 io .1.02.4

2 year fre9uency

(e) Discharge is given by the relationQ = 0.028 x x II x A1

0.08 x 1)46 x i~x A1

A1 30 m from start of grade

L = 31) m, t 10 mIs (from b above)A1 = 0.129 hectare (from c above)

10.26 cum (from d above)Q 2 years frequency 0.028 x 0.46 x 10.26 x 0.129 = 0.0170 cum/sec.Q S year frequency Q2 x 1.53 0.026 cumlsec.Q 10 year frequency Q2 x 1.72 = 0,029 cumlsec.Q 25 year frequency Q2 x 2.22 0.0378 cumlsec.Q 50 year frequency = Q2 x 2.56 = 0.0435 cumlsec.Similarly discharge at various distances from start of the grade will be as shown inthe Table A.

II. Chsenel Section CalculationFlow in a trapezoidal channel with 0.6 m flat bottom and sides on 21 slopes are

assumed for design calculations.

For easily eroded soil with average turf

n = 0.03

v max = 0.9 * 102 m/sec.For ground slope of I in 100

S = 0.01For 10 years frequency & t 10 minutes

Q 0.029 cum/sec.

44

Let depth of channel be d m. then

Area of Channel A = 0.6 + (0.6 + 4d) x d

Wetted per metre ofchannel = 0.6 +J~d x 2

(0.6 + 2d) .dHydraulic radius = _____________0.6+,/~dx2

1 AR2~E3 S112

& \T _1R213 S12

(0.6 + 2d) d

I r(0.6 + 2d) di2~311 i~2~Q = ~ (0.6 + 2d) d ~.0.6 -b15 x 2d i L~f~j

Solving theabove equation we find by trial & error that d = 8cm and = 0.55 rn/secwhich is within the permissible value and flow is not super critical.

Similarly

Q = ((.098Q = 0.17Q = 0.223Q = 0.260 cum/sec dnQ = 0.301 curn/sec dnQ = 0.3 18 curn/sec dnQ = 0.419 curn/sec dnQ 1)454 curn/see dn

= 0.15 m, vn = 0.72 rn/sec= 0.185m. vn = 0.81 m/sec= 0.22Sm, vn = 0.9 rn/sec.= (1.24 m, vn = 0.9 rn/sec= 0.27 m, vn = 0.99 rn/sec= 2.75 m. v = 0.99 rn/sec= 0.3 m, v I.OS rn/sec= 0.33 rn, v = 1.125 rn/sec

Similarly, the sections for other dischargeshave been worked out and presented in the

Table .\,

Example -2

curn/sec dncum/sec dncurn/sec dn

A concrete triangular gutter is to he designed for 0.03 curn/sec.discharge with I in 40cross slope when n 0.014 and channel slope is I in 1(X).

45

Soludon

From equation

Q ~ (~ Sn

Where

= reciprocal of cross slope i.e. side slope of channel in ~ horizontal:1 Vertical

d = depth of channel in metresand

5/31 ~ +~/f~J2/3

And Q, n and S have the following meaningsQ = discharge in cumlsecn mannings roughness coefficientS = energy slope which is roughly taken as slope of the bed ofroad drainage

= g F1 (.~f8/3 ~4~5/3

and F1 (~) Li ~

Solving the equation we get3.388 x l0~

or d 0.05 m

The spread of water (Zxd, fIgure below) is0.05 x40 2.00 m

46

Example -3

For designing a V-shaped channel section (figure below).

a

Shallow right triangular channel

B

Shallow ~-shapedchannel

L 30 10 0.129 10.26 0.017 0.09 4~.026 0.09 0.029 0.09 0.038

1 120 15 0.516 8.64 0.0574 011 0.088 0.19 0.098 0.15 0.127

3. 210 20 1.032 7.488 0.099 0.14 0.151 0.17 0.170 0.19 0.220

4. 390 30 1.677 6.012 0.1298 0.16 0.199 0.21 0.223 0.23 0288

5. 570 40 2.451 4.788 0.1512 0.18 0.233 0.23 0260 0.24 0.535

6. 750 50 3.225 4212 0.175 0.21 0.267 ~.25 0.301 0.27 0.388

7. 930 60 3.999 3.6 0.185 0.22 0283 0.23 0.318 0.27 0.411

Table A Discharge sad 1Mrectio~at vszlo~iloc*da.s sloug the HIg~way

S. Distance Time of Area Intensi- 2 Years Frequency5 Years Frequency 10 Years Fftquency25 Years Frequency50 Years FrequencyNo, in m from concent- contri- ty of Dis- Design Dis- Design Dis- Design Dis- Design Dis- Design

the start ration butory of Rain- charge depth charge depth charge depth charge depth charge depthof the t the flow fall for curn ~cc (m) cum! (in) cumJ (in) sum! (m) curn! On)grade (Minutes) Hectares 2 years Sec sec. sec. sec.

cms

8. 1410

9. 2210

0.10

0.17

0.21

025

0.28

0.3

0.3

0.044 0.11

0.146 0.18

0.253 0.23

0.332 027

0.387 0.3

0.448 0.31

0.474 0.32(From Map)

90 6,381 3 0.244 0.23 0.373 0.3 0.419

120 8.643 2.376 0.254 0.24 0.404 0.31 0.454

0.3 0.542 0.36 0.625

0.33 0.586 0.4 0.675

0.38

040

48

Shallow V-shaped Channel

The following equations will he used

Q = i/n r~(~)d8~3S112Where : F, (~) ~

And other elements Q, n S. ~ and d are as defined in Example 2.

01: