Unit 8 ( HIGHWAY DRAINAGE )

Highway Engineering Engineering

C3010 / UNIT 8

UNIT 8

HIGHWAY DRAINAGE

OBJECTIVES

General Objective To understand the basic highway drainage system. Specific Objectives At the end of the unit you should be able to :• • •

state the types of drainage. identify the drainage’s location and its functions. describe the differences between surface drainage and sub-soil drain


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INPUT

8.0 8.0

INTR INTROD ODUC UCTI TION ON TO HIG HIGHW HWAY AY DRAIN RAINAG AGE E

Highway drainage may be defined as the process of interception and removal of water from over, under and the vicinity of the road surface. Road drainage is very important for safe and efficient design of the road way and hence is an essential part of highway design and construction.

A part of rainwater falling on road surface and adjoining area is lost by evaporation and percolation. The remaining water known as surface water , either remains on the surface of the road and adjoining area, or flows away from it, depending upon the topography and general slope of the area. Removal and diversion of this surface water from highway and adjoining land is known as surface drainage.

Due to percolation, if water table does not rise near of the road sub-grade, it does not create any problem as it does not affect the road sub-grade. If water table rises to the vicinity of road sub-grade, it requires to be lowered as it will definitely affect road sub-grade. Measures adopted to lower the subsoil water table are called sub surface drainage.


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Some of the retained water which cannot be drained off by normal methods of drainage is called held water and special measures have to be taken either to drain it off or keep it low so that it may not affect the road sub-grade.

8.1 8.1

EFF EFFECTS ECTS OF IMPR IMPRO OPER PER DRAI DRAINA NAG GE

One of the major causes of road failure is its improper drainage. Improper drainage of the road causes destruction in the following ways:

a.

Road Road surfa urface ce if made made of soil soil,, gra grave vell or or wat water er boun bound d macadam, it will becomes soft and losses strength.

b.

The The roa road d sub sub-g -gra rade de may be soft soften ened ed and and its its bear bearin ing g capacity reduced.

c.

Vari Variat atio ion n in in mois moistu ture re cont conten entt in in exp expen ensi sive ve soil soils, s, caus causes es variation in the volume of sub-grade and thus causes failure of roads.

d.

Fail Failur ure e of of for forma mati tion on slop slopes es is also also attr attrib ibut uted ed to poor poor drainage.

e.

If rain rain wate waterr is is not not prop proper erly ly drai draine ned d and and allo allowe wed d to to flo flow w alo along ng the road side for long distances, slip and land slides may occur causing road failures.

f.

Eros Erosio ion n of of side side slop slopes es,, sid side e dra drain ins, s, form format atio ion n of of gul gulli lies es may may result if proper drainage conditions are not maintained.

g.

Flex Flexib ible le pave paveme ment nt’s ’s fail failur ure e by by for forma mati tion on of wave waves s and and corrugations is due to poor drainage.

h.

Cont Contin inuo uous us cont contac actt of of wat water er,, wit with h bitu bitumi mino nous us pave paveme ment nts s causes failures due to stripping of bitumen from aggregates like loosening or detachment of some of the bituminuos pavement layers and formation of pot holes.

i.

Rigi Rigid d pav pavem emen ent’ t’s s pri prim me cau cause se of fail failur ure e in in by by mud mud pump pumpin ing g which occures due to water in fine sub-grade soil.

Highway Engineering Engineering j. j.

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Exce Excess ss mois moistu ture re caus causes es incr increa ease se in weig weight ht and and thu thus s increase in stress and simultaneous reduction in strength of the soil mass. This main reason of failure of earth slopes and embankment foundation.

k.

Eros Erosio ion n of of soi soill fro from m the the top top of of unun-su surf rfac ace ed roa roads ds and and embankment slopes in also due to surface water.

8.2 8.2

HIGH HIGHWA WAY Y DRA DRAIN INAG AGE E REQ REQUI UIRE REME MENT NTS S

a.

Surf Surfac ace e wat water er shou should ld not not be be all allow owed ed to rema remain in stan standi ding ng on the

road pavement and shoulders. Measures should be

taken to drains off this water, immediately. b. The surface surface rain rain water water from the adjoining adjoining area, area, should should not be allowed to come towards the road surface. For this, general slope of the ground adjoining road, should be made slopping away from the road. This objective can be achieved by aligning road on ridge. c. Side drains drains should be of sufficient capacity and having sufficient longitudinal slope so that it may drain of all the collected surface water, efficiently. d. Surface water flowing across the road road pavement pavement should not develop cross ruts or erosions on road surface and shoulders. For this, high embankment slopes should be protected either by turfing or pitching. e. Seepage water and and other other capillary capillary waters waters should should be drained off by suitable underground drainage system. f. Maximum Maximum level level of under under ground ground water water table table shou should ld be maintained well below the sub-grade level of the road. Under ground water table should remain at least 1 m to 2 m below the road sub-grade.


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g. In water water logged logged areas, areas, special special measures measures should should be taken to keep down the harmful salts.


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INPUT SURFACE DRAINAGE 8.3 8.3

INTR INTROD ODUC UCTI TION ON TO TO SUR SURFA FACE CE DRA DRAIN INAG AGE E

The problem of surface drainage is first tackled at the location survey stage. An ideal location for a highway from drainage point of view is along the divides between large drainage areas. All the streams would then flow away from the highway and the drainage problem would be reduced to tackling the water that falls within the roadway boundary only.

Except in hilly areas, where easy gradients are available along the main streams, location of a highway along the streams involves construction of a large number of drainage structures at immense cost, which, if possible should be carefully avoided. Construction of high embankments involves erosion problems which require careful handling. This fact may also be taken into account while locating a highway. Drainage is, thus one of the essential considerations for the location of a highway.

8.4 8.4

SIDE SIDE DIT DITCHES CHES FOR FOR SUR SURFA FACE CE DRAI DRAIN NAGE AGE

Side ditches are used along the roadway in cuts and on ground that is fairly level and sometimes adjacent to embankments to prevent road water from flowing over cultivated or otherwise improved land. These ditches are usually Vshaped or trapezoidal, the former being generally preferred. However, the trapezoidal ditches have a higher capacity.


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The quantity of water to be carried by a side ditch along a highway is the run-off from the area contributing there to. Primarily this water comes from the portion of the road between the ditches and from shoulders flanking the roadway surface. As a broad principle, it may be accepted that for storms of short duration say of less than 15 minutes, about 75% of the water and that for storms exceeding 15 minutes duration, all the water that falls on this area would run to the ditches. Some more water might also reach the ditches from the adjoining lands sloping towards the roadway. Rate of run-off of water from these lands would depend upon the type of soil, type of vegetation and the slope of the land.

All these items should be carefully studied before arriving at the Design Discharge. For practical purposes, it may be assumed that for storms lasting 40 minutes or more a portion of water that falls on the adjoining lands also comes to the ditches. The design of cross section whether V-shaped or trapezoidal should be based upon these considerations.

Side-ditches should preferably be lined if financially feasible. The paving may consist of rubble masonry work, which is either laid dry or the joints filled with grout. The stones should be 15 to 20 cm deep and at least 5 cm wide for the purpose. This may be under-laid with a 10 cm layer of crushed rock or gravel, the particles of which may range in size from 10 to 20 mm. A typical section of such a ditch is shown in figure below.


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0.3 to 0.45m

0.6 to 1.0m

Shoulder Slope

45 to 60 cm

15 to 20 cm Stones laid dry or joint – grouted over 10 cm thick layer of crushed rock gravel

Figure 6.1 : Section of a typical V-shaped side ditch

8.5

INTERCEPTING DRAINS

Intercepting drains are used on natural slopes to prevent erosion of the cut-slopes in the hilly areas. They are sometimes also used to relieve the side ditches of greater discharge than the one for which they are designed. The water from the intercepting drain is carried down to the side ditch at specified points along spillways. Typical intercepting drain are shown as figure below.

Intercepting Ditch

Figure 6.2 : Cross section of intercepting ditch

Highway Engineering Engineering 8.6

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DITCH CHECKS

The primary function of a road side drainage ditch is to remove the surface water efficiently. The ditch, therefore, should have adequate capacity to carry the peak rate of run-off throughout the life of the roadway. This means that ditch should maintain its originally designed cross-section and should neither erode nor silt up. For this purpose, the slopes of the ditches are kept mild to protect the ditch against erosion grass may be grown in it.

In hilly or rolling areas, the longitudinal slopes along the ditches sometimes are large which induce high velocities causing erosion of the bed and the sides of the ditches. In order, therefore, are used to restrict the slopes. A ditch check is a solid barrier across the ditch with a fall of 25 to 30 cm. Water runs in the section between two consecutive ditch checks along a mild slope and then drops through this fall. The spacing of the ditch checks depends upon the general gradient of the ditch, the intervening gradient and the vertical drop available at the structure.

4.7

KERBS AND GUTTERS

Kerbs and gutters are essential for the city streets to drain off the water quickly and to maintain the cleanliness required. Usually, a kerb and gutter combination is employed for the purpose. It is cast in concrete as one piece though kerb and gutter combinations cut from stones are also employed for the purpose.

Concrete should have a compressive strength of about 80 kg/cm 2 at 7 days and about 200 kg/cm 2 at 28 days. The essential requirement of the stones from which the kerb and gutter combinations are cut is that they should be free from any laminations. Granite or any other hard stone may be used.


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The kerb and gutter combination are fixed at the edge of the roadway and gullies are placed at suitable intervals. The gutters slope towards gully from both sides and a longitudinal gradient of 1 in 100 to 1 in 120 is given to these. The water from the gutter flows through the grating into the gully and is drained off through an outlet pipe to a water sewer, which takes the water away to a natural water channel.

The arrangement is of the type shown in figure 6.3. Sometimes V-shaped shallow concrete gutters are used for concrete or other high type of pavements. The shape is shown in figure 6.4. Such gutters do not create any traffic hazard. Sometimes, separate kerb and gutters are employed for draining off water from the city streets. Such as arrangement is shown in figure 6.5 below. 8 cm

8 cm

Road Surface

Road Surface

10 cm 22.5 cm

3 cm

53 cm

53 cm SECTION OF CHANNEL AT SUMMIT

SECTION AT MID POINT BETWEEN SUMMIT AND GULYY 8 cm

Road Surface 45 cm

Outlet Pipe

14 cm

27 cm SECTION AT GULLY

Figure 6.3 : The arrangement of side surface drainage


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SLOPE: 1 IN 12

4 cm 20 cm

16 cm

15 cm 60 cm

Figure 6.4 : Section of a shallow concrete gutter

Pavement Surface

10 cm 30 cm

30 cm

8 cm

Figure 6.5 : Section of a kerb and gutter as separate units.

4.8PIPE DRAINS


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Pipe drains as water sewers are widely used for removing surface water and are sometimes desirable for carrying the water along roadways when sufficient widths of right-of-way are not available for suitable ditch construction. Vitrified clay sewer pipes and concrete sewer pipes are the most suitable. Sewers properly designed and constructed in brick masonry are also used for the purpose. In soils where settlement may occur, corrugated metal pipe is used. All sections of corrugated metal pipe are connected with metallic bands.

4.9INLETS, CATCH BASINS AND MANHOLES

Inlets are the openings from gutters or ditches into pipe drains or culverts and are of various types. Along a rural highway, it may simply be a V-shaped or straight concrete headwall with or without a paved approach. Drop inlets are used where water has to be discharged into a drain or a culvert at a lower level.

Catch basins and manholes are usually of circular shape and are generally connected to the drop inlets of pipe drains. They are spaced 150 to 300 meters apart depending upon the individual design requirement and are meant for inspection.


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INPUT

SUB - SURFACE DRAINAGE 4.10 4.10

INTR INTROD ODUC UCTI TION ON TO SUB-S SUB-SUR URFA FACE CE DRAIN DRAINAG AGE E

Stability and strength of the road surface depends upon the strength of the sub grade. Sub-grade is the foundation layer of the road whose strength largely depends upon its moisture content. With increase in moisture content, the strength of the sub-grade decreases.

Variations in moisture content of sub-grade are caused by following:

a. See page page of water water from from higher higher adjoin adjoining ing ground ground.. b. Penetra Penetration tion of of moisture moisture throug through h the paveme pavement. nt. c. Percolation Percolation of water from shoulders, shoulders, pavement pavement edges and soil soil formation slopes. d. Rise Rise or fall fall of under undergro ground und water water table. table. e. Capillary rise of moisture in case of retentive retentive type type of soils. soils. f. Trans Transfe ferr of moist moistur ure e vapour vapour thro through ugh soi soil. l.

The first three methods of entrance of moisture are concerned with free water, and last three methods are connected with ground water. In the case of sub-surface drainage of the roads, every effort should be made to reduce the change or variation in moisture content to minimum. It is better if sub-grade is always kept dry, but in rainy season, some change in moisture content if subgrade is inevitable. It should be remembered that by provision of sub-soil drainage, only gravitational water can be drained off, vapour water and cappillary water cannot de drained by this system.

Highway Engineering Engineering 4.11 4.11

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SUBSUB-SU SURF RFAC ACE E DRAI DRAINA NAGE GE SYST SYSTEM EMS S

If underground water table is more than 1.5 m below the sub-grade of the road, it does not require any sub soil drainage. But if it is closer than this, the best remedy is to raise the road formation to such a height that sub grade remains at least 1.2 m above the highest water table. But if due to other consideration, it is not possible to increase the height of formation, and subgrade soil being drainable, deep side trenches should be constructed on both sides of the road to lower the water table. These trenches are provided with drain pipes and filled at the top by filter sand. The depth of trenches depends upon amount of lowering of water table, lateral distance between trenches and type of sub-grade soil.

If the sub-grade soil is of retentive type, only side trench drains may not prove adequate, as at the centre of the pavement lowering of water table may not take place up to the desired level. In that case, transverse drains may have to be provided at suitable intervals along with the side longitudinal drains. Transverse drains may be pipe drains or trapezoidal trench drains filled with stone or rubble. Stone or rubble filled transverse drains are also called french or blind drains. Transverse drains collect percolating water from the width of the pavement and discharge it into the side drains. Transverse pipe drains are given a little slope towards the side drains. The filter material surrounds the transverse trenches and the top of these trenches is filled with graded rubble, the bigger size rubble being nearer the pipe.

The diameter of the lateral pipe drain may be 10 cm and that of longitudinal pipe drain 20 cm or even more according to the requirements. The cross drains are placed staggered in plan, in the herringbone fashion.

Highway Engineering Engineering 4.12 4.12

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CONT CONTRO ROL L ON ON SEE SEE PAGE PAGE FLOW FLOW

Where surface of the ground and impervious embedded below it are sloping towards the road, the seepage flow is likely to reach road sub-grade and affect its strength characteristics. If the see page level reaches a depth less than 60 cm to 90 cm from the road sub-grade, it should be intercepted to keep seepage line at a safe depth below road sub-grade. See figure 6.6 below.

Sloping Ground Pavement Clay seal

Zone of seepage flow

Filter Material

Original Seepage Flow

Lowered Seepage

Impervious Sloping Straum

Line Perforated Drain Pipe

Figure 6.6 : Lowering of water table in case of sloping ground

4.13 4.13

CONT CONTRO ROL L ON ON CAPI CAPILL LLAR ARY Y RIS RISE E


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If capillary rising water in very near to sub-grade of the road and is likely to affect its strength, steps should be taken to arrest the capillary rise of water. In the case of seepage, arresting of capillary rise is more useful than lowering the water table. In fact lowering of under ground water table is only economical method when sub-grade soil is of permeable type. In case of sub-grade having retentive type of soil, drainage is very difficult and costly and checking its rise by capillary cut-off’s proves more economical.

Capillary cut-off’s can be of three types as follows: a.

Prov Provis isio ion n of of a gran granul ular ar laye layerr of of sui suita tabl ble e thi thick ckne ness ss or of sand sand blanket.

b.

Prov Provis isio ion n of of imp imper erme meab able le laye layerr of of bit bitum umin inou ous s mate materi rial al..

c.

Heav Heavy y dut duty y tar tar felt felt,, pol polyt ythe hene ne enve envelo lop, p, are are the the othe other r measures which can be adopted.

During the construction of embankment, a granular layer of suitable thickness is inserted between the sub-grade and highest level of water table. Thickness of granular layer should be such that capillary rise of water remains within this layer. If this layer is too thin, it is likely that capillary rising water may rise above this layer and affect the road sub-grade. Refer to figure 6.7 and 6.8.

Granular Material

Capillary Rise

Highest Water Table


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Figure 6.7 : Prevention of a capillary rise ( Using Granular Material ) Impermeable Layer

Capillary Rise Highest Water Table

Figure 6.8 : Prevention of a capillary rise ( Using Impermeable Method ) In second method, in place of granular layer, an impermeable layer generally of bituminous material is inserted to arrest the capillary rise. 50% straight rum bitumen 80/100 grade with 50% diesel oil at the rate 1kg/m 2 is used for this layer.


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INPUT CROSS DRAINAGE 4.14 4.14

CROS CROSS S DRAI DRAINA NAGE GE WORK WORKS S

When a stream or river happens to cross the road alignment, facilities for cross drainage has to be provided. When road alignment gets away from the ridge line, the rain water on the area incircled by ridge and road alignment has to be passed from one side of the road to the other lower side. At the point of crossing, mansory structure known as cross drainage work or drainage structure has to be constructed.

The commonly used cross drainage structure are the culverts and small bridges. When linear water way between the inside faces of abutments of a drainage structure is less than 6m, it is called a culvert. When linear water way exceeds 3m, the cross drainage work is called a bridge. Thus culvert is just a small bridge. Across small streams, culverts are constructed but in case of large streams or rivers, bridges are constructed.

Slab culvert, Boc culvert, Arch culvert and Pipe culvert are the usual types of culverts most commonly used for cross drainage works.

a. Pip Pipe Culv Culver erts ts

When the stream carries low discharge and is having high embankment, pipe culverts are considered more suitable. Pipe is laid slightly inclined. For ease in inspection minimum diameter of pipe should be 75 cm. There should be at least 90 cm cover of soil so that


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traffic load transmitted on pipe is of small intensity and also without vibrations.

Pipes may be made of stone ware, concrete, etc. Pipes should be laid on 15 cm cement concrete bedding. A protective rubble apron should be provided in sandy or clay beds.

Road Surface

Figure 6.9 : Pipe Culverts

b. Slab Slab Culv Culver ertt

These culverts have masonry abutments with stone slab over them. In localities where stone is easily available, these culvert are mostly used. These culverts are mostly used up to about 2 m span. Where stone patties are not available, Rein-concrete slabs are used. Rein-concrete slabs are designed as simply supported slabs. Span of rein-concrete slab may be about 3 m.


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Bed Pitching

Figure 6.10 : Slab Culvert c. Box Box Cu Culvert ert

These culverts are constructed where the nature of the soil below the foundation is not suitable for individual footing under piers and abutments. It is a monolithic rectangular drainage structure. The size of the rectangular passage should not be less than 60cm X 60 cm for easy cleaning of debris. Short span box cilverts can be precast.

Figure 6.11 : Box Culvert


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d. Arch Arch Cu Culver lvertt

This culvert is preferred under the conditions where high fillings are envolved and there are heavier loadings on the culvert. Arches may be built from brick, or stone masonry, or plain cement concrete. Span of each arch should be kept less than 3 m.

Road Surface Arch

Abutment Bed Pitching

Figure 6.12 : Arch Culvert

Selection of culvert to be used is done on the basis of availability of construction materials and economic considerations.

ACTIVITY 8


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TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT

Question 1.

State the type of culvert that are used for cross

drainage system. i. ___________________________ ii. ___________________________ iii. ___________________________ 2.

Describe about kerb and gutter that are used for

surface drainage.

FEEDBACK ON


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Answer 1.

Slab Slab culv culver ert, t, Box Box cul culve vert rt,, Arch Arch culv culver ertt and and Pipe Pipe culv culver ertt are are the the usual types of culverts most commonly used for cross drainage works.

2.

Kerb Kerbs s and and gutt gutter ers s are are esse essent ntia iall for for the the city city str stree eets ts to to drai drain n off off the the water quickly and to maintain the cleanliness required. Usually, a kerb and gutter combination is employed for the purpose. It is cast in concrete as one piece though kerb and gutter combinations cut from stones are also employed for the purpose.

Concrete should have a compressive strength of about 80 kg/cm

2

at 7 days and about 200 kg/cm 2 at 28 days. The essential requirement of the stones from which the kerb and gutter combinations are cut is that they should be free from any laminations. Granite or any other hard stone may be used.

The kerb and gutter combination are fixed at the edge of the roadway and gullies are placed at suitable intervals. The gutters slope towards gully from both sides and a longitudinal gradient of 1 in 100 to 1 in 120 is given to these. The water from the gutter flows through the grating into the gully and is drained off through an outlet pipe to a water sewer, which takes the water away to a natural water channel.


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Question 1.

What What are are the the req requi uire reme ment nts s for for high highwa way y dra drain inag age? e? Desc Descri ribe be the the entire requirement.

2.

Dra Draw and and desc descri ribe be the the foll follo owing wing aspe aspect cts: s: a. Box Box culv culver ert. t. b. Arch Arch culv culver ertt


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Answer 1.

Highway Drainage Requirements

a.

Surf Surfac ace e wat water er shou should ld not not be be all allow owed ed to rema remain in stan standi ding ng on the

road pavement and shoulders. Measures should be

taken to drains off this water, immediately. b.

The The sur surfa face ce rain rain wate waterr fro from m the the adjo adjoin inin ing g are area, a, shou should ld not not be allowed to come towards the road surface. For this, general slope of the ground adjoining road, should be made slopping away from the road. This objective can be achieved by aligning road on ridge.

c.

Side Side drai drains ns shou should ld be of suff suffic icie ient nt capa capaci city ty and and hav havin ing g sufficient longitudinal slope so that it may drain of all the collected surface water, efficiently.

c.

Surf Surfac ace e wat water er flow flowin ing g acr acros oss s the the road road pave paveme ment nt shou should ld not not develop cross ruts or erosions on road surface and shoulders. For this, high embankment slopes should be protected either by turfing or pitching.

d.

Seep Seepag age e wat water er and and oth other er capi capill llar ary y wat water ers s sho shoul uld d be be dra drain ined ed off by suitable underground drainage system.

e.

Maxi Maxim mum leve levell of of und under er grou groun nd wat water er tabl table e sho shoul uld d be be maintained well below the sub-grade level of the road. Under ground water table should remain at least 1 m to 2 m below the road sub-grade.

Highway Engineering Engineering f.

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In water ater logg logged ed area areas, s, spec specia iall mea measu sure res s sho shoul uld d be be tak taken en to keep down the harmful salts.

2.

Box Culvert

These culverts are constructed where the nature of the soil below the foundation is not suitable for individual footing under piers and abutments. It is a monolithic rectangular drainage structure. The size of the rectangular passage should not be less than 60cm X 60 cm for easy cleaning of debris. Short span box cilverts can be precast.

Figure 6.11 : Box Culvert

Arch Culvert

This culvert is preferred under the conditions where high fillings are envolved and there are heavier loadings on the culvert. Arches may be built from brick, or stone masonry, or plain cement concrete. Span of each arch should be kept less than 3 m.

Road Surface


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Arch

Abutment Bed Pitching

Figure 6.12 : Arch Culvert

Selection of culvert to be used is done on the basis of availability of construction materials and economic considerations.

Unit 8 ( HIGHWAY DRAINAGE )

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