Unit 8 ( HIGHWAY DRAINAGE )

HIGHWAY DRAINAGEHighway Engineering C3010 / 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

UNIT 8UNIT 8


8.0 INTRODUCTION TO HIGHWAY DRAINAGE

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.

INPUT


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 EFFECTS OF IMPROPER DRAINAGE

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 surface if made of soil, gravel or water bound

macadam, it will becomes soft and losses strength.

b. The road sub-grade may be softened and its bearing

capacity reduced.

c. Variation in moisture content in expensive soils, causes

variation in the volume of sub-grade and thus causes failure

of roads.

d. Failure of formation slopes is also attributed to poor

drainage.

e. If rain water is not properly drained and allowed to flow along

the road side for long distances, slip and land slides may

occur causing road failures.

f. Erosion of side slopes, side drains, formation of gullies may

result if proper drainage conditions are not maintained.

g. Flexible pavement’s failure by formation of waves and

corrugations is due to poor drainage.

h. Continuous contact of water, with bituminous pavements

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. Rigid pavement’s prime cause of failure in by mud pumping

which occures due to water in fine sub-grade soil.


j. Excess moisture causes increase in weight and thus

increase in stress and simultaneous reduction in strength of

the soil mass. This main reason of failure of earth slopes and

embankment foundation.

k. Erosion of soil from the top of un-surfaced roads and

embankment slopes in also due to surface water.

8.2 HIGHWAY DRAINAGE REQUIREMENTS

a. Surface water should not be allowed to remain standing on

the road pavement and shoulders. Measures should be

taken to drains off this water, immediately.

b. The surface rain water from the adjoining area, 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 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 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 other capillary waters should be drained off

by suitable underground drainage system.

f. Maximum level of under ground water table should 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.


g. In water logged areas, special measures should be taken to

keep down the harmful salts.


SURFACE DRAINAGE

8.3 INTRODUCTION TO SURFACE DRAINAGE

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 SIDE DITCHES FOR SURFACE DRAINAGE

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 V-

shaped or trapezoidal, the former being generally preferred. However, the

trapezoidal ditches have a higher capacity.

INPUT


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.


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.

Figure 6.2 : Cross section of intercepting ditch

Shoulder Slope

0.3to 0.45m

0.6to

1.0m

45 to 60 cm

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

Intercepting Ditch


8.6 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/cm2 at 7

days and about 200 kg/cm2 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.

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.

Figure 6.3 : The arrangement of side surface drainage

3 cm

Road Surface

8 cm

10 cm

22.5 cm

53 cm

SECTION OF CHANNEL AT SUMMIT

8 cm

53 cm

SECTION AT MID POINT BETWEEN SUMMIT AND GULYY

Road Surface

8 cm

Outlet Pipe

27 cm

14 cm

45 cm

Road Surface

SECTION AT GULLY


Figure 6.4 : Section of a shallow concrete gutter

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

4.8PIPE DRAINS

20 cm16 cm

SLOPE: 1 IN 12

4 cm

15 cm

60 cm

30 cm

10 cm

8 cm

30 cm

Pavement Surface


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.


SUB - SURFACE DRAINAGE

4.10 INTRODUCTION TO SUB-SURFACE DRAINAGE

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 of water from higher adjoining ground.

b. Penetration of moisture through the pavement.

c. Percolation of water from shoulders, pavement edges and soil

formation slopes.

d. Rise or fall of underground water table.

e. Capillary rise of moisture in case of retentive type of soils.

f. Transfer of moisture vapour through soil.

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 sub-

grade 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.

INPUT


4.11 SUB-SURFACE DRAINAGE SYSTEMS

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 sub-

grade 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.


4.12 CONTROL ON SEE PAGE 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.

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

4.13 CONTROL ON CAPILLARY RISE

Lowered Seepage

Zone of seepage flow

Impervious Sloping Straum

Line Perforated Drain Pipe

Filter Material

Pavement

Clay seal

Original Seepage Flow

Sloping Ground


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. Provision of a granular layer of suitable thickness or of sand

blanket.

b. Provision of impermeable layer of bituminous material.

c. Heavy duty tar felt, polythene envelop, are the other

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


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

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/m2 is used

for this layer.

Impermeable Layer

Capillary Rise

Highest Water Table


CROSS DRAINAGE

4.14 CROSS DRAINAGE WORKS

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. Pipe Culverts

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

INPUT


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.

Figure 6.9 : Pipe Culverts

b. Slab Culvert

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.

Road Surface


Figure 6.10 : Slab Culvert

c. 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

Bed Pitching


d. 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.

Figure 6.12 : Arch Culvert

Selection of culvert to be used is done on the basis of availability of

construction materials and economic considerations.

Arch

Abutment

Bed Pitching

Road Surface

ACTIVITY 8


TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT

INPUT

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.

Question

FEEDBACK ON ACTIVITY 8


1. Slab culvert, Box culvert, Arch culvert and Pipe culvert are the

usual types of culverts most commonly used for cross drainage

works.

2. 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/cm2

at 7 days and about 200 kg/cm2 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.

Answer


1. What are the requirements for highway drainage? Describe the

entire requirement.

2. Draw and describe the following aspects: -

a. Box culvert.

b. Arch culvert

Question


1. Highway Drainage Requirements

a. Surface water should not be allowed to remain standing on

the road pavement and shoulders. Measures should be

taken to drains off this water, immediately.

b. The surface rain water from the adjoining area, 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 should be of sufficient capacity and having

sufficient longitudinal slope so that it may drain of all the

collected surface water, efficiently.

c. Surface water flowing across the road 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.

d. Seepage water and other capillary waters should be drained

off by suitable underground drainage system.

e. Maximum level of under ground water table should 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.

f. In water logged areas, special measures should be taken to

keep down the harmful salts.

Answer


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.

Arch

Abutment

Road Surface


Figure 6.12 : Arch Culvert

Selection of culvert to be used is done on the basis of availability of

construction materials and economic considerations.

Bed Pitching

Unit 8 ( HIGHWAY DRAINAGE )

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