Significance of Drainage
Requirements of Highway Drainage
Methods of Surface Drainage
Methods Of Sub-Surface Drainage
Road Construction in water logged area
Highway drainage is a process of removing and controlling excess
surface and sub-soil water within the right of way.
It includes interception and diversion of water from the road surface
and sub grade.
Excess moisture in soil subgrade causes considerable lowering of its
Increase in moisture cause reduction in strength of many pavement
materials like stabilized soil and water bound macadam.
Sustained contact of water with bituminous pavements causes failure
due to stripping of bitumen from aggregates like loosening or
detachment of some of the bituminous pavement layers and formations
of pot holes
Excess water on shoulders and pavement edges causes considerable
In clayey soil variation in moisture content causes considerable variation
in volume of sub grade.
High moisture content causes increases in weight and thus increase in
stress and simultaneous reduction in strength of soil mass.
Erosion of soil from top of unsurfaced roads and slopes of embankment,
cut and hill side is also due to surface water.
In cold regions presence of water in the subgrade and a continuous
supply of water from the ground water can cause considerable damage
to the pavement due to frost action.
The surface water from the carriageway and shoulder should effectively
be drained off without allowing it to percolate to sub grade.
Surface water from adjoining land should be prevented form entering
Side drain should have sufficient capacity and longitudinal slope to carry
away all surface water collected.
Flow of surface water across the road and shoulders and along slopes
should not cause formation of cross ruts or erosion.
Seepage and other sources of under ground water should be drained off
by the subsurface drainage system.
Highest level of ground water table should be kept well below the level
of subgrade, preferably by atleast 1.2m.
In waterlogged areas special precautions should be taken, especially if
detrimental salts are present or if flooring is likely to occur.
During rains, part of the rain water flows on surface and part of it
percolate through the soil mass as gravitational water until it reaches the
ground water below the water table.
Removal and diversion of surface water from the road way and adjoining
land is termed as surface drainage.
Diversion and removal of excess of soil water from the subgrade is
termed as Surface Drainage.
1. By Longitudinal side drains.
2. Catch basins and Inlets in urban areas
3. Providing damp proof course
4. Providing proper camber
5. Providing sufficient slope to the sides
6. Keeping the level of carriage way at least 60 cm above the HFL.
It is necessary to provide side drain on one side or both sides, when road
is constructed in embankment.
Side drains should be at least 2.0 m away from bottom edge of an
Depth of side drains is kept 1.0 to 1.5 m to prevent the entry of drain
water into the embankment.
Catch basin is a structure like chamber constructed on a sewer line.
Water from pavement surface is collected in catch basin and discharged
to the sewer line.
The catch basin (catch pit) is provided with grating to prevent the entry
of rubbish into the drainage system.
Inlets is a concrete box with grating either at the top or in the side.
For a quick drainage it is necessary to ensure that shoulder surface is
properly sloped and free from irregularities and depressions.
In impervious type, it is practiced to extend the subbase course with
drainage across the shoulder upto side drain.
Alternatively we can provide cont layer of 75- 100 mm thick laid under
the shoulder at the bottom layer of sub base about 150 mm thick
extended upto the edge.
The paved shoulder should be at least 0.5% steeper then camber
subjected to min of 3%
The unpaved shoulder should be further 0.5% steeper along the
horizontal curve, Shoulder on inner side of the curve should have slope
steeper than that of pavement.
On the outer side, the shoulder should be made to drain away from the
pavement, a low rate of super elevation are provided.
On the other hand, where higher rate of super elevation, the outside
shoulder are kept level or rounded.
Except for a median less than 5m wide, it is not desirable to drain the
median area towards the pavement surface.
1.2-1.8 m wide median – Provided with kerbs and always paved.
1.8-5 m wide median – Usually turfed and crowned so that surface
water could run towards the pavement maybe with or without kerbs.
Median wider than 5m are without any kerbs at edge
If carriage way slopes towards the median, central drain maybe made to
carry the runoff and slope should not be steeper than 6:1.
When the median has only earthen surface, it should not be crowned
because it can result in soil particles being carried to the pavement
surface making it slippery.
Removal or Diversion of excess soil-water from sub-grade is termed as
The change in moisture of sub-grade are caused by the following
Fluctuations in GroundWaterTable
Movement of CapillaryWater
In sub-surface drainage it is practiced to keep the variation of moisture in
sub-grade to a minimum.
When road is in cutting and water seeping from sides.
When road is near the foot or hill and is likely to be damaged by water
flowing down the hill.
When road is passing through plain area and water is likely to
accumulated on sides.
Where water rises up to sub-grade by capillary action.
Where sub-grade soil is affected by the drain passing near the road.
In order to that the sub-grade and pavement are not subjected to
excessive moisture the water table should kept at least 1.0 to 1.2 m
below the sub-grade.
In places where water table is high to take the road formation on
embankment of height not less than 1.0-1.2m is the best approach.
ButWhen the formation level is at or below the general ground level it is
necessary to lower the water table.
(a) If soil is relatively permeable, it may be possible to lowerWT by
constructing longitudinal drainage trenches with drain pipe and filter
sand and top of trenches is covered with clay seal
The depth of trench depend on:
(i) The required lowering of water table
(ii) distance b/w the drainage trenches
(iii) Types of soil
(b) If the soil is relatively
less permeable, the
lowering of groundWT
may not be adequate.
Hence in addition to the
traverse drains have to be
installed at suitable
intervals in effectively
drain off the water.
When the general ground as well as impervious strata below are sloping,
seepage flow is likely to exist.
If seepage zone is at depth less than 0.6-0.9 m from sub-grade level,
longitudinal pipe drain in trench filled with filter material and clay seal
may be constructed to intercept the flow.
If the water reaching the sub-grade due to capillary rise is likely to be
detrimental, it is possible to solve the problem by arresting the capillary
It can be done by following methods:
(a) A layer of granular materials of suitable thickness is provided during
the construction of embankment, between the sub-grade and the
highest level of sub surface water table.
(b) Alternate method is providing the capillary cut off is by inserting an
impermeable or a bituminous layer in place of granular blanket.
The size and spacing of the sub-surface drainage system depend on the
quantity of water to be drained off, the type of soil and type of drain.
The filter material used in sub-surface drains should be designed to have
sufficient permeability offering negligible resistance to the flow.
And also resist the flow of the fine foundation soil resulting in soil piping.
The Steps are as follows:
Draw grain size distribution curve on log scale %passing vs. particles size
for the foundation soil.
Find the value of D15 size of foundation soil.
*Note D15- particle size such that 15%of soil is finer than this size
The size of filter material should be 5 times more than the size of
D15 of filter/D15 of foundation > 5
To fulfill the condition to prevent piping.
D15 of filter/D85 of foundation < 5
Hence plot a point represent the upper limit of D15 size of filter given by
5D85 of foundation.
Find the size of the perforation in the drain pipe or the gap in the open
jointed pipes and let this be = Dp
Plot a point representing D85 size of filter given by the size 2Dp.
The Shaded area thus obtained represents the region within which the
grain size distribution curve of satisfactory filter material should lie.
In case of sub-grade subjected soil water to soaking cond., capillary rise,
and in extreme cond. Even flooding for prolonged periods is termed as
water logged areas.
In such areas construction and maintenance of road is a problem.
Methods adopted to Overcome
Raising the road level by constructing embankment
Providing a capillary cut off to arrest capillary water
Providing Cement Concrete Block
Depressing the sub surface water level by drainage system