Highway drainage is the process of removing and controlling the excess surface and sub-surface water within the road pavement and shoulder so, design, protection and maintenance of approximate highway drainage system are one of the most important tasks in the highway engineering.
1.2: Types of the Drainage system
There are two types of highway drainage system:
It enables the water to drain off from the pavement surface and shoulders of the road. This is carried out by providing the chamber on the road, longitudinal drain and cross drainage structure.
2)Subsurface drainage system
The sub-surface drainage system deals with the diversion or removal of water from the sub-grade soil. This is carried out by providing pipes in sub-grade soil and providing the impermeable layer.
1.3: Importance of highway drainage
Increase in moisture contains causes a decrease in strength and stability of soil mass which supports the pavement layer. Proper highway drainage is important because of the following reason:-
1) Excess moisture contains causes decreases in stability.
2)Excess water in the pavement causes damage to the road structure.
3)Stagnation of the water on the pavement surface can cause a reduction in strength characteristics.
4)In some soil variation in moisture contain cause differential swelling and shrinkage.
5)Sustained contact of water with bituminous pavement causes stripping of aggregate from the road surface.
6)Ineffective and improper drainage system cause erosion of soil mass from the top surface of the pavement.
1.4: Requirement of a good drainage system
1)The surface water of the carriageway and shoulder should be drained off before it percolates into the sub-grade.
2)The surface water from the adjoining land should be pavement from entering on the road.
3)Side drain should have sufficient capacity and longitudinal slope.
4)The flow of surface water towards the longitudinal drain should not cause erosion.
5)Surface and sub-surface water source should be intercepted and drained off by a suitable system.
6)Highest level of groundwater table should be at least 1-2 meter below the pavement surface.
7)In the waterlogged area, special precautions should be taken.
1.5: Field construction procedure
1)Surface drainage construction
The surface water from the roadway and adjoining land are to be collected are disposed off with the help of the surface drainage system. It consists of three components:-
c)Cross drainage structure
Camber is constructed based on the type of road pavement surface. It shall be constructed as smaller as possible. The size of camber varies from 1.7% to 4%.
These are constructed parallelly with the road alignment. The size of the side drain should be such that it can carry water from the right of way. The slope of the side drain should be such that it will be within non-silting and non-scouring flow.
c)Cross drainage structure
These are constructed to dispose of the water from side drain to natural water courses.
Hence surface drainage shall be constructed based on:-
1) Hydrological Analysis
-The topography of the land
-size and shape of the drainage
-non-silting and non-scouring velocity
2)Subsurface drainage construction
Change in moisture contained is caused due to the fluctuation in the groundwater table, percolation of rainwater and movement of capillary water. It is attempted to keep the moisture contained variation as minimum as possible. It can be achieved by;-
a) Lowering the water table.
b)control of seepage flow.
c)Control of capillary rise.
a) Lowering the water table
The water table must be at least 1-2 meter below the road pavement surface. In permeable soil, longitudinal drainage should be constructed while for less permeable soil transverse drainage should be constructed along with the longitudinal drain.
b)control of seepage flow
When the road is constructed on the ground as well as on impervious strata below sloping, seepage flow is likely to exit. If seepage zone is not at least below 0.9-meter longitudinal drains shall be constructed.
c)Control of capillary rise
If the water reaches sub-grade soil due to capillary rise, it is possible to stop water instead of lowering the water table. It is done by constructing the impervious layer.
Hence, sub-surface drainage construction is controlled by:-
1)Soil grain characteristics
2)The groundwater table of the land