Sub – Surface Drainage.

$\rightarrow$ (MU – Dec. 13, May 14)

$\rightarrow$ Definition.

When the system by which sub soil water from underside of a road pavement is collected and removed then it is called sub surface drainage or sub soil drainage.

• Sub – Surface drainage is specially used to control the moisture content of the road sub grade.

• If the moisture content accumulating in the sub soil is excessive, then road structure becomes weak. Hence it is more essential to control the moisture of the road sub grade.

• The factors which increases the sub soil moisture content are:

1] Rise in ground water table.

2] Seepage of water from adjoining areas.

3] Capillary action which causes the rise of moisture above ground water table.

4] Percolation of surface water through joints and cracks.

• The factors which causes the rise in sub soil moisture or sub surface moisture can be well controlled by providing longitudinal drains, cross drain and impervious bituminous layer which specially controls the capillary rise.

$\rightarrow$ Functions of Sub – Surface drainage

1. To control the moisture capacity of the road sub grade.

2. To maintain the bearing capacity of the sub grade soil by preventing the entry of water into it.

3. To reduce the capillary rise, sometimes due to capillary action, the water rises into the sub grade from the ground water. It can be controlled by introducing a cut off layer of granular material or impervious bituminous layer.

4. Lowering the water table by providing L – drains (Longitudinal drains) and C – drains (Cross drains).