Sub-surface drainage is intended to take away the water seeping through the pervious layers of the soil, causing a high water table, or in cut slopes of hills. The arrangement usually consists of a trench, at the bottom of which is laid a perforated or porous pipe. The trench is filled with stone aggregates. Figure shows a sub-surface drain in a hill slope, which drains away the water and prevents landslides and slips. Figure shows a sub-surface drainage system for lowering the water table.
Figure : Subsurface Drain at Cut Slopes
Figure : Sub-drains for Lowering Ground Water Table