Subsurface drainage and its importance

Sub-surface drainage is the system of structures that intercepts, lowers and removes water below the ground surface, such as ground water, seepage flow, and capillary water, so that the road subgrade and pavement layers stay dry. It includes longitudinal and transverse trench drains with perforated pipes, interceptor drains, drainage blankets, horizontal drains and capillary cut-off layers.

Importance

1. Keeps the subgrade strong. A subgrade at high moisture content loses its CBR and bearing value, so the pavement deflects and fails.
2. Prevents pavement damage: pumping of fines, stripping of bitumen, frost heave and volume change of expansive soils.
3. Stabilises slopes in cuttings and hills by lowering pore-water pressure and so preventing slides.
4. Controls seepage and springs, which cause wet patches, weak formation and erosion.
5. Increases pavement life and reduces maintenance cost, since a drained pavement carries many times more traffic than a poorly drained one.
6. Allows lower thickness design because a higher soaked CBR value can be used.