Abstract
The purpose of longitudinal drains under roads and their effectiveness in lowering the water table are discussed. The effect of lowering the water table on the strength of road subgrades, and hence on the design thickness of flexible pavements, is examined. Shallow longitudinal drains are used to drain the road structure and also to remove surface water. Deeper drains in permeable soils can be used to lower the water-table and thus to increase the strength of the sub-grade. However in saturated soils of low permeability, such as heavy clays, deep drains are ineffective in lowering the water-table below formation level and do not significantly increase the strength of the soil. (Author/publisher)