Effect of Backfill on Drain Flow in Layered Soils

Transient flow into drains is studied by obtaining computer solutions of the Richards equation by the Alternating Direction Implicit (ADI) method. The flow towards drains placed in two-layered soils is examined for two backfill conditions. In the first case the drain is installed so that the soil layers remain unchanged. This might correspond to a drain pulled into the ground by a plow. In the second case the excavated trench is backfilled with surface soil. The entire backfill is assumed to have the hydraulic conductivity of the surface soil. The study shows that the backfill has a very significant effect on the flow into drains in layered soils. The computer results are also compared to several well-known transient drainage equations for homogeneous soil.