Tube Drainage in Stratified Soil above an Aquifer

Potential flow theory is developed to give the relation of the drain spacing to the water-table height midway between equally spaced tube drains in a two-layer stratified soil overlying an artesian aquifer under pressure. The tube drains, which are located in the upper layer, receive both steady downward recharge from the soil surface and upward seepage from the aquifer. The ratio of the drain spacing for a layered soil to the spacing for a homogeneous soil is 3.8 for these conditions: (1)The hydraulic conductivity ratio of the upper soil layer to the lower is 10; (2)the upper layer is one-fifth the combined thickness of the two; (3)the recharge is zero; (4)the ratio of the artesian head (measured with respect to the level of the drain tubes) to the desired maximum water table head above the drain centers is 2; and (5)the drain diameter is 0.02 times the combined thickness of the two layers. If the conductivity ratio is 100, for these conditions, the drain spacing ratio changes from 3.8 to 12.7, and if 1,000, to 41.1.