Control of Sediment Flow into Subsurface Drains

Sediment inflow into subsurface drains in noncohesive soils causes serious maintenance problems. An impervious hemispherical cover over a drain having inflow openings only at the top was previously proposed to solve this problem. Water flows between the cover and the outer drain surface in an upward direction before entering the openings at the top of the drain. Sediment inflow is prevented if the flow velocity is less than the critical boiling velocity. In this paper the shape of the upflow channel openings and the velocity distribution across the openings are evaluated. Terzaghi's concept of effective stress is the basis for the theoretical velocity distribution. An electric analog model is employed to verify this theory. A special laboratory model is used to determine critical velocities. Critical boiling velocities for fine sand and glass beads are about 1% of Stokes' settling velocity. Within reasonable limits the upflow channel width and shape can be designed to prevent movement of sediment. High water tables have the greatest influence on velocity.