Hydraulic Considerations in Geosynthetic and Aggregate Subsurface Drains

Abstract

The hydraulic design of certain types of subsurface drains has recently been put on a more rational footing, and deficiencies in earlier design methods have been demonstrated. However, significant limitations remain in hydraulic design methods for geosynthetic and aggregate subsurface drains. It is important to decouple the groundwater hydrology from the internal hydraulics of the drain, and properly design subsurface drains for open-channel rather than pressurized conditions. Present design methods can inadvertently result in pressurized flow. The assumption of uniform flow (Manning’s equation alone) is also improperly made in some present design methods. Consequences can include unintended pressurized flow and attendant nonuniformity of inflow on the one hand and uneconomical design on the other. Current standard guidelines provide relatively little guidance for the design of geosynthetic and aggregate drains. A current ASTM standard, commonly referenced by geosynthetic manufacturers, has significant limitations. Deficiencies and qualifications are identified for present design methods. Guidance is given for the improved design of geosynthetic and aggregate subsurface drains based on sound hydraulic principles.