Laboratory Evaluation of Sand Underdrains

Constant-head hydraulic conductivity tests are performed on layered heterogeneous porous media to evaluate the use of underdrains to calculate the hydraulic conductivity of an overlying, less permeable medium. The layered profiles consist of a barrier layer comprising sand mixed with 10% kaolin, overlying a foundation layer comprising sand mixed with only 5% kaolin. Underdrains are evaluated by replacing excavated portions of the foundation layer with only sand. The results indicate that preferential flow of water occurs around, rather than through, the sand underdrains resulting in an underestimate of the measured hydraulic conductivity of the barrier layer assuming 1D, saturated flow in accordance with standard practice. The observed preferential flow effect is consistent with previously published numerical simulations of unsaturated flow through similarly layered heterogeneous soil profiles that indicate lateral flow around underdrains due to the contrast in unsaturated properties of the soils. The results of this study have important ramifications with respect to the use of underdrains to measure in situ hydraulic conductivity of compacted clay liners for waste containment.