Effects of confining pressure and effective stress on hydraulic conductivity of sand-clay mixtures

Abstract

The hydraulic conductivity is highly influenced by the state of confinement and effective pressure applied. Hydraulic conductivity evaluated using indirect (i.e., oedometer tests) or direct techniques such as constant or falling head methods induces lateral confinement. The most reliable direct measurement tests are those considering triaxial or flexible wall setup such that field conditions are simulated. The ASTM D5084 and BS1337-6: 1990 are appropriate approaches to give reliable hydraulic conductivity values. Both tests require considerable time due to various stages of saturation, consolidation, and permeation. This study provides a comparison between indirect hydraulic conductivity tests and standard flexible wall tests in order to cut down the time needed to perform the test for different sand-Alqatif clay mixtures. Sand-clay mixtures used as liners for waste containment or water barriers are very sensitive to the hydraulic conductivity and need to be designed for different values of effective pressures. This research presents prediction equations derived from a series of direct and indirect tests carried out for sand-Alqatif clay mixtures. The results indicated that oedometer tests underestimated the hydraulic conductivity by 2 to 3 orders of magnitude at different effective pressures. The hydraulic conductivity was found to vary with effective pressures indicating that near surface material yields higher hydraulic conductivity than deeply seated similar soils.