Hydraulic conductivity of clays in confined tests under low hydraulic gradients

Clay barriers normally function at hydraulic gradients much lower than are commonly used in laboratory tests for hydraulic conductivity. This paper de scribes low-gradient tests on compacted illite, Na-bentonite, and sand-bent onite specimens at a range of dry densities. The tests examined the effects of deionized or saline pore fluid, the initial degree of saturation at the time of compaction, and back-pressuring to achieve saturation during perme ation. No "critical" or "threshold" gradients were observed. In low-density materials, "transitional" gradients de-fined two separate regions of Darci an flow. Low flow rates and low hydraulic conductivities were associated wi th hydraulic gradients below the transitional gradient. Higher conductiviti es were associated with gradients above the transitional gradient. Hydrauli c conductivities decreased with increasing initial density. They appeared i ndependent of initial degree of saturation or back-pressuring. Tests on Na- rich bentonite showed that deionized and saline permeants produced similar hydraulic conductivities. Measured hydraulic conductivities were in reasona ble agreement with values computed using the Poiseuille and Kozeny-Carman e quations.