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.