Critical gradients and pressures in dense swelling clays

Abstract

Uncertainties exist with regard to the applicability of Darcy's law to dense swelling clays. These clays may not allow water to pass through them when the hydraulic gradient is below a critical value. Preliminary results are presented from a series of constant-head permeability tests on dry, confined, densely compacted bentonite clays. The tests are intended to clarify the applicability of Darcy's law to dense bentonites: these materials may be used for isolation of nuclear fuel wastes in deep geologic disposal vaults. On wetting and with increasing hydraulic gradient, the clays develop swelling pressures, and some specimens appear to exhibit a critical hydraulic gradient or pressure. Below these gradients and pressures, water does not appear to flow through the materials. Once the apparent critical gradient is exceeded, water flux through the materials increases linearly and directly with gradient. Water continues to flow if the gradient is subsequently decreased to values below the original critical value. The possible importance of this finding to effective stress testing of dense bentonite materials is briefly discussed. The hydraulic performance of dense bentonite clay barriers over the range of conditions anticipated in a nuclear-fuel-waste disposal vault remains uncertain. Studies of material behaviour within the anticipated constraints of emplacement state and hydraulic boundary conditions are required. Key words : clay, bentonite, Darcy's law, effective stress, compacted clay, swelling pressure.