Shear displacements induced by decrease in pore solution concentration on a pre-existing slip surface

• The residual strength τr of a bentonite reconstituted with 1 M NaCl solution is higher than that of the clay with water.
• Stresses lower than τr in solution and higher than τr in water don’t cause creep along the slip surface even at the residual.
• Exposure to water causes pore ion concentration decrease, τr decrease, and thus displacement rate increase until failure.
• The processes of shear strength decrease/ displacement rate increase due to ion diffusion seem to be drained.

This paper reports on experimental results that show how variations in pore fluid composition can induce time dependent shear displacements in clay soils under constant effective stresses. The experimentation has been carried out on a bentonite, mainly composed of Na-montmorillonite. The clay, reconstituted with distilled water and with NaCl solutions at various concentrations, was first sheared to the residual state under constant displacement rate. The residual friction angle was found to increase greatly with the pore solution concentration. Then, the solution-saturated specimens were submitted to shear tests under constant driving shear stresses lower than the residual strength obtained with the salt solutions and higher than the residual strength obtained with distilled water. As a consequence, shear displacements with decreasing rate occurred. On the subsequent exposure to distilled water, the displacement rate increased progressively and the specimens re-experimented failure. The observed behaviour has been attributed to the loss of strength caused by the decrease in pore solution concentration following exposure to water. During this transient phase, shear strength and average pore ion concentration on the slip surface seemed to be related by the same relation as that evaluated in the absence of chemical gradients.