Influence of Displacement Rate on Residual Shear Strength of Clays

This paper reports on the results of direct shear tests carried out under controlled displacement rate in the range of 10-4 - 102 mm/min, under different normal stresses, with different shear devices. The tests were carried out on a kaolin, a bentonite, their mixtures with sand at various percentages, and the clayey soil of the Costa della Gaveta earthflow. The tests were performed on specimens reconstituted with distilled water as well as with NaCl solutions at various concentrations. Positive rate effects were exhibited by mixtures with c.f. higher than 50% and, consistently, by the natural clayey soil the c.f. of which is about 50%. The residual shear strength increases significantly for shear displacement rate higher than about 1 mm/min. The rate effect increases with the pore solution concentration. The residual shear strength independence of displacement rate has been confirmed in the range 10-6 - 10-1 mm/min by the results of shear tests performed under controlled shear stress, with varying chemical conditions of the pore fluid.