Influence of rate effects on the residual strength of moving slopes

Abstract

The existing literature on the rate effects on residual strength is reviewed. Clearwater clay-shale, present in the foundation of Syncrude Tailings Dyke situated in northern Alberta, is tested under different rates, in both ring shear apparatus and the direct shear box. The clay-shale, which has a Plasticity Index of 107%, a Liquid Limit of 135%, natural water content of 23% and clay content of 49% shows an improved residual shear resistance at higher rates. It is found that the residual strength is increased by 3.4 - 3.5%, for a tenfold increase in the strain rate. The tested range of displacement rates is 0.185 mm/day to 7.0 mm/day. Judging from the existing literature on such rate effects, a broad correlation between the plasticity of clay and rate effects is obtained. By extension of the plasticity theory, a constitutive relationship is developed to model the strain rate-dependence of residual strength. It can be easily incorporated into existing finite element plasticity codes. The model is coded in the program PISATM (Chan and Morgenstem, 1992) and calibrated using an illustrative example. A simulation of construction movements at Cell 23 of Syncrude Tailings Dyke has been done. Cell 23 of the tailings dyke has been identified as a problem area where more than 38 cm of horizontal movement has been observed at some locations over a period of 11 years. Clearwater clay-shale, present beneath portions of the dyke, is found previously sheared, due to glacial drag forces of the overlying till. The use of the rate-dependent model, for the shear zone, was found to provide a satisfactory basis for simulation of the movement pattern. The results of the deformation analysis provide the anticipated field velocities and show how they deteriorate with time to a reasonable accuracy.