Rate and acceleration effects on residual strength of kaolin and kaolin–bentonite mixtures in ring shearing

Abstract

Rate effects on residual strength play an important role in predicting and evaluating the behaviour of reactivated landslides and in the application of test results and procedures. Although the rate dependency of residual strength has been investigated extensively, there is a lack of consistency in the experimental results. In particular, the effect of the shear rate on the residual strength of high-plasticity, low-permeability soils is not well understood. Additionally, field observations suggest that the sliding velocity of a soil block in the residual state is not constant and that its acceleration may affect the residual strength of the soil. However, the effect of acceleration changes on the residual strength of soil has not yet been investigated. The main objectives of this study are to elucidate the rate dependency of the residual strength of high-plasticity soils and investigate the effect of acceleration. In addition, the effect of the test procedure on the rate effect is also examined. A number of kaolin and kaolin–bentonite samples were tested using the Bishop-type ring shear device. The single-stage procedure which the shear rate was not changed during shearing, was conducted on individual specimens with shear rates from 0.02 to 20 mm/min to investigate the rate effect. The accelerations of 0.028 and 0.014 mm/min2 were applied in multi-stage procedure with shear rates increasing gradually from 0.002 to 20 mm/min to examine the acceleration effect. The test results show that the bentonite content significantly affects the rate effect on the residual strength of kaolin–bentonite mixtures. For instance, in the single-stage procedure, the presence of just 10% bentonite changes the rate effect from positive to negative. This may be due to the generation of unknown excess pore water pressure under fast shearing. Furthermore, the test results also show that the type of rate effect observed depends on the test procedure. For mixture samples of 90% kaolin and 10% bentonite, the rate effect is negative in the single-stage procedure but positive in the multi-stage procedure. The analysis of the experimental results suggests that the effect of acceleration can be neglected in determining the residual strength.