Variation in Shearing Rate Effect on Residual Strength of Slip Zone Soils Due to Test Conditions

Abstract

Shearing rate effect on the residual strength of slip zone soils is considerable for sliding or reactivated landslides. It is complicated and variably affected by external inducing factors, and its variation has not attracted much attention. Using drained ring shear tests on the slip zone soils from two selected landslides with the shearing rates of 0.1–20 mm/min, this study investigated the variation in the shearing rate effect on the residual strength under the influences of the clay fraction, water content, and normal stress. In addition, the shear zone structures were analysed using microscopic observations. The results show that the residual strength decreases with increasing shearing rate (i.e., negative rate effect) and exists a good linear relationship with the logarithmic shearing rates. The slope of the fitting lines of the residual stress ratios versus shearing rates is proposed to assess the shearing rate effects. Furthermore, the effect is significantly affected by the clay fraction, water content, and normal stress. The samples with the low clay fraction exhibit a more significant negative rate effect, and the effect weakens with decreasing water content and normal stress, which are associated with the variations in the shear zone structures due to particle breakage. Additionally, increasing shearing rates and shear displacement change the shear zone structures and thus induce a negative rate effect on the residual strength of the soils.