Effect of Mineral Composition and Shearing Rates on the Undrained Shear Strength of Expansive Clays

Evaluation of undrained shear strength is very important in geotechnical engineering. Undrained shear strength is widely used in the slope stability analysis or foundation design for undrained loading. Several articles in the literature address the undrained shear strength of natural soils through the use of both laboratory procedures as well as in-situ testing results. However, quantification of the undrained shear strength characteristics for different mineral compositions as well as the effect of the shearing rate on the strength is rarely available. This study provides a systematic evaluation of the influence of mineral compositions and shearing rates on the undrained shear strength of a soil mass. Fifteen samples composed of different proportions of quartz and clay minerals such as montmorillonite and kaolinite, were prepared. The undrained and drained friction angles and cohesions of the mineral mixtures were obtained with the use of a fully automated constant-volume direct simple shear device, using the effective vertical stresses of 50 kPa, 100 kPa, 150 kPa, and 200 kPa. The effective shear strength parameters obtained from the undrained simple shear tests were similar to the drained shear strength parameters obtained from the direct shear testing. Moreover, increasing the shearing rate resulted in reductions in the shear strength. BACKGROUND