Effects of test conditions on shear behaviour of composite soil

Test conditions, such as shearing rate and normal stress, have been long recognised as influencing the measured shear strength of clays and sands. However, their influence on composite soils, which have a wide range of particle sizes, has attracted much less attention from researchers. In this study, a total of 35 direct shear tests at different shearing rates under different normal stresses were conducted on specimens prepared by mixing different proportions of kaolin and glass beads. The changes in volume and water content of sheared specimens and the mesostructure of shear surfaces were studied. The results reveal a positive correlation between volume change and deviation of water content between the shear zone and outer zones, suggesting that the shear-induced volume change occurred primarily in the shear zones. Moreover, high normal stress and low shearing rate produced a relatively small void ratio in the shear zones, facilitating volumetric contraction of the specimen and the development of polished and evenly slickensided shear surfaces. In addition, the residual friction angle of the specimens tested was found to decrease with the increase in normal stress. The high shearing rate caused an increase in residual strength in specimens with low fines fraction, and reduced it in specimens with high fines fraction.