Study on Large-Scale Direct Shear Test on Soil–Rock Mixture in an Immersion State under Water

Soil–rock mixture (S–RM) is a natural geomaterial with multiple grain grades. It has dramatic trans-scale and hierarchical natural characteristics, and its mechanical properties are dramatically influenced by water. Various S–RM samples with multiple grain grades in engineering in situ state were prepared for large-scale and conventional direct shear tests in a native state and an immersion state under water to investigate the shear strength and deformation characteristics of S–RMs. The experimental results show that the S–RM in the natural state has high shear strength; the cohesion c is 44.0 kPa, and the internal friction angle φ is 34.7°. The shear strength of the S–RM after water immersion drops significantly: the cohesion c is 45.7 kPa, and the internal friction angle φ is 7.7°. The research results indicate that the cohesion of S–RM is mainly provided by the shear bond strength of the coarse grains, the development of which is governed by the gradation and surface topography of mineral grain and affected by constraint conditions. The interpretation of the sharp drop in the shear strength of S–RM after water immersion is that the sliding friction resistance between coarse grains and clay membrane and the occlusion friction resistance generated by the rotation transfer of coarse grains through clay membrane decreases sharply as the clay membrane around coarse grains becomes substantially soft. The shear strength of S–RM presents a dramatic scale effect and engineering design may come across major deviation and adventure when it adopts shear strength parameters determined by small-scale testing apparatus.