Study on large scale direct shear test on soil-rock mixture with multiple grain grades in high fill slope engineering

Soil-rock mixtures are multiphase natural geological materials composed of mineral particles of multiple grain sizes, and have significant natural characteristics of multiscale material groups. In order to investigate the shear strength characteristics and deformation failure characteristics of soil-rock mixtures with multiple grain sizes under in-situ conditions in fill engineering, samples of soil-rock mixtures with multiple grain sizes under in-situ conditions were prepared, and large-scale in-situ direct shear tests were conducted under natural and water-soaked conditions. The test results showed that the shear strength of soil-rock mixtures is relatively high, and after saturated by water immersion, its shear strength decreases significantly, mainly manifested in the decrease of internal friction angle while the cohesion remains basically unchanged. The cohesion of soil-rock mixtures with multiple grain sizes is mainly provided by the shear-breaking cohesion of coarse particles such as gravel in the soil-rock mixture, and its degree of exertion is controlled by the grading characteristics and particle morphology of the soil-rock mixture and affected by constraints. The reason for the significant decrease in shear strength of soil-rock mixtures with multiple grain sizes after water-soaked saturation can be explained as follows: the clay film covering the surface of coarse particles becomes soft after water immersion, resulting in a significant decrease in the sliding friction resistance between coarse particles and the clay film, as well as the occlusion friction resistance generated by the rotation transfer between coarse particles through the clay film interface.