Effects of Surface Roughness, Gradation, Relative Density, and Normal Stiffness on Global and Local Behaviors of the Sand–Steel Interface

Sand–steel interaction directly influences foundation stability. To study the global and local behaviors of the sand–steel interface (SSI) under monotonic loading, a series of modified SSI shear tests were conducted with the aid of the particle image velocity technique. The factors of surface roughness, sand properties, and loading conditions were comprehensively investigated. For the tests on rough steel plates, the interface exhibited distinct strain softening and dilatancy, whereas tests on smooth plates only showed strain hardening and negligible compression. Sand dilatancy on rough surfaces increased with relative density and mean grain size. The interface friction angles increased with relative density and exhibited a certain change from constant normal load to constant normal stiffness boundary conditions. Due to varying grain interlocking levels, the evolving trends of friction angle versus surface roughness were divergent in sands with different grain size distributions. Further image analysis provided evidence of significant shear band evolution and grain rearrangement at the SSI. The shear band thickness was notably influenced by surface roughness, gradation, and grain size. Two scalars based on grain size probability entropy and grain orientation probability entropy were proposed to capture microstructure changes during shearing, where they were highly related to the intrinsic arrangement of sand grains at the interface but irrelevant to the loading conditions. No shear band and negligible fluctuation in the microstructural scalars were observed at the SSI with a smooth surface, where only sliding motion occurred.