Global and local behaviors of sand-steel interfaces subjected to cyclic and post-cyclic shear loading

• Developed a modified direct shear apparatus enabling cyclic and post-cyclic interface shear tests. • Applied mathematical expectation to quantify soil microstructural characteristics from image analysis. • Examined how gradation, surface roughness, and displacement amplitude affect sand-steel interface response. The sand-steel interface (SSI) behavior is crucial for the stability of offshore structures in marine environments. A series of cyclic and post-cyclic shear tests were conducted using a modified direct shear apparatus (MDSA), to examine both global and local SSI behaviors. Experimental visualization and image processing improvements enabled a comprehensive investigation of the influences of material properties and loading characteristics. Cyclic loading induced significant dilation-contraction alternations, with overall contraction predominating. Boundary conditions influenced the trends of cyclic shear stress, which stabilized after a specific loading cycles. Post-cyclic SSI exhibited notable dilation and stress-softening. Sands with improved gradation showed poor SSI strength development and more pronounced cyclic contraction. An optimal surface roughness for SSI strength development was identified. Dilation state lines, reflecting cyclic deformation, displayed a downward trend with progressive loading cycles. Friction angles, affected by displacement amplitude, underwent a weakening-strengthing-stabilizing process with cumulated shear displacement. Shear band thicknesses, sensitive to normal stress, surface roughness, and displacement amplitude, gradually stabilized with increased shear displacement. Grain microstructure evolution during cyclic shearing depended on the initial sand fabric. Fine grains migrated and collapsed during cyclic loading, combined with grains lying down, lubricating sand-sand friction. Insignificant macro-micro changes in SSI with a smooth plate indicated only sliding motion occurring.