Effects of confining stress and roughness on mechanical behavior of sand-steel structure interface

This study presents an experimental investigation into the macro- and micro-scale shear mechanical behavior of sand-structure interfaces. Objectives include characterizing stress-displacement and volumetric responses, identifying optimal interface roughness, and understanding strain localization and kinematics failure mechanisms within shear and dilation zones. Direct shear tests were performed on sand interfaced with steel plates exhibiting varying, well-defined trapezoidal sawtooth roughness profiles ( R _n ranging from 0 to 21.6) under normal stresses from 50 to 350 kPa. A modified direct shear apparatus integrated with PIV technology enabled real-time, non-contact monitoring and quantitative analysis of the sand deformation field, correlating macroscopic mechanical responses with microscale observations. Results showed that interface peak shear strength decreased in stress ratio ( τ _η /σ _η ) with increasing normal stress, with R _n = 1.35 yielding the highest strength. Volumetric behavior transitioned from dilative to contractive-dilative modes as normal stress increased, with peak contraction near peak strength. Interface shear strength efficiency ( α ) generally decreased with increasing normal stress, indicating a transition from internal shearing within adjacent sand to predominantly interfacial sliding failure mode. PIV analysis provided direct visualization and quantification of shear band and dilation zone formation and evolution. The thickness and morphology of these zones were affected by both normal stress and interface roughness; higher normal stress generally suppressed dilatancy, while specific roughness profiles modulated strain localization. Microscale kinematics observations confirmed non-uniform deformation patterns, highlighting the critical role of particle overriding and rearrangement. The findings underscore the importance of integrating macro- and meso-scale to achieve a comprehensive understanding of sand-structure interface behavior.