Large-Displacement Shear Behavior and Particle Breakage Evolution of a Crushable Coral Sand–Steel Interface

The large-displacement shear behavior of crushable granular interfaces is critical to pile foundations in reef engineering, but the coupled evolution of strength, deformation, and particle breakage at coral sand–steel interfaces remains poorly understood, especially regarding shear rate effects and the transition from coral-sand-dominated to quartz-sand-dominated behavior. Ring shear tests were conducted on dry coral sand–steel interfaces under constant normal load (CNL) conditions, varying normal stress, relative density, shear rate, and quartz-sand content. Post-shear particle-size distributions were used to quantify breakage via Hardin’s relative breakage index. Results show that peak and residual strengths increase with normal stress, with relative density effects becoming more evident at higher normal stress. Dilatancy is suppressed as normal stress increases. Peak strength depends non-monotonically on shear rate, peaking near 6°/min, while residual strength is much less rate-sensitive. Increasing quartz-sand content reduces strength and breakage and promotes dilation. Transition ranges at approximately 30% and 85–90% quartz-sand content indicate a shift from coral-sand-dominated to quartz-sand-dominated behavior. Persistent contraction at large displacements, attributed to particle breakage and fines infilling, may cause additional settlement. These findings clarify the coupled roles of breakage and composition in interface behavior.