Macroscopic and mesoscopic mechanical behavior of the interface between piles and unsaturated sand under vertical cyclic loading

Understanding the degradation of pile bearing capacity under vertical cyclic loading requires an in-depth investigation of the pile–unsaturated sand interface. This study employs a multi-scale approach—combining visualized cyclic loading tests, particle image velocimetry (PIV), and discrete element method (DEM) simulations—to elucidate the underlying macro–meso evolutionary mechanisms. Laboratory results identify an optimal water content of 8%, which maximizes interfacial effective stress and bearing capacity, while cumulative settlement is shown to stabilize beyond a critical threshold of 400 cycles. A key transitional mechanism is observed at a depth of 180 mm, where an initial spike in excess pore water pressure triggers transient strength degradation, followed by localized particle densification that restores stability. Kinematic analysis via PIV confirms that soil deformation propagates radially from the pile tip, mitigated significantly by matric suction. Crucially, DEM simulations correlate these macroscopic behaviors to mesoscopic load-transfer mechanisms, revealing the formation of a structural compaction core zone at the pile tip sustained by dense force chains. By clarifying the synergistic process of initial cyclic weakening and subsequent stabilization, this study provides a robust theoretical framework for the long-term performance assessment and design optimization of pile foundations.