Experimental study of quasi-static collapse and flow dynamics in low-saturation wet granular columns

The transient collapse behavior of low-saturation wet granular materials under quasi-static conditions is crucial for geophysical and engineering applications but remains poorly understood. This study investigates the collapse dynamics of wet granular columns using two-dimensional and three-dimensional experiments with precisely controlled release velocities. A servo-driven lifting mechanism enables accurate regulation of release conditions, and a flow-state identification approach based on the advancing front velocity is proposed to distinguish between quasi-static and inertial flow states. Three distinct quasi-static collapse regimes—delayed transient collapse, intermittent collapse, and prolonged collapse—are identified, with transitions governed primarily by initial column height and release velocity, forming well-defined regions in the proposed phase diagram. A theoretical model is developed to predict the restabilizing height at which the collapsed material temporarily stabilizes, and its predictions, incorporating a velocity-dependent failure angle, show strong agreement with experimental results. Additionally, particle size is found to significantly influence capillary cohesion and collapse regime boundaries. The proposed framework captures the combined effects of column geometry, release velocity, and particle size, offering a predictive tool for wet granular flows and advancing the understanding of their behavior in both natural and engineered systems.