Dynamic mechanical response of unsaturated beach sand under confined one-dimensional compression

This study investigates the dynamic mechanical behavior of unsaturated beach sand under high-strain-rate loading (300–1000 s⁻1) using a modified Split-Hopkinson Pressure Bar (SHPB) system with passive confinement. Experiments were conducted on specimens with three distinct particle size distributions (fine, medium, and coarse) and varying moisture contents (0%, 10%, and 15%) to decouple the effects of strain rate, moisture contents, and density. The results reveal a pronounced strain rate dependence, where yield stress increases while the compression index and energy absorption efficiency decrease with increasing strain rate. Moisture content exhibits a competitive dual-mechanism effect: capillary cohesion enhances the initial dynamic shear modulus at small strains, whereas pore water lubrication reduces yield stress and energy absorption capacity at larger strains. A quantitative analysis of particle breakage, supported by multivariable regression, identifies a hierarchical control mechanism where initial dry density serves as the primary governor of mechanical response and particle crushing intensity, while particle size distribution (PSD) acts as a secondary modulator at fixed densities. Specifically, denser specimens exhibit higher stiffness and greater particle breakage, marking a transition in energy dissipation from frictional sliding to grain fragmentation. Furthermore, the study characterizes the viscoplastic nature of beach sand through the distinct temporal lag between peak stress and peak strain. These findings provide critical constitutive data and theoretical guidance for the design of coastal infrastructure and protective structures subjected to dynamic impact.