A state-dependent dilatancy constitutive model and parametric analysis for dense sand–structure interfaces

To describe the pronounced dilatancy, normal-stress evolution under constrained conditions, and post-peak softening of dense sand–structure interfaces, this paper develops a simplified state-dependent dilatancy-coupled constitutive model within the framework of critical-state soil mechanics. The state parameter is introduced to characterize the density state of the interface, while the dilatancy rate is used to describe the normal plastic deformation induced by interface shearing. Under constant normal stiffness (CNS) conditions, the constrained dilatancy is converted into an increment of normal stress, enabling the model to continuously capture the complete interface response from pre-peak hardening and peak mobilization to post-peak softening. An incremental numerical scheme is formulated, and the model is calibrated and evaluated through comparisons with published CNS direct-shear test results for dense sand–structure interfaces. The comparisons show that the proposed model can reasonably reproduce the main characteristics of interface shearing, including the evolution of shear stress, the increase in normal stress, and post-peak strength degradation. Parametric analyses indicate that the critical-state-line parameters and the initial void ratio primarily govern the initial dilatancy potential of the interface; the dilatancy coefficient has the most significant influence on the peak shear strength and the magnitude of normal-stress increase; and the shear-band thickness mainly controls the rate of state evolution and the post-peak response. The proposed model provides a concise constitutive framework for describing the coupled evolution of dilatancy, normal stress, and shear strength of dense sand–structure interfaces under CNS conditions.