Dynamic shear behavior of geogrid–coral sand interface under cyclic loading

In this study, a series of displacement-controlled large-scale cyclic direct shear tests was conducted to investigate the dynamic shear behavior of the geogrid–coral sand interface. The effects of normal stress, horizontal displacement amplitude, and number of cycles were examined. The results show that the geogrid–coral sand interface exhibits stress-dependent shear contraction during dynamic shearing, with rapid contraction occurring at high normal stresses and progressive contraction at low stresses. With increasing horizontal displacement amplitude, the shear mechanism undergoes a dynamic transition from hardening to stabilization and eventually to softening. The peak and residual dynamic shear strengths increase with horizontal displacement amplitude and stabilize when the amplitude reaches 3 mm. An amplitude of 3 mm is sufficient to trigger interface softening, which causes a significant reduction in residual dynamic shear strength. Using the proposed hyperbolic relationship between internal friction angle and horizontal displacement amplitude, both peak and residual shear strengths can be predicted. Shear stiffness decreases with horizontal displacement amplitude, while the damping ratio shows the opposite trend. Higher normal stress increases shear stiffness, particularly at small horizontal displacement amplitudes, whereas no clear relationship exists between damping ratio and normal stress. Finally, formulas were developed to predict the decay of shear stiffness and the variation of the damping ratio, enabling more accurate dynamic analyses of geogrid-reinforced coral sand geotechnical structures.