Investigation of Cyclic Shear Characteristics of Geogrid–Calcareous Sand Interface under Large-Scale Direct Shear Test

The cyclic shear properties of the geosynthetic–calcareous sand interface are critical for the design and stability analysis of geosynthetic-reinforced calcareous sand structures subjected to seismic loading. In this study, large-scale cyclic shear tests were conducted to investigate the mechanical behaviors of geogrid–calcareous sand and unreinforced calcareous sand under the influence of normal stress (σn), shear displacement amplitude (Aw), and the number of cycles (N). The evolution of cyclic shear characteristics of geogrid–calcareous sand interface is revealed. The results indicated that the backbone curve and development of shear stiffness and damping ratio for the geogrid–calcareous sand interface exhibit significant differences compared to the geosynthetics–siliceous sand (saline soil) interfaces. The backbone curve of geogrid–calcareous sand interface exhibits greater symmetry than that of unreinforced calcareous sand. Further, the area of the hysteresis loop is larger, facilitating a more effective transfer of dissipated energy. The hysteresis loop area and interface shear strength exhibit a clear nonlinear relationship with σn and Aw, respectively. Additionally, the relationship between the shear contraction coefficient and N conforms to the power function model. These findings provide theoretical support for investigating the internal stability of geosynthetic-reinforced calcareous sand structures in coral reef areas subjected to seismic loads.