Elastoplastic Mechanical Behaviors of Hydrate-Saturated Sand: Insights from Cyclic Loading–Unloading Triaxial Shear Tests

Cyclic loads induced by artificial and natural disturbances alter strength and deformation characteristics of hydrate-saturated sediments, which are more prone to cause potential geological risks. However, its deformation regularity and control mechanisms are still unclear. Here we redesign a triaxial shear apparatus to exam elastoplastic mechanical characteristics of hydrate-saturated sediments under monotonic/cyclic loads. Stress–strain, volumetric strain, and lateral strain curves exhibit hysteresis loop with various shapes and size, depending on hydrate formation and stress states. Cyclic loads increase the strength and secant modulus by 51.51% and 32.03% on average compared with monotonic loads, indicating significant enhancement effects. Also, it brings in plastic strain accumulations with a higher proportion up to 91.47% of total strain that can be described by an improved Monismith’s model. Cyclic loads enhance plastic deformation and microstructure damage simultaneously through altering hydrate–sediment interactions as well as promoting microcrack developments. Plastic strain accumulations induce unexpected geological risks within a safe range of total deformation. Our findings suggest that plastic-strain-dominated assessment can perform reliable failure determination instead of a traditional method based on total strain, providing a new perspective for potential geological control during hydrate exploration.