Mechanical Behavior of Hydrate-Bearing Clayey-Silty Sediments under an Unloading-Reloading Triaxial Test

During the extraction of natural gas hydrates, artificial engineering disturbances (equipment erection, discontinuous fluid migration, etc.) will impose cyclical loads on sediments, altering stress conditions, and these would lead to potential geo-disasters (collapses, landslides, etc.). In this study, a series of unloading-reloading triaxial tests were conducted on the consolidated drained hydrate-bearing clayey-silty sediments (HBCS), finding that (1) the mechanical properties of HBCS are influenced by unloading-reloading behavior, exhibiting a strong strain rate dependency. Moreover, the higher the hydrate saturation, the more pronounced the stress–strain response; (2) for hydrate-freeing clayey-silty sediments (HFCS), the volumetric change increases with decreasing strain rate. HBCS exhibit the same trend, and its volumetric change decreases with increasing saturation; (3) despite the fact that the mechanical properties of HBCS show a decreasing response to the strain rate within a narrow range of variation, the failure strength of HBCS still increases with higher strain rates and saturation levels; (4) the yield lines were drawn for sediment samples in the q-p, plane using the end-of-test points of HBCS and HFCS at the same final axial strain (εa = 20%), and the M value of the sediment sample increases with higher strain rates and greater hydrate saturations. Finally, this study reveals the microstructural evolution mechanisms during shearing of sediment samples and elucidates the underlying mechanisms of the mechanical behavior of HBCS and HFCS. Therefore, determining gas hydrate saturation distribution and monitoring formation strain rates in real time are essential.