Mechanical Behavior of Hydrate-Bearing Clayey-Silty Sediments in the South China Sea: Strain-Rate Dependency

Evaluating the mechanical behavior of hydrate-bearing sediment is a prerequisite and guarantee for achieving the safe exploitation and utilization of hydrates, and it is also important for the stability assessment of suction piles, anchor piles, and cable laying during the exploitation process. In this study, focusing on the strain-rate dependence of mechanical properties of methane hydrate-bearing clayey-silty sediments, 15 sets of drained triaxial shear tests with different strain rates and hydrate saturations were carried out, and X-ray computed tomography (CT) scans were performed on the specimens after deformation and dissociation. The results show that (1) all specimens exhibit characteristics of strain hardening and volumetric compression. With hydrate saturation increasing, the strain hardening phenomenon changes insignificantly, and the sensitivity of the hydrate-bearing sediment to strain rate increases due to the strong strain-rate dependency of the hydrate itself. The increased strain rate prevents small particles from adequately filling the pores, resulting in smaller volumetric compression; (2) as the strain rate increases, due to the coupled effect of hydrate cementation strengthening mechanism and decrease of local effective confining pressure, the shear strength decreases for hydrate-free sediments and increases for hydrate-bearing sediments; (3) when the hydrate saturation increases, the failure strength increases linearly, and the Secant Young's modulus E-50 increases nonlinearly. A nonlinear relationship formula between strain rate and E-50 is given; (4) similar to previous studies in clay mechanics, the strain-rate dependence of hydrate-bearing clayey-silty sediments tends to decrease or disappear at lower strain rates; (5) for converted strain-rate tests, the loading method with strain rates from low to high progressively can make the filling and interlocking between particles more sufficient, resulting in greater shear strength of the sediment.