Strain Rate-Dependent Hardening-Softening Characteristics of Gas Hydrate-Bearing Sediments

The presence of gas hydrates (GHs) increases the stiffness and strength of marine sediments. In elasto-plastic constitutive models, it is common to consider GH saturation (S_h) as key internal variable for defining the contribution of GHs to composite soil mechanical behavior. However, the stress-strain behavior of GH-bearing sediments (GHBS) also depends on the microscale distribution of GH and on GH-sediment fabrics. A thorough analysis of GHBS is difficult, because there is no unique relation between S_h and GH morphology. To improve the understanding of stress-strain behavior of GHBS in terms of established soil models, this study summarizes results from triaxial compression tests with different S_h, pore fluids, effective confining stresses, and strain histories. Our data indicate that the mechanical behavior of GHBS strongly depends on S_h and GH morphology, and also on the strain-induced alteration of GH-sediment fabrics. Hardening-softening characteristics of GHBS are strain rate-dependent, which suggests that GH-sediment fabrics dynamically rearrange during plastic yielding events. We hypothesize that rearrangement of GH-sediment fabrics, through viscous deformation or transient dissociation and reformation of GHs, results in kinematic hardening, suppressed softening, and secondary strength recovery, which could potentially mitigate or counteract large-strain failure events. For constitutive modeling approaches, we suggest that strain rate-dependent micromechanical effects from alterations of the GH-sediment fabrics can be lumped into a nonconstant residual friction parameter. We propose simple empirical evolution functions for the mechanical properties and calibrate the model parameters against the experimental data.