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

Abstract

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 (Sh) 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 Sh 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 Sh, pore fluids, effective confining stresses, and strain histories. Our data indicate that the mechanical behavior of GHBS strongly depends on Sh 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.