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.

Mechanical Characteristics of Hydrate-Bearing Sediment: A Review

“…For HFS, especially that made of fine-graded sand, the loading rate dependence is quite weak and could be negligible in most cases. However, HBS presents a considerable loading rate dependence similar to frozen sediment, and the higher the loading rate is, the larger the strength of HBS, the larger the volumetric strain, and the larger the residual strength. ,, In addition, with the increase in loading rate, HBS shows a strain-hardening tendency . These are mainly due to the following three aspects: (1) Under a high loading rate, a larger force is required to balance the corresponding resistance during shearing because the sediment has to be destroyed quickly; thus, a larger strength would be exhibited at the macroscale .…”

“…(2) Under a high loading rate, the sand particles would not have sufficient time to rearrange into the pores formed by the neighboring particles, resulting in a lower axial displacement and larger volumetric expansion of the specimen under the same deviator stress; that is, the HBS with a greater fine component would show a larger loading rate dependency . (3) Load-induced heating at particle contacts would cause hydrate dissociation to some extent, and the transient dissociation and hydrate reformation at a fast loading rate could explain the suppressed dilatancy and higher postpeak residual strength …”

“…67 (3) Load-induced heating at particle contacts would cause hydrate dissociation to some extent, and the transient dissociation and hydrate reformation at a fast loading rate could explain the suppressed dilatancy and higher postpeak residual strength. 66 3.2.5. Influences of Temperature and Pore Pressure.…”

A Geomechanical Model for Gas Hydrate Bearing Sediments Incorporating High Dilatancy, Temperature, and Rate Effects

“…The geomechanical behavior of MHBS is particularly challenging. Experimental studies in this area have primarily focused on how the strength, stiffness, and others mechanical properties of MHBS are affected by the following factors: hydrate saturation (e.g., [6,[18][19][20]), effective mean stress (e.g., [21]), confining pressure (e.g., [22]), temperature (e.g., [23][24][25][26][27][28][29][30]), pore pressure e.g., [31]), strain rate (e.g., [32][33][34][35][36][37][38][39][40][41]), drainage conditions (e.g., [42]), and sediment skeleton (e.g., [20,43]).…”

“…The THM formulations for MHBS proposed in Kimoto et al [38] and Akaki et al [39] adopted a mechanical model based on viscoplastic concepts, however the mechanical law was not validated. In Deusner et al [40], the rate-dependent behavior of MHBS was explained considering a mechanism based on the kinematic rearrangement of the material fabric.…”