TIME-DEPENDENT BEHAVIOR OF CO2-HYDRATE-BEARING SEDIMENTS AND APPLICATION OF ELASTO-VISCOPLASTIC CONSTITUTIVE MODEL

It has been recognized that gas hydrate-bearing sediments clearly exhibit time-dependent behavior such as long-term consolidation, strain rate dependency, and creep deformation due to its viscoplasticity of gas hydrates. The main objective of the present study is to understand the time-dependent behavior of gas-hydrate-bearing sediments by a series of experiment and to establish a constitutive equation that can de-scribe the time-dependent behavior of gas-hydrate-bearing soils. First, a series of undrained triaxial compression tests with step-changed strain rate is conducted on synthetic CO 2 -hydrate-bearing sand specimens in order to investigate the strain rate dependency of the hydrate-bearing soils. Second, the elasto-viscoplastic constitutive equation has been developed based on the obtained experimental results. The proposed model is then applied to the experimental results to find that the model can reproduce the mechanical behavior of the CO 2 -hydrate-bearing specimens. The main findings obtained from the present study are as follows: First, the unique stress-strain-strain rate relations, namely, Isotaches, is observed for both the CO 2 -hydrate-bearing sand specimens and the sand sample without hydrates. In addition, the CO 2 -hydrate-bearing specimens clearly show the strain rate dependency, and it increases with increase in the hydrate saturation. Second, from the experimental results, it is found that dependency of hydrate saturation on both the strength and the stiffness does not appear in the case where the strain rate is quite slow. This is because the strength of the CDH-bearing sediments may be determined by the balance of the hardening effect and viscoplasticity of the hydrates. In the case of the slow strain rate, the viscoplasticity is more dominant on the mechanical behavior of the CDH-bearing sand than the hardening effect. The proposed constitutive model can well express those mechanical responses of the CO 2 -hydrate-bearing specimens by introducing the effect