A Viscoelastic Constitutive Equation for Methane-Hydrate-Bearing Sand

Geomechanical behaviors of methane-hydrate-bearing sub-seabed layers have not been sufficiently clarified, although they are essential to ensure sustainable exploration of methane hydrate in marine sediments. In particular, the viscoelastic properties of methane-hydrate-bearing layers are thought to have great significance in the long-term prediction of the geomechanical behaviors. It has been clarified that methane-hydrate-bearing sand has significantly strong time dependence for a geomaterial. Thus a constitutive equation used for the long-term prediction of the geomechanical behaviors is required to consider the viscoelasticity (time dependence) of methane-hydrate-bearing sand. To date, however, there has not been any viscoelastic constitutive equation modeled based on experimental results. This study aims to discuss the viscoelastic properties of methane-hydrate-bearing sand specimens obtained from drained triaxial compression tests and propose a viscoelastic constitutive equation. The results of three types of triaxial compression tests, e.g., constant-strain-rate test, constant-stress-rate test and creep test, on artificial methane-hydrate-bearing sand specimens are reviewed, mainly focusing on the viscoelastic behaviors. Then, a nonlinear viscoelastic constitutive equation is proposed for methane-hydrate-bearing sand, considering the strain-rate and stress-rate dependences of mechanical properties and creep behaviors. The model prediction fits well with the test results of stress-strain relationships in constant-strain-rate and constant-stress-rate tests before the peak failure point and primary creep behavior.