Plane strain compression behaviour and localization of deformation of MH-bearing sand

Abstract

As a result of recent investigations, carried out mainly by the MH21 research consortium of Japan, methane hydrate, referred to hereafter as ''MH'', is expected to become a possible future energy resource. In order to evaluate the stability of the seabed during MH production, previous research on the mechanical properties of MH-bearing sand have been conducted under triaxial stress conditions. However, these were very specific conditions whereas several stress conditions are assumed to exist in the Nankai Trough where MH production is carried out. Therefore it is essential to consider these stress conditions in order to understand the changes in stress and deformation behavior of the seabed. In this study, we describe the effects on MH-bearing sand under plane strain conditions with changing confining pressure. The apparatus used for plane strain tests has both a confining plate and an observation window. Local deformation of MH-bearing sand is shown using photographs of specimens taken during shear tests.A marked increase in the stress ratio of the host sand (Toyoura sand) and MH-bearing sand was more apparent in the plane strain tests than in the triaxial tests. The stress ratio decreased dramatically after the appearance of the peak stress ratio. Moreover, the peak stress ratio of MH-bearing sand was higher than that of the host sand as shown in previous research. After shear, in the case of the host sand, particle crushing increased as the effective confining pressure increased. Additionally, more particle crushing occurred inside the shear band than outside. In the case of the host sand, the failure strength for plane strain tests shifted from the SMP criterion to the Mohr-Coulomb criterion as the effective confining pressure increased, due to particle crushing. In the case of MH-bearing sand, the failure strength for every effective confining pressure lay between the SMP criterion and Mohr-Coulomb criterion. Expansive volumetric dilation increased within the shear band as the axial strain increased for each case, with more dilation occurring for MH-bearing sand compared with host sand.