Abstract: Exploiting methane hydrate induces deformation and damage to the reservoir, leading to a series of geotechnical engineering problems. Therefore, to achieve safe and effective extraction of hydrates, it is necessary to research the shear deformation characteristics of methane hydrate-bearing sediments(MHBS). The thermo-hydro-mechanical-chemical (THMC) microscopic contact model was utilized to account for hydrate cementation effects and their sensitivity to environmental temperature and pressure. In addition, the flexible boundary was applied to triaxial numerical tests to ensure the full evolution of shear bands. The microscopic mechanism of onset and development of shear bands was studied by considering macro and micro variables such as strain localization, porosity, average pure rotation rate (APR), bond failure, and spatial distribution of shear band. The results indicate that the flexible boundary used in the triaxial shear test effectively represents the stress-strain and volumetric response of MHBS while ensuring the free deformation of the sample. The shear band has already begun to germinate and to develop during the initial strain hardening stage and becomes more pronounced once it reaches the strain softening stage. Considerable disparities exist in macro- and microscopic parameters, including particle rotation and porosity alterations, both within and outside the shear band. Furthermore, hydrate cementation exerts a dual impact on its host sand. On the one hand, it enhances its strength characteristics, and on the other hand, as a weak link in the shear process, it takes the lead in failure, thus contributing to the emergence of shear bands. The research results have reference value for understanding the mesoscale evolution mechanism of MHBS deformation.

Key words: methane hydrate-bearing sediments, discrete element method, strain localization, macro- and meso- mechanism