Abstract: Extraction-induced mechanical weakening of methane hydrate-bearing sediments can potentially trigger submarine landslides, but a comprehensive understanding of the underlying progressive failure mechanisms remains lacking. This study used the discrete element method (DEM) to simulate direct shear tests on methane hydrate-bearing sand (MHBS) and systematically investigated how peak strength, structural yield strength, and residual strength varied with vertical stress and saturation. We examined shear-band evolution in MHBS across different hydrate saturations and established links between macro-mechanical responses and microstructure parameters during progressive shear failure. The numerical results revealed that: (1) The numerical simulation results effectively captured the macro-mechanical response characteristics of MHBS. Peak strength, residual strength, and structural yield strengths, all showed consistent positive correlations with hydrate saturation. Additionally, hydrate cementation led to significant nonlinearity in the peak strength envelope of MHBS. (2) The mechanical properties of MHBS intrinsically depended on hydrate cementation. Under a vertical stress ( σ_v) of 1 MPa, the hydrate cementation acted as the primary mechanism resisting external stresses. Upon reaching peak stress, the number of hydrate cementation failures increased sharply, leading to a progressive transition in which interparticle frictional contacts became the dominant stress-resistance mechanism. At = σ_v 10 MPa, the load-bearing role shifted predominantly to the sandy soil particles, with a corresponding reduction in the influence of hydrate cementation, consequently leading to enhanced overall compaction. (3) Macro- and microstructural parameters exhibited significant correlation with the initiation and evolution of shear band. Under lower confining pressures, the higher the hydrate saturation, the more concentrated the cementation failure within the shear zone. Additionally, the formation of the shear zone was accompanied by substantial cementation failure. The particle rotation rate and void ratio within the band were significantly higher than those outside, whereas the mechanical coordination number and residual cementation rate were lower. Under high confining pressures, hydrate cementation experienced severe damage, while the specimen exhibited pronounced volume reduction characteristics.

Key words: methane hydrate-bearing sand (MHBS), discrete element method, direct shear test, shear band