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 ( ) 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 = 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.