The shear strength and stiffness characteristics of methane hydrate sediments in seabed reservoirs are crucial for the safe and efficient exploration and exploitation of hydrates. To investigate the impact of changes in occurrence environments on the shear strength characteristics of muddy hydrate-bearing sediments

a series of drained shear tests have been conducted on methane hydrate-bearing muddy silt sediments under varying conditions of saturation (water-saturated and gas-saturated) and confining pressure based on the improved triaxial shear tester for hydrate-bearing sediments. The experimental results show that changes in the occurrence environment in sediments have significant effects on the shear strength and stiffness characteristics of hydrate-bearing muddy silt sediments. Compared to gas-saturated sediments

the shear strength of hydrate-bearing sediments with the same hydrate saturation greatly reduces under water-saturated condition

with a maximum reduction ratio of 52.0%. Although the effect of occurrence environmental changes on the internal friction angle of sediment is limited

the effect on the cohesion of sediment is significant. Compared to the results from gas-saturated sediments

the reduction ratio of the effective cohesion for the water-saturated hydrate-bearing sediments can reach 68.1%. Furthermore

the exponential equation is developed for describing the relation between the effective cohesion and hydrate saturation. Under water-saturated conditions

the deformation modulus of hydrate-bearing sediments decreases significantly

with a maximum reduction ratio of 72.7% for E50. Additionally

the deformation modulus increases linearly with hydrate saturation under water-saturated conditions. Lastly

based on the stress analysis of the skeleton of hydrate-bearing sediments under gas-saturated and water-saturated conditions

the evolution mechanism of the mechanical properties of hydrate-bearing sediments under these two different occurrence environments is preliminarily revealed. The observations can offer fundamental data and methods for recovering the occurrence mechanism of methane hydrate-bearing sediments and assessing the stability of the reservoir.