When natural gas hydrate is completely decomposed

the gas generated in this process makes the pore pressure increased rapidly and the effective stress decreases in energy soil

which leads to liquefaction failure of soil. At this time

the stress state and static liquefaction instability of deep sea energy soil slope can be simplified as the failure of gas-bearing soil under a constant shear stress drained or undrained stress path. Based on this

an improved gas tube method is proposed to prepare the gassy sandy soil samples

and a series of triaxial tests with constant shear stress path is carried out. The results of the 22 sets of experiments show that the gassy dense sand with the same void ratio has the same instability line under different cell pressures or different constant shear stress paths. The stress ratio during instability and volume strain increase with the increase of initial relative compactness. The regularity of saturation effect on the instability of gassy dense sand under the constant shear stress path is not obvious in both drained and undrained conditions

but the sensitivity of pore pressure (or volume deformation) of gassy dense sand to deformation decreases under undrained condition. After the constant shear stress path reaches the point of instability

the transient liquefaction bulging failure occurs in gassy dense sand under drainage condition and gradual shear failure occurs under undrained condition.