Experimental study on mechanical response characteristics of gassy sand under constant shear stress paths

The stress state changes in shallow gassy sand strata under anthropogenic disturbances can be simplified as failure problems of gassy sand under constant shear stress drained (or undrained) stress paths. However, related research remains relatively limited. To investigate the mechanical properties of gassy sand, this study utilized a high-pressure gas dissolution saturator and employed a CO₂-saturated aqueous solution degassing method to prepare high-saturation gassy sand specimens. A series of triaxial tests under constant shear stress paths were conducted under both drained and undrained conditions, focusing on the effects of relative density, saturation, and deviatoric stress levels in constant shear stress paths. The effective stress paths and instability time-response patterns of gassy sand under constant external loads with internal stress variations were obtained. Experimental results revealed that in the p'-q plane, for loose or dense gassy sand with varying saturations, the stress path entry points into an unstable state consistently lie within the potential instability zone bounded by the critical state line (CSL) and instability line (IL). In constant shear stress drained (CSD) tests, lower sand saturation corresponds to longer times to reach instability, whereas the opposite trend was observed in constant shear stress undrained (CSU) tests. These findings enhance the understanding of the mechanical behavior of gassy sand and provide theoretical support for safety assessments in engineering projects involving shallow gassy strata.