Experimental investigation on the triaxial unloading mechanical characteristics of sandstone immersed in different brines

Abstract In this study, the stress–strain curves of sandstone soaked in 5% NaCl and 5% K2SO4 solutions under different loading and unloading paths were obtained by carrying out different paths of unloading confining pressure tests on sandstone specimens. Thereafter, the strength, deformation, and failure characteristics of the specimens were analysed. First, the unloading capacity of sandstone specimens soaked in 5% NaCl was found to be greater than those soaked in 5% K2SO4, and the unloading amount increased in the order of increasing axial pressure unloading confining pressure < constant axial pressure unloading confining pressure < constant deviatoric stress unloading confining pressure. When unloading the confining pressure, the strain change rate of the sandstone specimens soaked in 5% K2SO4 was greater than that of the sandstone specimen soaked in 5% NaCl, and the strain change rate had the following trend: increasing adding axial pressure and unloading confining pressure > constant axial pressure unloading confining pressure > constant deviatoric stress unloading confining pressure. The deformation modulus and Poisson's ratio of the sandstone specimens during unloading exhibited an exponential change with increasing unloading ratio. The deformation modulus and Poisson’s ratio of the sandstone soaked in 5% NaCl during the unloading period was respectively greater and less than those of the sandstone soaked in 5% K2SO4. Second, the total energy absorbed by 5% NaCl soaked sandstone specimens was always less than that soaked in 5% K2SO4 under different unloading paths. The effect of immersing the specimens in different solution on acoustic emission and failure characteristics was unclear. Compared with the dry sandstone specimens, the sandstone specimens soaked in brine solutions lower mineral composition and higher, clay mineral content. The contents of plagioclase and calcite contents in the sandstone specimens soaked in 5% NaCl was similar to those in dry sandstone, but no plagioclase or calcite was detected in the specimens soaked in 5% K2SO4.