Study on variation law of rock mass dilatancy and stress threshold during loading and unloading

In order to further understand the dilatancy phenomenon of rock, the conventional triaxial compression test and unloading confining pressure test are carried out with coarse sandstone, and the rock sample underwent single slope shear failure, and the deviatoric stress-strain curve of rock sample without confining pressure is obtained. Combined with the deviatoric stress-axial strain curve, the rock dilatancy process is divided into linear elastic stage, slow relative dilatancy stage, accelerated relative dilatancy stage and absolute dilatancy stage. The concepts of narrow dilatancy (absolute dilatancy) and generalized dilatancy (relative dilatancy) are proposed, and the fracture evolution law of different dilatancy stages is analyzed by numerical simulation. According to the bilinear dilatancy model, the general rules of apparent dilatancy angle and dilatancy index are analyzed. The results show that the apparent dilatancy angle in the conventional triaxial compression test decreases from 66.12° to 39.15°, with a decrease rate of 40.79%;the apparent dilatancy angle decreased from 63.44° to 48.22° in the unloading confining pressure test, with a decrease rate of 23.99%; and the confining pressure of 15 MPa is the demarcation point of the dilatancy index.In both tests, the characteristic stress thresholds (crack initiation stress σ_ci, damage stress σ_cd, peak strength σ_p) increased with the increase of confining pressure. The dilatancy strain at different dilatancy stages decreases as the confining pressure increases, and the volume strain after unloading confining pressure is greater than that after the conventional triaxial loading. The phenomenon of rock dilatancy mainly occurs in the accelerated relative dilatancy stage and the absolute dilatancy stage, and the proportion of volume strain in the two stages is 76.73%. The research results provide an effective method for analyzing the phenomenon of rock dilatancy.