Abstract: This study is aimed to investigate the unloading and dilatancy characteristics of rock under high-stress level and the variation law of the dilatancy angle. A series of triaxial pre-peak unloading confining pressure tests was carried out at different initial confining pressure levels, and the conventional triaxial compression tests were also conducted at corresponding confining pressure levels. Based on the experimental results, the effect of the unloading stress path on the dilatancy characteristics of sandstone was analysed, the variation law of dilatancy angle under the unloading stress path was summarized as well. Moreover, the dilatancy angle function under the unloading stress path was proposed, and its numerical realisation method was established. Finally, the feasibility and rationality of the proposed dilatancy angle function was verified by the numerical simulation of the triaxial pre-peak unloading confining pressure tests. Under different confining pressure levels, the volumetric strain values of pre-peak dilatation were similar under the loading paths. While under the unloading stress paths, the volumetric strain of pre-peak dilatation increased from the minimum value of 3.15×10?3 to the maximum value of 9.65×10?3 with increasing the confining pressure level. The volumetric strain of pre-peak dilatation under the unloading path was about 1.1-4.0 times than that under the loading path. The volumetric strain corresponding to the peak of the deviatoric stress was substantially zero or close to zero under the unloading condition, while the volumetric strain was negative under the loading condition. Under the unloading stress path condition, the proportion of the volume expansion strain before the peak of deviatoric stress in the total volume expansion strain was significantly larger than that under the loading path. The dilatancy angle firstly increased and then decreased with increasing the plastic shear strain under the two stress paths. Under lower confining pressure conditions, both the maximum dilatancy angle and the dilatancy angle corresponding to the peak were greater than those under the higher confining pressure conditions. Compared with the loading path, the dilatancy angle reached the maximum value faster, and the maximum dilatancy angle and the dilatancy angle corresponding to the peak were larger under the unloading path. According to the unloading test results, the linear fitting method was used to establish the dilatancy angle function with the confining pressure and post-peak plastic shear strain as independent variables. The experimental results showed good agreements with the simulation results from the established function and the strain softening constitutive model under the unloading path. The results indicate that the function can better describe the expansion characteristics of sandstone under the conditions of the triaxial pre-peak unloading path. This study can provide a theoretical basis for deformation prediction, stability analysis and support design of deep underground engineering.

Key words: dilatation, dilatancy angle, unloading, high stress, triaxial test