Evaluation Method of Rock Brittleness under True Triaxial Stress States Based on Pre-peak Deformation Characteristic and Post-peak Energy Evolution

Deep rock masses are in true triaxial stress (TTS) fields, and few studies have examined the brittleness evaluation of rock under TTS thus far. In particular, research has not been conducted on the brittleness evaluation index and the effect of TTS. Therefore, based on a summary and analysis of pre-existing brittleness indexes, this paper proposes a brittleness index of rock under TTS that considers both stress-induced anisotropy and energy conversion characteristics during Class I behavior and Class II behavior. Meanwhile, the studied brittleness index is analyzed and verified. The brittleness of rock was found to increase with increasing σ 2 while σ 3 is constant; when σ 2 is constant, the rock brittleness decreases with the increase in σ 3 . When σ 3 is high, the increase in the rock brittleness amplitude with increasing σ 2 is large; conversely, when σ 3 is low, the amplitude is small. For the same σ 3 ( σ 3 = 5 MPa), the order of brittleness of the three rocks is basalt > granite > marble. Finally, this paper shows that the proposed brittleness index can be applied to conventional triaxial and uniaxial stresses. In addition, under different TTS, a brittleness threshold of σ 2 / σ 3 = 5 was found for the rocks, and when σ 2 / σ 3 < 5, the rock brittleness enters a stress sensitive zone. The research results in this study can provide a relevant basis and guidance for the study of the brittle fracture process and the brittleness of rock under TTS.