Influences of initial damage on the saturated granite failure characteristics under true triaxial stress with a free face: Insights into the rockburst prevention and control

The moisture content and damage conditions significantly influenced the mechanical properties of hard rock, which are closely associated with rockburst tendency. However, there was limited prior knowledge of this issue due to the scarcity of physical experiments. In this study, a true triaxial experiment with a free face was conducted on granite specimens to investigate the influence of the initial damage and moisture content on rockburst control, utilizing AE monitoring, rock NMR imaging, and photography techniques. Experimental results showed that with the increase of the initial damage factor k and moisture content omega, the rock strength and AE activity decreased. The static crack extension concentrated around the initial damage zone, and the dynamic ejection decreased, leading to an increase in the fracture surface fracture index. When k and omega increased to a certain extent, the tensile mechanism was significantly reduced, and then the rockburst was efficiently prevented. The failure mechanism lay the initial damage zone weakened the strength of the granite matrix, inducing the FPZ nucleation and crack extension. Furthermore, the rockburst envelope of the granite was altered, with the reduced envelope area indicating a decrease in stored energy, resulting in the granite having insufficient energy to trigger a rockburst upon failure. In addition, due to the "water wedge" effect, the moisture content further enhanced the weakening effect of the initial damage factor on rockburst tendency.