Fracture Angle Analysis of Rock Burst Faulting Planes Based on True-Triaxial Experiment

The aim of this paper is to estimate fracture angles in deep-seated rock bursts encountered in intact hard rock tunnels. The fracture angles of fault planes in rock burst failure are analytically formulated by employing stress analysis based on Mohr's circle construction. Mohr's circle construction suits well for representing the rock burst stress states including the static loading and dynamic unloading processes existing at or near the excavation surface. Four fracture angles can be precisely predicted using the proposed mathematical models, including two minimum angles for two conjugate planes where the shear stress is equal to the maximum static shear stress tau (max) while the normal stress approaches to zero, and two maximum angles for two conjugate planes where the normal stress is reduced from sigma (1) to sigma (1/2) while shear stress increases markedly from +/-(sigma (1)-sigma (3))/2 to the maximum dynamic shear tau (dmax) = +/-sigma (1)/2. For validation of the analytical solutions to fracture angles, rock burst experiments on Laizhou granite were conducted using a modified true-triaxial apparatus. The predicted fracture angles are compared very well with the results obtained from the laboratory rock burst tests and are in good agreement with the in situ observations. The proposed solutions to the fracture angle are a function of the static stresses only which can be known a priori from a field survey.