Effect of dynamic disturbance frequency on brittle failure of granite in deep excavation

Dynamic disturbances with various frequencies could trigger different failure modes of deep excavations. Superimposed on this static stress are dynamic disturbances due to various dynamic vibrations, e.g. excavation blasting, blasting, tunnel boring machine (TBM) vibration, rockburst wave, earthquakes. Specifically, these dynamic sources are characterized by a wide range of wave frequencies f , resulting in differences in failure modes. A series of true-triaxial compression tests were conducted on granite to simulate the excavation-induced stress path in three-dimensional (3D) stresses. Subsequently, a dynamic disturbance with various frequencies was applied to a cuboid specimen, to reveal the behavior associated with brittle failure. The dynamic disturbance with frequencies f of 5 Hz, 10 Hz, and 40 Hz generates less disturbed energy components in the granite together with higher peak strength. However, dynamic disturbances with f of 20 Hz and 30 Hz resulted in a lower peak strength; the peak strength of the rock increases σ p albeit it decreases at first, then increases. This U-shaped phenomenon relates to the natural frequency of the granite under such stress conditions. Different rock lithologies consisting of diverse mineral composition, respond differently to each sensitive resonance frequency. Interestingly, the weak disturbance stress with a high frequency f and low amplitude A increases the ratio of crack damage to peak strength ( σ cd / σ p ) in the granite. This leads to the inhibition of the expansion of the granite during the dynamic disturbance process. Multiple penetrating tensile–shear cracks appear in the σ 3 -direction as the disturbance frequency f increases.