Study on initiation and propagation mechanism of internal cracks caused by dynamic and static action of shaped charge blasting under in-situ stress

Deep hard rock blasting is a free surface blasting process under the coupling effect of explosive dynamic and static load and in-situ stress. In-situ stress has a strong inhibition effect on blasting crack propagation, which greatly limits the application of deep hole blasting in the engineering of hard rock pre-weakening and roof cutting and pressure relief. In order to clarify the action mechanism of stress wave and explosive gas on crack initiation and propagation in infinite rock blasting under the influence of in-situ stress, based on explosion mechanics and fracture mechanics, the length of guided crack in shaped charge blasting action was theoretically deduced. Combined with LS-DYNA software, the process of crack initiation and propagation in rock under dynamic and static action of shaped charge blasting with or without the influence of ground stress is simulated and analyzed visually. The results show that: 1) The numerical simulation results indicate that the time cut-off point of dynamic and static action is 32 μs, and the phase of shaped charge blasting action is dominant for crack initiation and initial propagation. The static action of explosive gas is the dominant factor for crack propagation. 2) When there is no in-situ stress, the crack propagation length in the phase of static action is 11 times longer than that in the phase of stress wave action. Compared with the condition without in-situ stress, when the in-situ stress is 20 MPa, the crack length in the action stage of shaped blasting is inhibited by 12.4%, and the crack length in the static action stage is inhibited by 86.3%. The in-situ stress mainly inhibits the crack growth in the static action stage of the later explosive gas. When the lateral stress is perpendicular to the direction of crack growth, the inhibitory effect decreases with the increase of the lateral stress. 3) Based on the background of roof cutting and pressure relief engineering of N1302 working face in Gucheng Mine, the crack propagation mechanism of in-situ stress and dynamic and static action of deep-hole blasting was considered through numerical simulation, and the optimal decoupling coefficient was finally determined to be 1.3, and the hole spacing was 1000 mm. The engineering inspection effect after explosion was good, and directional pre-cracks were formed in the hole.