Impacts of delayed detonation of empty hole bottom charge on the cavity forming effects of deep hole parallel cutting

The speed of underground blasting driving is primarily determined by the effectiveness of cutting blasting. To address the low cavity forming efficiency of parallel cutting in deep hole blasting driving, a solution involving delayed detonation of the bottom charge in the empty hole was proposed. Firstly, theoretical analysis, model experiments, and numerical simulations were conducted to investigate the influences of delayed detonation of empty hole bottom charge on the cavity forming efficiency. Subsequently, combined with high speed photography and numerical simulations of rock throwing process, the cavity forming mechanism under the synergistic effect of charge blasting in two types of blasting holes was revealed. Finally, field tests were performed to verify its actual application effects. The results indicate that delayed detonation of the empty hole bottom charge could significantly eliminate residual stumps at the bottom of the cutting cavity, increasing the cavity forming efficiency from less than 80 % to over 95 %. During the blasting stage of cutting holes, the strong clamping effect of surrounding rock leads to residual stumps in the cavity, but the newly formed free face could facilitate the throw of stumps subsequently. During the blasting stage of empty hole bottom charge, the weakened clamping effect of surrounding rock enables the residual stumps to be thrown out at the relatively high velocity, thereby achieving an excellent cavity forming effect. The rock throwing velocity during the empty hole blasting is about 2.0 m·s⁻1 higher than that during cutting hole blasting. Compared with the traditional parallel cutting technique, the proposed method increases the cycle footage by 0.35 m and reduces the specific charge by 0.43 kg·m⁻³. This confirms that the improved parallel cutting technique is conducive to enhancing driving efficiency and reducing driving costs. The research findings could provide theoretical supports and practical references for the optimization of deep hole cutting blasting in underground engineering.