Blasting-induced rock damage control in a soft broken roadway excavation using an air deck at the blasthole bottom

The charge structure in the cutting area has an important practical engineering significance in improving the blasting performance and increasing the excavation work efficiency, especially in tunneling practices with soft and broken surrounding rocks. To contain the excessive damage in the tunnel face of the Meishan Iron Mine, the finite element program LS-DYNA is used to investigate the effect of the charge structure of cutting holes (with an air deck and without an air deck at the blasthole bottom) on the blasting performance in roadway excavation. Numerical results indicated that the blasthole bottom was seriously damaged by blasting under the condition of charge without an air deck. Accordingly, 80-mm bottom air deck length is determined based on analytical calculation, and a numerical simulation was conducted and showed that the air deck could effectively weaken the excessive damage around the blasthole bottom. Specifically, the peak pressure at the monitoring point (10 cm horizontally from the center of the blasthole bottom) with an air deck is 22.1 MPa, which is much smaller than that of the no-air-deck case of 207.1 MPa. The field experimental results indicated that the tunnel face was obviously damaged under the condition of holes without an air deck and the tunnel face was relatively flat under the condition of a blasthole bottom with an air deck, which is consistent with the numerical results. The workload for cleaning loose rock for the next excavation cycle in a 3.5 m advance was reduced from 40 min to around 25 min.