Study on the Effects of Empty Hole Spacing on Fracture Propagation and Damage Evolution Mechanisms in Multi‐Hole Blasting

ABSTRACT The smooth blasting of hard granite tunnel will have problems such as poor fragmentation of the tunnel face. To address these issues, this study investigates the mechanism of empty hole blasting. According to the blasting theory, by deducing the formula of the equivalent damage zone radius of the group hole, and a criterion for defining the blasting effect is proposed. The rationality is verified by numerical simulation and field test. The results show that the empty hole can change the stress distribution of the group hole blasting, and has the effect of directional to crack. With the increase of empty hole‐group hole spacing, the peak stress decreases gradually. Aiming at the damage change, the damage feature transformation coefficient m is proposed to define the blasting effect and m = 0.6 is determined as the damage feature transformation threshold. With the increase of distance, the propagation speed and length of rock cracks gradually weaken, and the regional penetration effect gradually deteriorates. The relationship between damage threshold and vibration velocity is constructed. It is concluded that when the space is greater than the damage threshold, the maximum vibration velocity fluctuation interval gradually increases, and the stress wave propagation velocity gradually weakens. The Sadovsky formula of spacing and vibration velocity peak is constructed to characterize the change law. It provides a reference for the optimization of smooth blasting parameters in hard granite tunnels.