Experimental study of crack propagation and failure around a horseshoe tunnel during nearby blasting

To investigate the effects of nearby blasting on a horseshoe tunnel and to analyze dynamic response and crack growth with different loading directions, five laboratory experiments were performed using granite specimens of a rectangle plate containing a blast hole and horseshoe tunnel structure. HS photography and digital image correlation (DIC) methods were used to visualize the highly transient process of crack propagation in the surrounding rocks when the sidewall, spandrel, vault, corner, and bottom floor were selected as the front blasting faces. According to the experimental results, the shock wave attenuated to stress wave when the wave arrived at the front blasting face of the tunnel, and the radial fracturing played a significant role around the tunnel. The radial cracks originated from the borehole and merged with the cracks which from the tunnel. The central damage area between the borehole and the tunnel were similar for different loading directions. The initial stress wave became weak when it arrived at the tunnel back blasting face, the combined action of the reflected and diffracted stress waves played an important role in the crack growth. The side cracks were more easily generated in the orthogonal corner, and their growth was strongly influenced by the loading direction. Therefore, the reinforcement of the existing tunnel was valuable along the connection direction and corners.