Abstract: In order to study the damage and failure of the surrounding rock of the existing tunnel caused by the blasting excavation of the new overlapping tunnel,the physical test and numerical simulation methods were used to analyze the damage law of the blasting position of the floor hole of the new tunnel and the distance between the two tunnels to the existing tunnel. Both the test and simulation results show that the existing tunnel damage mainly occurs at the top of the tunnel and its two bottom corners. The damage at the top of the tunnel is mainly composed of spallation cracks and radial cracks. Through the analysis of the crack propagation results of the two bottom corners of the existing tunnel,it is found that:① After the initiation of the central bottom hole and the left bottom hole of the new tunnel,the two bottom corners of the existing tunnel show symmetrical and asymmetrical ′八′ shape crack propagation morphology respectively. ② When the central bottom hole is detonated,the length of the two bottom corner cracks in the lower tunnel decreases with the increase of the tunnel spacing. When the left bottom hole is detonated,the length of the left bottom corner crack in the lower tunnel is mostly greater than the length of the right bottom corner crack. ③ The numerical analysis further shows that the distance between the two tunnels has a significant effect on the bottom corner force and crack initiation of the existing tunnel. With the increase of the spacing,the bottom corner force and crack initiation stress of the existing tunnel change nonlinearly. The appropriate spacing can optimize the construction structure of the overlapping tunnel to improve the safety. The coincidence of numerical simulation and experiment in phenomena and laws verifies the reliability of the results to a certain extent. The research results have certain reference significance for the reasonable support of existing tunnels and the safe blasting construction of new tunnels. 

Key words: tunnel driving,blast construction,existing tunnel,blasting crack,crack growth angle