# Dynamic Mechanical Characteristics of Horseshoe Tunnel Subjected to Blasting and Confining Pressure
## 1 Introduction  
The stability of the adjacent existing tunnel is affected by the disturbance from nearby blasting during new tunnel excavation. The prestress formed around the tunnel under the in-situ stress field. The dynamic loading from nearby blasting combined with the prestress cause the crack initiation and development in surrounding rock masses [1, 2]. The spalling or breaking along the tunnel's free surfaces becomes a potential threat to the safety of construction and support [3, 4]. Before the dynamic loading from nearby blasting, the static stress field is produced by the confining pressure. The static stress concentrates at the top and bottom of the tunnel, and the prestress distribution is controlled by the in-situ stress field. During the new tunnel excavation, the pulverized zone is formed by the shock wave around the borehole [5]. Shock waves gradually decay to stress waves as the spreading distance increase, and the radial cracks are obviously under circumferential tensile loading because the rock tensile strength is lower than the compressive strength [6]. In addition, the adjacent tunnel provides the free face, the reflected wave is formed, and the existing cracks further extend to the free boundary. The damage is produced along the tunnel under the nearby blasting [7, 8]. [PERSON] et al. [9] performed the hard rock pit blasting test under static stress and found that static stress had a significant effect on crack initiation and propagation when the stress strength ratio was 0.15. [PERSON] et al. [10] experimented with the cracked tunnel specimen by drop weight impacting and found the speed of crack propagation was increased with the loading rates. Different from the shallow tunnel, the fragmentation was influenced by the state of stresswhen the underground tunnel was subjected to blasting [11; 12; 13]. The blast load direction had a significant influence on the final damage, and the stress superposition around the tunnel was influenced by the interconnected cracks [14; 15]. The external confining stresses increase the compressive strengths of hollow cylindrical granite [16]. [PERSON] [17] found the three-dimensional cracks were initiated from the internal wall because of the shear slippage under triaxial compression. [PERSON] et al. [18] proposed a pressure relief method by grooves on the top and bottom of the circular hole to solve the problem of uneven stress distribution around the circular hole under anisotropic far-field stress. A series of models was established, and the stability of tunnel parameters was analyzed by using the finite element program [19]. [PERSON] et al. [20] studied the failure process around the tunnel by AUTODYN and found the deformation and destruction were serious on the floor and shoulder. [PERSON] et al. [21] found that the crack propagation was similar under the ground stress of 0.5-3 MPa. [PERSON] et al. [22] investigated the failure mechanism of rock masses around circular tunnels by RFPA. [PERSON] et al. [23] investigated the rock fragmentation mechanisms under dynamic disturbance and different static conditions by using the AUTODYN code. [PERSON] et al. [24] explored the destruction process during underground excavations with three different geomechanical scenarios by FDEM. [PERSON] and [PERSON] [25] developed failure criteria for FLAC3D to study zonal disintegration. [PERSON] and [PERSON] [26] analyzed the dynamic response of the rock surrounding tunnels during earthquakes in the FLAC program. [PERSON] et al. [27] found that high ground temperature and ground stress had a significant influence on the cracking mode of the surrounding rock of the hydraulic tunnel. [PERSON] et al. [28] used LS-DYNA to explore the crack growth of jointed rock mass under different ground stresses and verified that in-situ stress has an obvious guiding effect on crack growth. In the paper, the effect of blast loading direction and confining pressure on fracturing propagation of adjacent tunnel surrounding rock will be investigated.