## 1 Introduction
Also, due to the limited number of monitoring devices and the limitations of the monitoring instruments, the model test cannot obtain the full-field multivariate information during the process of the damage and destabilization of the water-resistant rock mass of the tunnel adjacent to the fault. Numerical simulation methods can solve the above problems by visualizing the process of a water inrush disaster occurring in the tunnel adjacent to the fault. Regarding numerical simulation, [PERSON] et al. [15] used finite element software ANSYS 11.0 to study the deformation and stress distribution characteristics of the rock mass when tunnel excavation passed through the fault fracture zone, and analyzed influence of fault dip angle and width on the stability of tunnel rock mass. [PERSON] et al. [16] performed a 3D finite element analysis to investigate the evolution of a fault mud outburst of a tunnel and classified the evolution process into three stages, i.e., the prepared stage, latent stage, and explosive stage. [PERSON] et al. [17] investigated the influences of the dip angle, permeability, and flow pattern of the fault on the pressure and velocity of water inrush in a tunnel by means of finite element software COMSOL 5.6. [PERSON] et al. [18] used the finite difference programme to analyze the distribution of the rock seepage field, face extrusion deformation, plastic zone volume, water yield, and shear deformation of the grouting reinforcement circle ahead of the face when the face advanced to different positions, and they finally determined a reasonable thickness of the grouting reinforcement circle. [PERSON] et al. [19] used FLAC 3D to simulate the evolution of seepage, stress, and displacement fields in the process of water inrush and mud outburst in a fault-crossing tunnel and summarized the disaster conditions and catastrophic characteristics of water inrush and mud outburst in a water-rich section. The above numerical simulations have greatly promoted the research on the catastrophic process of water inrush in tunnels adjacent to faults. However, the numerical simulations based on the finite element method and finite difference method generally regard the water-resistant rock mass at the face as a porous continuous medium, which is unable to simulate the process of water inrush and the destruction of the water-resistant rock mass due to the formation, expansion, and dynamic evolution of the seepage channel. In recent years, scholars often study the law and mechanism of water inrush in tunnels with the DEM, which offers more obvious advantages in simulating the damage of tunnel rock mass [20; 21; 22; 23]. Most of these studies focus on the mechanism of rock mass damage caused by groundwater in fault fracture zones, and there are fewer studies on analyzing the damage evolution law of the water-resistant rock mass of the tunnel adjacent to the fault and the minimum safe thickness. The Central Yunnan Water Diversion Project is the largest and most invested water resource allocation project in Southwest China and is also one of the largest water diversion projects under construction in China. Its construction is faced with major engineering geological problems, such as active fault, karst groundwater, high geo-stress, and a large deformation of soft rock, which brings great challenges to the tunnel construction [24]. Xianglushan tunnel is the first structure in the Dali I section of the Central Yunnan Water Diversion Project, and it is also the most complicated deep long tunnel in the whole line with the most complicated geological conditions. Since its location is close to several fault zones, the groundwater flow is irregular, and water inrush disasters are prone to occur at the tunnel face during the excavation. In view of this, this paper took the 2# adit of Xianglushan tunnel as the research object and used the 3D DEM to analyze the evolution characteristics of displacement and seepage-pressure fields during the catastrophic process of water inrush at the face.