# Mechanical Response Law and Parameter Influence Analysis of Karst Tunnel Dynamic Excavation
## 1 Introduction  
Karst collapse is a karst geological action and phenomenon in which the rock around a cave is damaged by deformation caused by external factors and eventually causes a collapse [1, 2]. It is one of the most common geological hazards in karst areas [3]. China is a vast country with complex geology. According to statistics, the karst area in China could be up to 3.65 million km\({}^{2}\), accounting for more than one-third of the national territory, and it is the country with the largest karst area in the world. The occurrence of karst subsidence accidents is also very common, having been widely seen in 22 provinces and regions, and is most common in the southern region, while serious karst disasters have also occurred in Hebei, Shandong, Liaoning and other places [4]. In China, the problem of karst collapse has become a problem that cannot be ignored in the process of modern construction. The situation of karst collapse is complex and diverse, and the geological hazards caused by karst collapse are universal [5, 6, 7, 8]. It threatens people's lives and property safety and causes significant economic loss, but also brings a lot of construction and operational problems for engineering construction [9, 10]. Karst collapse is difficult to predict because of the complex causes of collapse and the following characteristics: territoriality, hiddenness, suddenness and mass occurrence [11]. In addition, with the steady progress of rail transit construction projects, the number of shield tunneling projects through karst areas is gradually increasing. Karst collapse is easily triggered by factors such as construction disturbance, construction precipitation and water intrusion in karst areas [12, 13, 14]. The karst collapse induced by shield tunneling in karst strata is affected by the combination of stratigraphic structure, geologicalconditions, construction control and other factors, which makes its karst collapse mechanism complex, thus leading to the complexity and urgency of karst collapse prevention and control during construction [15; 16]. Therefore, in karst areas, it is very important and difficult to maintain the stability of the tunnel's surrounding rock. To resolve these difficult problems, investigations based on theoretical derivations, numerical simulations, and physics experiments have been conducted by many scholars [17; 18; 19; 20]. [PERSON] et al. [21] used the distinct lattice spring model (DLSM) to numerically investigate the effect of karst caves on tunnel stability. Numerical simulation demonstrated that the closer the cave was to the tunnel, the weaker the tunnel was under loading. The larger the cave, the weaker the tunnel and its surrounding rock mass. [PERSON] et al. [22] established a three-dimensional numerical model of the upper cavern, studied the influence of the shape of the cavern on the stability of the surrounding rock, and analyzed the change law of the stress field and displacement field of the surrounding rock during the tunnel excavation. [PERSON] et al. [23] studied the law and degree of influence of karst cave span, high span ratio and filling degree on the safety thickness of karst cave rooves by numerical simulation and theoretical calculation and established a prediction model of the safety thickness of karst cave rooves. By means of mechanical analysis, simulation calculations and field tests, [PERSON] [24] studied the stability of cavities of different orientations and sizes on the tunnel envelope (displacement, stress) and support (axial force, bending moment). [PERSON] [25] used similar model tests and numerical simulations to study the stability of the surrounding rock and considered the effects of different cavern locations, water pressure, surrounding rock, and spacing on tunnel stability. In addition, other scholars have studied the stability of tunnels from other perspectives. [PERSON] et al.