## 2 Theoretical Analysis and Definition of Surrounding Rock Loose Zone  
After the rock mass in the center of a tunnel (chamber) is excavated, the initial stress state of the stratum is disturbed, and the original three-dimensional stress state changes to a two-dimensional stress state. This leads to stress redistribution and damage to the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Method Type** & **Source** & **Theoretical Calculation Method** & **Shortcoming** \\ \hline Empirical formula & Dong [14] & \(L=2.22\frac{B^{2.49}H^{4.30}R^{-1.05}10^{-11}}{(P_{\text{f}}+c\cot\varphi)(1- \sin\varphi)}\) & Poor applicability \\ Damage theory & Li [22] & \(\rho_{\text{k}}=R_{0}\left\{\begin{array}{ll}\frac{\sin\varphi}{\sin\varphi} \left[\left(\varphi_{0}+\left(2+\frac{1}{\right)}\right)\frac{\cos\varphi}{ \sin\varphi}\right]\\ \left(\frac{1+\sin\lambda}{2\pi\lambda}\right)\frac{\sin\varphi}{\sin\varphi} \end{array}\right.\left(\frac{2-\frac{1.88\sin\varphi}{1-\sin\varphi}}{1- \sin\varphi}+\frac{\lambda}{2\pi\lambda}\right)\\ &\text{[PERSON] [26], [PERSON] [27]}\\ & r=\rho_{0}\Big{[}\frac{(P_{0}+c\cot\varphi)(1-\sin\varphi)}{(P_{\text{f}}+c \cot\varphi)(1-\sin\varphi)}\Big{]}^{\frac{1-\sin\varphi}{2\sin\varphi}}\\ & \text{Complex parameter values}\\ Strength theory & [PERSON] [32] & \(R_{p}=r_{\text{f}}\Big{[}\frac{(m+n-a)(6m+2)}{\left[\sin^{2}-m+a)(6m+4)(M+ \frac{1}{2})}\Big{]}^{\frac{1-\sin\varphi}{2\sin\varphi}}\\ & \text{[PERSON] [33]}& R_{p}=a\left[\frac{D_{1}(1+\xi_{\text{f}})}{D_{2}(5-\xi_{\text{f}})} \right]^{\frac{1}{1+\sqrt{2}}}\Big{[}\frac{(m_{\text{f}}+M)T^{1-\Delta}-M+N}{P _{\text{f}}+N}\Big{]}^{\frac{1}{1-\sqrt{2}}}\end{array}\) & Ambiguous parameter value \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the scope of the existing surrounding rock loose zone. surrounding rock in the process. After the surrounding rock is damaged, the surrounding rock stress decreases along the radial depth of the tunnel until it becomes the initial stress. Based on the damage degree, the surrounding rock is divided from inside to outside into the loose zone, plastic zone, elastic zone, and original rock stress zone, as shown in Figure 1.  
During the excavation of the chamber, the formation of the loose zone is regarded as occurring in two parts. The first part is the stress release caused by the unloading and excavation of the chamber. Because the concentrated stress of some surrounding rock exceeds the strength of the surrounding rock, there will be a broken area along the radius of the chamber. The loose zone formed at this time is called the stress loose zone. In addition, the explosion produces a shock wave. When the shock wave enters the surrounding rock, it will attenuate into a stress wave for transmission. In the transmission process, it will produce a stress field in the surrounding rock. When the stress acts on the fracture zone of the loose zone and penetrates the crack tip, it will produce stress concentration at the crack tip. The singularity of the stress field at the crack tip leads to large failure stress caused by a small external load, which makes the crack in the loose zone of the surrounding rock expand and extend. At the same time, new cracks are generated in the original stress loose zone, and finally, a wider range of the loose zone is formed after expansion. At this time, the newly formed annular loose zone is called the blasting loose zone. Here, the final loose zone of the surrounding rock is regarded as consisting of two parts: a stress loose zone and a blasting-expansion loose zone. Here, the loose zone is defined as the boundary range of the annular rock damage area around the chamber, which is formed by the damage of the rock mass under the combined action of the in situ stress and the blasting load after the chamber is excavated.