# A Finite Element Analysis of Tunnel Lining Demolition by Blasting for Subway Tunnel Expansion
## 5 Conclusions and Discussion  
Based on a practical engineering case of the in situ expansion excavation of an existing subway tunnel to a subway station, the CD construction process of the drilling and blasting method is studied using a numerical simulation, and a rapid demolition method of the existing tunnel lining is proposed. The rationality of the blasting parameters used in this paper is verified from three angles, namely, the overbreak and underbreak of the surrounding rock, the PPVs of the surrounding rock, and the mechanical response of the existing tunnel lining during the demolition process. The following conclusions are drawn:  
1. Using the blasting parameters proposed in this paper to in situ expansion excavate an existing subway tunnel to a subway station, the maximum overbreak of the tunnel vault, side wall, and inverted arch are 15 cm, 7 cm, and 5 cm, respectively, meeting the requirements of tunnel overbreak and underbreak quality.
2. The blasting tensile stress is greater than the tensile strength of the concrete material, and the radius of the blasting fracture zone is 0.93 m, which is greater than the thickness of the tunnel lining, so penetrating cracks occur in the tunnel lining. Therefore, the existing subway tunnel lining will collapse under the action of the blasting wave load and the gravity of the surrounding rock, which makes it possible to quickly demolish the tunnel lining.
3. The maximum principal stress \(S_{1}\) and the minimum principal stress \(S_{3}\) produced by the blasting reach the limit values of the surrounding rock, with the \(S_{1}\) stress being 1030 MPa and the \(S_{3}\) stress being 886 MPa from the initiation of the explosion to the time of 0.1 \(\times\) 10\({}^{-3}\) s. The surrounding rock is damaged by the action of the blasting stress. The \(S_{1}\) stress and the \(S_{3}\) stress gradually decrease from the time of 0.1 \(\times\) 10\({}^{-3}\) s to 0.4 \(\times\) 10\({}^{-3}\) s. The \(S_{1}\) stress tends to be stable; the \(S_{3}\) stress increases and the value is greater than that of \(S_{1}\) until the time of 0.5 \(\times\) 10\({}^{-3}\) s. Lastly, the \(S_{3}\) stress tends to be stable at the time of 0.8 \(\times\) 10\({}^{-3}\) s.
4. The PPVs decrease with the increase in the distance from the blasting center. The relationships between the PPVs and the vertical distance H and the horizontal distance L of the explosion center are as follows: \(V_{H}\)\(=0.01H^{2}-0.734H+16.45\) and \(V_{L}\)\(=0.0024L^{2}-0.165L+5.2\). Under the condition of the same stratum and charge quantity, the PPVs at different positions from the blasting center can be predicted using Equations (4) and (5) so as to provide a theoretical and design basis for the in situ expansion of tunnel blasting.  
Conceptualization, J.Z. and B.Z.; investigation, P.S. and B.D.; methodology, B.Z., P.S. and Y.W.; resources, P.S.; data curation, J.Z. and P.S.; writing, J.Z. and B.Z.; formal analysis, B.D. and Y.W.; writing--review and editing, J.Z., B.D. and Y.W.; funding acquisition, B.Z.; project administration, B.Z. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Chongqing Doctor Direct Online Fund (Nos. CSTR2022 BSXM-JSX0024), the Chongqing Education Commission Science and Technology Fund (Nos. KJQN202101506), and the State Key Laboratory of Mountain Bridge and Tunnel Engineering Open Foundation (Nos. SKLBT-2110).