# A Finite Element Analysis of Tunnel Lining Demolition by Blasting for Subway Tunnel Expansion
## 3 Parameter Design of Blasting Expansion  
The thickness of the existing subway tunnel lining structure is 70 cm, which needs to be cut in longitudinal and circular directions by a machine, and then the existing subway tunnel's cross-section needs to be expanded using the drilling and blasting method with optimized blasting parameters; thus, the existing tunnel lining will be crushed by blasting shock waves and the collapsed surrounding rock mass. In order to achieve the purposes of quickly demolishing the tunnel lining and reducing the expansion blasting load influence on the adjacent non-expanded subway tunnel structure, the center diaphragm (CD) excavation method is adopted for the construction of a subway station that is in situ expanded from an existing subway tunnel. The construction process is as follows:  
1. Cut the existing subway tunnel lining, and the average spacing of the longitudinal and circular cutting joints is 3 m and 1.5 m, respectively. 2. Expand the excavation of the left drift heading of the station section using the drilling and blasting method, and support the excavated left drift heading using an anchor and shotcrete. 3. Expand the excavation of the right drift heading of the station section using the drilling and blasting method, and support the excavated right drift heading using an anchor and shotcrete. 4. Excavate and support the inverted arch. 5. Build the tunnel lining structure of the platform section after the expansion excavation process. The subway tunnel station is in IV-class surrounding rock mass of relatively broken sandy mudstone, and the value of compressive strength is 24.7 MPa. The blasting footage of the existing subway tunnel in the expansion construction is 1.5 m, which can ensure the safety of subway station construction. Moreover, the distance between the adjacent  
Figure 1: The plan of subway tunnel in situ expansion excavation to a subway station. Figure 2: A schematic diagram of in situ expansion excavation method for subway station (unit: mm). non-expanded subway tunnel structure and the expanded blasting surface is about 80 m. Some research results suggest that the blasting vibration velocity must be controlled within 5.0 cm/s during the blasting process [3; 4; 7]. The permitted maximum explosive charge of each excavation cycle is 25.6 kg in the CD excavation method of the tunnel construction process, which is calculated by the formula of [PERSON]'s maximum vibration velocity shown in formula 1 [4; 7; 15]. \[V=K(\frac{Q^{1/3}}{R})^{\alpha} \tag{1}\]  
where \(V\) is the maximum vibration velocity (unit: cm/s), \(Q\) is the explosive charge quality in each blasting cycling (unit: kg), \(R\) is the distance between the blasting face and the control particle (unit: m), \(K\) is the site coefficient, and \(\alpha\) is the attenuation coefficient. The explosive charge in the controlled blasting construction is listed in Table 1, which is determined using Equation (1). In order to achieve a smooth blasting excavation outline and a small vibration velocity, it is necessary to design the blasting hole spacing. Based on a numerical simulation calculation, the optimized blasting parameters are determined as follows: the spacing between peripheral boreholes is 50 cm, the spacing between auxiliary holes is 80 cm, and the spacing between the bottom holes is 70 cm. The optimized borehole layout is shown in Figure 3.  
The diameter of the borehole is 42 mm, and using the No.2 rock ammonium nitrate explosive, the density is 1.05-1.25 g/cm\({}^{3}\), the explosion intensity is 295 mL, the explosive intensity is 12 mm, and the detonation velocity is 5000 m/s in the blasting process. Moreover, a millisecond detonator is used to initiate the explosive. The explosive charges per 1.5 m of excavation footage are listed in Table 2.