# Research on Optimization Design of Tunnel Blasting Scheme Adjacent to Buildings
## 4 Optimal Design of Construction Parameters
### Optimized Design of Blasting Parameters  
In the project on which this paper is based, mechanical excavation combined with a blasting construction program is used. There are two drilling methods for blasting: one is the conventional palm face drilling method, and the other is similar to the surface mining bench blasting method [34,35]. After mechanical excavation of the upper bench, the lower bench has a free surface condition; meanwhile, due to the large tunnel section, the resulting hole is very deep, which will increase the difficulty in drilling and charging, thus increasing the construction cost, and there is no way to ensure the shaping of the tunnel contour line if the surface mining bench blasting method is used. In contrast, using the method of tunnel step blasting, it is less difficult to drill holes or load charges. The blasting method was used to control the quality of the tunnel profile. The blasting parameters of the perimeter hole, including the charge and the minimum resistance length, are the main determinants of the quality of light face blasting.  
Since a new proscinium is created through the mechanical excavation of the upper step, there is no need to arrange trenching holes, as auxiliary and peripheral holes are enough.  
(1) The determination of the charge amount.  
The charge length per meter of the perimeter hole is as follows:  
\[L\leq\left(8k_{b}S_{c}/n\rho_{0}D^{2}\right)\left(d_{b}/d_{c}\right)^{2} \tag{4}\]  
where \(L\) is the length of charge per meter of the hole, m/m; \(S_{c}\) is the uniaxial compressive strength of rocks, Pa; \(K_{b}\) is the coefficient of increase in the compressive strength of rocks, generally taken as \(K_{b}\) = 10; \(n\) is the collision pressure increase factor of the hole wall, generally taken as 8 to 11; \(\rho_{0}\) is the explosive density, kg/m\({}^{3}\); \(D\) is the explosive blast speed, m/s; \(d_{b}\) is the diameter of the hole, m; and \(d_{c}\) is the diameter of the explosive roll, m. Then, the charge per meter of the blast hole is as follows:  
\[q_{L}\leq\left(\pi{d_{c}}^{2}/4\right)L\rho_{0} \tag{5}\]  
where \(q_{L}\) is the charge per meter of the cannon hole, kg/m;  
(2) Determination of the minimum resistance line.  
In order to make the light explosion layer from the original rock, the minimum resistance line of the surrounding hole must be determined.  
The determination of the minimum resistance line lies in a reasonable choice of the proximity factor, m. The value of m should be chosen according to the nature of the rock, which is taken as 0.8 in this paper, and the minimum resistance line is calculated according to the following formula:  
\[W=a/m \tag{6}\]  
where \(W\) is the minimum resistance line length; and \(a\) is peripheral hole shell hole spacing, which is taken as 0.6 m according to experience.  
Figure 11: Plastic zone of surrounding rocks under different working conditions: (**a**) circular footage of 0.5 m; (**b**) circular footage of 0.75 m.  
Based on several field experiments and calculations performed in the above manner, the lower step blasting parameters were finally determined. With rock emulsion explosives used as explosives, the specifications of the volume were 32 mm \(\times\) 300 mm (diameter \(\times\) length), the weight of the volume was 300 g, and the minimum resistance line length was 500 mm. Digital electronic detonators are used in the holes, and then the terminal equipment is used for field delay. According to the blast hole layout and blasting parameters, the detonator of the corresponding holes is selected, and the delay time is set by sweeping the code with a handheld device, with its interval in each section set as 50 ms. The blasting parameters of the lower step section are shown in Table 3, the hole layout is shown in Figure 12, and the schematic diagram of the perimeter hole charging structure is shown in Figure 13.