# Research on Excavation Method for Soft Rock Tunnel Based on Stress Release Rate
## 3 Model Test Arrangement and Result Analysis
### Experiment Results and Analysis
Figure 24: Contact pressure of the first primary support with different excavation methods: (**a**) Three-bench; (**b**) three-bench with reserved core soil; (**c**) three-bench seven-step. In order to intuitively compare the contact pressure values under each excavation method, the final envelope diagram is drawn, as shown in Figure 26. The envelope diagram shows that the supporting structure is under overall pressure. From the quantitative point of view, the contact pressure of the first and second layers of the initial support is about 1100 kpa and 210 kpa. The pressure of the first layer of the initial support is much larger than that of the second layer, which is close to the field monitoring results, which further verifies the reliability of the model test. From the comparison diagram of contact pressure, it can be seen that the distribution shape of the surrounding rock pressure under different excavation methods is relatively close. The pressure is mainly concentrated in the arch shoulder and arch waist, and the pressure at the arch foot and arch bottom is small. The first layer of the initial support of the three-bench method, the three-bench core soil, and the three-bench seven-step method all reached the maximum pressure at the arch shoulder, which were 1163.17 kPa, 1223.95 kPa, and 1262.93 kPa, respectively. The maximum pressure of the primary support of the second layer is distributed at the spandrel and haunch. The three excavation methods are 248.52 kPa, 222.96 kPa, and 216.83 kPa, respectively. It can be seen that the three-bench core soil method and the three-bench seven-step method are more complicated due to their processes. The stress release is more sufficient during the tunnel excavation process. The  
Figure 25: Contact pressure of the second primary support with different excavation methods: (**a**) Three-bench; (**b**) three-bench reserve core soil; (**c**) three-bench seven-step. primary support structure of the first layer bears a greater load, while the load borne by the primary support of the second layer is reduced by 10.28% and 12.75%, respectively, compared with the three-bench method. 3 Analysis of internal force monitoring results of steel arch  
In order to study the mechanical characteristics of the supporting structure of the soft rock tunnel under different excavation methods, the internal force of the supporting steel arch is monitored and analyzed. Due to the symmetry of the left and right sides of the model test, the strain values measured at the measuring points of one side (vault, arch shoulder, arch waist, arch foot, arch bottom) are extracted, and the axial force and bending moments of the copper sheet are calculated using Equation (5). The similarity ratio of the known axial force and bending moment is \(C_{N}\) = 125,000, \(C_{M}\) = 6,250,000, and the bending moment axial force of the actual steel arch is further calculated. \[\left\{ \begin{array}{l}F=\frac{1}{2}E(\varepsilon_{in}+\varepsilon_{out})bh \\ M=\frac{1}{12}E(\varepsilon_{out}-\varepsilon_{in})bh^{2}\end{array} \right. \tag{5}\]  
where \(b\) is the width of the copper strip; \(h\) is the thickness of the copper strip; \(E\) is the elastic modulus of the copper strip; \(\varepsilon_{in}\) is the measured strain value on the inner side of the copper strip; \(\varepsilon_{out}\) is the measured strain value of the outer side of the copper strip. At the same time, in order to further judge the safety of the steel arch and initial support, the safety factor of the steel arch is obtained according to Equation (6), and the bending moment axial force and safety factor of the steel arch under different excavation methods are sorted out in Table 10.  
\[K_{a}=R_{a}/\left(\frac{F}{A_{a}}\pm\frac{M}{W_{a}}\right) \tag{6}\]  
where \(K_{a}\) is the safety factor of the steel arch, \(R_{a}\) is the ultimate strength of the steel arch, and \(A_{a}\) and \(W_{a}\) are the cross-sectional area and bending stiffness of the steel arch, individually.