## 4 Engineering Application of the ASRB Parameter Design Method
### Engineering Background  
The ASRB design solutions vary depending on ground stress, tunnel size, lithology, and excavation method. To this end, the following is an example of a deep diversion tunnel project in southwestern China to illustrate the application of the above methodology. The engineering geological profile and tunnel cross-section design are shown in Figures 7 and 8, respectively. The tunnel engineering area was the one with the strongest tectonic activity and the fastest geomorphic evolution on the Earth today. The tunnel was 38 km long, and the maximum buried depth reached 1673.5 m. Moderate-to-intense rock bursts may occur along the route. Therefore, the project used the open TBM method as the primary construction method and the drill and blast method as the auxiliary method. The project also fully considered the impact of rockbursts on the TBM equipment, which was equipped with high-efficiency advance drilling equipment to facilitate deep surrounding rock stress release when necessary.  
This study was based on the rockburst assessment results along the tunnel and took the section near the high-stress-risk area of the tunnel (CK1247-CK1248, blue box position in Figure 7) as the study area. According to the results of in situ hydraulic fracturing and in situ stress field inversion, the initial geostress state of the area was \(\sigma_{H}=39.38\) MPa, \(\sigma_{h}=31.14\) MPa, and \(\sigma_{z}=27.63\) MPa. The minor deviation between the intermediate principal stress direction and the tunnel axis was ignored during the study. The maximum principal stress direction was set as the vertical direction, the intermediate principal stress direction was consistent with the tunnel axis direction, and the minimum principal stress was perpendicular to the tunnel  
Figure 8: Design drawing of the TBM-constructed section of a deeply buried headrace tunnel in southwest China.  
Figure 7: Engineering geological profile of a deeply buried headrace tunnel in southwest China.  
axis. The lithology of the study area was mainly Himalayan granite. To better simulate the true mechanical behavior of rock masses, a rock deterioration model (RDM) was selected for this simulation study [64]. This model is widely used in various deep hard rock engineering projects, such as for the marble in the pilot tunnels of the Jinping II Hydropower Station [63], and can accurately depict the high-stress fracture damage process of hard rock with weakened cohesion and strengthened friction, as shown in Figure 9. Based on the indoor test results, the elastic modulus, Poisson's ratio, initial friction angle, and initial cohesion parameters of the rock mass were obtained. On this basis, the remaining parameters were continuously adjusted to fit the depth results of the surrounding rock damage zone, and the simulation parameters were finally determined, as shown in Table 3. The bottom, right, and rear parts of the model (with the negative direction of the X-axis as the dominant visual direction) were constrained, and the initial element stress values were set to \(\sigma_{x}=31.14\) MPa, \(\sigma_{y}=27.62\) MPa, and \(\sigma_{z}=39.38\) MPa. The simulation results of tunnel excavation based on the above parameters are shown in Figure 10a. The figure shows that the depth of the tunnel surrounding the rock EDZ was 2\(\sim\)3 m, which is consistent with the measured results (Figure 10b,c).  
Figure 10: _Cont._  
Figure 9: Mechanical properties and model parameter variation characteristics of strain softening constitutive model: (**a**) example stress–strain curve; (**b**) variation in cohesion with plastic strain; (**c**) variation in friction with plastic strain.