# Study on the Damage Process and Numerical Simulation of Tunnel Excavation in Water-Rich Soft Rock
## 5 Validation of the Simulation Method Based on Dual Softening Effect
### Comparison of Numerical Simulation and Field Monitoring Results
#### 5.4.2 Pressure Analysis of Surrounding Rocks of the Tunnel  
Figure 22 shows the comparison between the field monitoring data and the calculated values of different models at each measuring point of section K14 + 885. At the vault, waist and foot of the tunnel, the field monitoring results are closer to the calculated results considering unloading softening, water-softening, and time effect, and the errors of the calculated values compared with the monitored values are 2.2, \(-\)1.0 and 4.2%, respectively.  
Figure 21: Curves of field monitoring and simulations of horizontal convergence at section K14 + 885.  
The calculation error is maximum at the foot of the arch and minimum at the waist of the arch. The results obtained from the fluid-solid coupling differ greatly from the field monitoring results, and the errors of the calculated values are \(-17.7\), \(-20.7\), and \(-11.7\%\), separately, with the largest error at the arch waist and the smallest error at the arch foot.  
By comparing the calculated results of different models, the results of the fluid-solid coupling without softening are much smaller than that of new-proposed method considering unloading, water-softening, and time and field monitoring results, which fully indicates that because in the fluid-solid coupling, the physical-mechanical parameters of the surrounding rock element (e.g., \(E,\varphi,c\)) do not deteriorated with the softening process. When some elements' parameters should be deteriorated due to softening but still keep the initial condition involved in the calculation, then local yielding damage of surrounding rock will not occur. Therefore, the excavation of rock elements still maintains a good integrity and deformation modulus, resulting in a smaller interaction pressure between the surrounding rock and support than the field monitoring data. On the contrary, in the model, considering unloading softening, water-softening and time effect, the mechanical parameters of surrounding rock units change accordingly with the construction process, environment, and time so that the final calculation results are in better agreement with the field monitored values. In addition, all the calculated results are consistent with the trend of the pressure distribution monitored in the field, i.e., the surrounding rock pressure at the foot of the arch is greater than the pressures at the waist of the arch and the pressure at the vault is the smallest, which also shows the reasonableness of both calculations. At the inverted, the multi-field coupling model calculated results have about 16.7% error with the monitoring results, while the fluid-solid coupling model calculated results have about 40.4% error. As the invert is subjected to much smaller surrounding rock pressure than other monitoring positions, and typically is not the weak position of the tunnel, it is not discussed in this model.  
Through the analysis of field monitoring data, fluid-solid coupling results and fluid-solid-damage multi-field coupling results, it can be found that the fluid-solid coupling often underestimates the softening of surrounding rock in water-rich soft rock, and overestimates the stability of surrounding rock after excavation, so that the deformation and pressure of surrounding rock after excavation are smaller than the actual situation on site. The new-proposed fluid-solid-damage multi-field coupling method considers the damage field under the combined effect of excavation unloading, water-softening, and time  
Figure 22: Field and calculated surrounding rock pressure at each measuring position.  
variation, which is better suited for the on-site construction steps and damage phenomena, and the final calculated results are closer to the field monitoring results.