## 3 Engineering Application
### Numerical Simulation Model  
FLAC3D software was used for the numerical simulation of excavation and support. The model included the embankment of the He-Da Expressway with a slope of 1:1.5 (Figure 3a). At the bottom of the model were X, Y and Z constraints, and around them were normal constraints. The model consisted of 67,336 nodes and 62,784 elements, and the Mohr-Coulomb yield criterion was adopted. Shell and cable structural elements simulated the primary support and bolt, respectively. Groundwater was located 6 m above the tunnel roof arch.  
In the early stages of construction, the rock and soil masses within the construction range of the station are drilled and sampled. The geological parameters of the station are investigated and the stratigraphic parameters are obtained. However, there are some limitations in the geological surveys. The process of geological surveying is interfered with by many factors. The rock mass and overlying rock mass parameters of the station are essential references in the construction process. The main body of the tunnel in the study area is in moderately weathered gneiss, and the strongly weathered gneiss above the tunnel affects the stability of the tunnel to a certain extent. Therefore, the relevant parameters of moderately weathered gneiss and strongly weathered gneiss are mainly identified.  
The elastic modulus (E1) and Poisson's (\(\mu\)1) ratio of moderately weathered gneiss and the elastic modulus (E2) and Poisson's ratio (\(\mu\)2) of strongly weathered gneiss were selected as the back analysis parameters. In addition, the permeability coefficients of moderately weathered gneiss (K1) and the permeability coefficients of strongly weathered gneiss (K2) that are difficult to measure were also added as the back analysis parameters. Other mechanical parameters are shown in Table 1. Input values were tunnel deformation value, pore water pressure and water inflow, and output values were surrounding rock parameters.  
The arch crown settlement AZ, arch bottom uplift DZ, arch waist convergence BC and the relative displacement AB of measuring points A and B were taken as the displacement monitoring values (Figure 3b). The pore water pressure P at point E and the unit seepage volume F of the tunnel were taken as the seepage monitoring values. Among them, point E was 1 m away from the arch foot of the tunnel, the pore water pressure P was measured by the pore water pressure gauge and the unit seepage F of the tunnel was calculated by dividing the sum of the seepage of all outlet points in a particular mileage section of the tunnel by the length.  
According to relevant specifications for tunnel engineering design and geological survey data, the value ranges of six parameters were as follows: E1 is 2.85 GPa\(-\)6.53 GPa; \(\mu\)1 is 0.21-0.41; E2 is 1.81 GPa\(-\)3.49 GPa; \(\mu\)2 is 0.25-0.45; K1 is 0.196 m/d\(-\)0.372 m/d; K2 is 0.4 m/d\(-\)0.78 m/d. The orthogonal design scheme and uniform design scheme were established through these six parameters for numerical calculation. The calculation results are shown in Table 2 (training samples) and Table 3 (test samples).  
Figure 3: The numerical calculation model of the Chenjiadian tunnel. (**a**) Numerical model. (**b**) Distribution of monitoring points.