## 5 3D Numerical Simulation Using FLAC 3D
### Numerical Simulation Results  
There are four weak structural planes close to each other at Stakes K0 + 500-K0 + 620 in the diversion tunnel, in which microseismic activities were concentrated in layer-1 excavation of the diversion tunnel. Therefore, this region is the key position to study surrounding rock stability of the diversion tunnel. To explore the impact of weak structural planes on the stability of surrounding rock in the diversion tunnel, no support measures were imposed on this numerical simulation model. The numerical model calculation took 5 days. Figure 11 illustrates the displacement of the diversion tunnel trunk at Stakes K0 + 500-K0 + 620. It can be seen that the maximum displacement of 17 cm is at Stake K0 + 568 near the diversion tunnel trunk.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Surrounding** & **Density** & **Deformation** & **Elastic** & **Poisson’s** & **Internal Friction** & **Cohesion** \\
**Rock Grade** & **(kg/m\({}^{3}\))** & **Modulus** & **(GPa)** & **(GPa)** & **Ratio** & **Angle (\({}^{\circ}\))** & **(MPa)** \\ \hline IV1 & 2600 & 4 & 4.5 & 0.3 & 35 & 0.4 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Surrounding rock parameters.  
Figure 10: The 3D numerical model. (**a**) Complete model; (**b**) half of the model.  
As shown in Figure 11, four weak structural planes form three types of structural plane combinations, namely, the \"nearly parallel\" structural plane combination by g16 and f34, \"upright triangle\" combination by f34 and f27, and \"inverted triangle\" combination by f27 and f31. After the tunnel excavation, the surrounding rock inside the \"upright triangle\" structural plane has the largest displacements, with the maximum displacement of 17 cm. The surrounding rock inside the \"inverted triangle\" structural plane has relatively small displacements, with the maximum displacement of 15 cm. The surrounding rock inside the \"nearly parallel\" structural plane has the smallest displacements, with the maximum displacement of 12 cm. It is presumed that the rock mass in the \"upright triangle\" structural plane forms an \"upright wedge\" with a large bottom, which has a greater tendency to slip downward under the action of gravity and stress adjustment. However, the rock mass in the \"inverted triangle\" structural plane forms an \"inverted wedge\" with a small bottom, and the lower surrounding rock plays a certain role in blocking the slip. In addition, the structural plane combination has an offset to the side of the structural plane with a larger dip angle at the maximum displacement of the surrounding rock. For example, the \"nearly parallel\" structural plane has an offset to g16; the \"upright triangle\" has an offset to f34 and the \"inverted triangle\" has an offset to f31.  
Figure 12 shows the front view profile of displacements at the intersection of four weak structural planes and the diversion tunnel trunk. Specifically, the displacement of the surrounding rock at the intersection of the diversion tunnel trunk and g16, f34, f27, and f31 is 11, 13, 15, and 16 cm, respectively. The largest displacement appears at the intersection of the diversion tunnel trunk and f31. It is assumed that the dip angle of f31 is too large, which leads to the large displacement of the rock mass at the intersection under the action of gravity and stress adjustment of the surrounding rock.  
As previously indicated, microseismic activities are concentrated at Stakes K0 + 540 and K0 + 570, the maximum displacement in the numerical simulation is at Stake K0 + 568, and a relatively large displacement is generated at stake K0 + 542, which indicates that the distribution of microseismic activities closely coincides with the numerical simulation results.  
Figure 11: Displacements at stakes K0 + 500–K0 + 620 of the diversion tunnel.