## 3 The Variation Law of Groundwater during Tunnel Construction
### Influence of Tunnel Drainage Mode on Groundwater Level  
Tunnel drainage modes include full drainage, limited drainage, and full sealing. The full drainage mode does not consider tunnel water plugging measures. Groundwater  
Figure 8: Water level changes at three measuring points near the surface in each construction stage. Figure 7: Tunnel drainage volume at each construction stage. that can be discharged after tunnel excavation is released through the tunnel drainage system. The numerical model is consistent with the permeability coefficient of the initial lining and the permeability coefficient of the surrounding rock. Limited discharge refers to using grouting layer measures to discharge a certain amount of groundwater through the drainage system. The permeability coefficient of the tunnel lining is set as shown in Table 2. Full sealing refers to not installing a drainage system after tunnel excavation and initial support, and setting the pressure boundary of the secondary lining unit as an impermeable boundary in the numerical model. Figure 9 shows the water level changes at each measuring point during the construction process under three modes. Under the full discharge mode, the groundwater level rapidly declined and did not recover. If the groundwater remained below the ecological burial depth for a long time, it would seriously threaten plant growth and even lead to plant death. Under the limited discharge mode, the groundwater level changes at each measuring point were relatively stable, with a high degree of recovery. Within 15 m from the free water level, the recovery degree exceeded 75%, with the water level at point F declining by 0.81 m. After the construction of the second lining in the fully sealed mode, the groundwater level of the tunnel was almost completely restored, with the most obvious fluctuation at measuring point A. The lowest groundwater level declined to 1.17 m. After restoration, the water level reached 28.24 m, with a recovery degree of 97.3%. The cost of the fully sealed mode is extremely high. Considering the impact of groundwater discharge and cost, the limited discharge mode is more reasonable for the normal growth of plants. The Influence of the Thickness of the Water Blocking Grouting Circle on the Groundwater Level of the Tunnel  
The same grouting material and different thicknesses of water-blocking grouting circles have a significant impact on groundwater discharge and water levels. To analyze the effect of different thicknesses of water-blocking grouting circles on the variation of groundwater levels during tunnel construction, the permeability coefficient of grouting circles with thicknesses of 4.9 \(\times\) 10\({}^{-1}\) m/d, 0 m, 2 m, 4 m, and 6 m were studied. The results are shown in Figure 10. Measurement point A was located inside the grouting circle, and the water-blocking effect of the grouting circle prevented the groundwater outside the grouting circle from being transported to point A in a timely manner, resulting in an increase in pore pressure inside the grouting circle. The thicker the grouting circle during construction, the higher the lowest water level at point A, with a maximum of 3.93 m. After the construction of the secondary lining, the pressure water level at point A rapidly declined, and the redistribution of the seepage field gradually stabilized. After stabilization, the water level was 28.92 m. Measurement point B was in the grouting range of 4-6 m. When the grouting thickness was 2 m or 4 m, point B was outside the grouting circle, and the water level was larger than that of without grouting. When grouting for 6 m, measurement point B was located inside the grouting circle, with a minimum water level of 19.61 m. The  
Figure 9: Changes in water level in different construction and discharge stages. (**a**) Full drainage. (**b**) Limited drainage. (**c**) Full sealing. water blocking effect of grouting caused the water level to be slightly lower than in the first two working conditions, but much higher than the situation without grouting.