## 5 Discussion  
The purpose of this study is to study the distribution of external water pressure under limited discharge in shield construction of soft rock tunnels, which provides a scientific basis for formulating a reasonable drainage strategy and ensuring the safety of tunnel excavation. Through numerical simulation analysis, it is concluded that with the increase in drainage rate, the external water pressure of the tunnel decreases significantly. When the drainage rate reaches 66.67% of the water inflow, the external water pressure of the tunnel can be reduced to 0.3 MPa. The results show that the appropriate drainage measures are effective means to control the external water pressure of the tunnel, and it is an important guarantee to ensure the safe tunneling of the Earth pressure balance shield machine and the smooth progress of the project. The results of this study are consistent with the findings of [PERSON]. [26], but there are some differences. This study clarifies the quantitative relationship between the specific drainage rate and the external water pressure, which provides more accurate technical guidance for tunnel construction. Secondly, the various exploration methods and conclusions adopted in this study have reference value for other tunnel projects under similar geological conditions and help to improve the scientificity and safety of engineering design and construction. This study also reveals the complexity of the Tertiary strata. The Paleogene sandstone and conglomerate strata are thick, and the hard and brittle sandstone layers have high mechanical strength and strong stress transfer ability. Under the action of tectonic movement, the stress is released in the form of brittle fracture, which is easy to produce cracks. The density of cracks is small, the opening is large, the extension is long, the penetration is strong, the filling is generally less, and the permeability is good. It has become a good channel for groundwater storage and migration. The soft and flexible mudstone layer has low mechanical strength, and the stress is mainly released in the form of plastic deformation after loading. The failure mode of the rock is mainly viscous shear, resulting in a large number of unobvious hidden cracks and closed cracks. The cracks are narrow and short, and the distribution is dense. Even if the tensile cracks are locally developed, they are often filled with their own broken argillaceous products. The permeability of the cracks is generally weak and becomes a relatively impermeable layer. Although there are aquifers such as clay rock and mudstone, the pinch-out phenomenon of the stratum causes a hydraulic connection between the originally isolated aquifers, forming a low-resistance water storage area. The reason for the polarization of water-richness in the stratum where  
Figure 12: Trend diagram of external water pressure under different drainage rates.  
the pumping well is located 30 m away from the middle line of the tunnel is explained. This complexity leads to the inhomogeneity of water-rich strata, which brings great challenges to the construction.  
However, this study also has some limitations. Specifically, when establishing the bottom boundary of the three-dimensional numerical model to 50 m at the bottom of the tunnel, some strata are generalized. However, in reality, the strata in the engineering area are often very complex, and there is still a certain gap between the numerical simulation model and the actual situation. Therefore, in future research, it is necessary to strengthen the field test and monitoring of the engineering properties of the stratum, obtain more accurate stratum parameters, provide more reliable input data for the numerical model, and further optimize the drainage strategy, considering the optimal drainage rate under different geological conditions, in order to improve the efficiency and safety of tunnel construction.