## 4 Engineering Background
### Initial Scheme of Foundation Pit Dewatering in the Study Area  
To achieve dewatering and maintain a dry excavation in the foundation pit of the inverted siphon section of the canal head, this study implements the tube well dewatering method instead of utilizing a waterproof curtain. Moreover, a circular arrangement approach is employed to uniformly position dewatering wells at a distance of 1 m outside the pit to effectively lower the groundwater level. In this paper, the foundation pit with a pile number of XZ0+326-XZ0+500 in the inverted siphon section of the canal head is selected for dewatering design in the pebbles stratum. The length of the foundation pit is 174 meters, and the width is 39 m. The ground elevation of the project area is about 122.5 m, the first layer of sandy loam is about 3 m thick, and the second layer of pebbles is about 20 m thick. The initial groundwater level is about 121.5 m, the bottom elevation of the phreatic aquifer is about 99.5 m, the thickness of the aquifer is about 22 m, the bottom elevation of the foundation pit is 110.3 m, the average excavation depth of the foundation pit is 12 m, the groundwater level is calculated as 1 m below the foundation pit bottom plate, and the drawdown of the water level in the project area is 11-12 m. The engineering geological longitudinal section and foundation pit location of the canal head inverted siphon XZ0+326-XZ0+500 section in the project area is shown in Figure 4. The red line in the figure shows the location of the foundation pit. Figure 3: Location distribution map of study area. The initial design scheme uses a group of phreatic completely penetrating wells for dewatering, which needs to lower the groundwater to 0.5-1 m below the bottom of the foundation pit. According to the technical specification for building a foundation pit (JGJ/120-2012) [33], calculate the total water inflow of the foundation pit:  
\[Q=\pi k\frac{(2H-s_{d})s_{d}}{\ln\left(1+\frac{R}{r_{0}}\right)} \tag{16}\]  
Among them, \(Q\) is the total water inflow, m\({}^{3}\)/d; \(k\) is the permeability coefficient, m/d; \(H\) is the thickness of the phreatic aquifer, m; \(s_{d}\) is the design drawdown of the groundwater level of the foundation pit, m; \(r_{0}\) is the equivalent radius of foundation pit, m, which can be calculated according to \(r_{0}=\sqrt{\frac{A}{\pi}}\); \(A\) is the area of foundation pit, m\({}^{2}\); and \(R\) is the influence radius of dewatering, m.  
The influence radius of dewatering can be calculated according to the drawdown value of the observation well arranged in the pumping test, combined with the graphical method. If there is no water level observation well, it can also be solved by using the phreatic aquifer empirical formula method with reference to the parameters obtained from the pumping test:  
\[R=2s_{w}\sqrt{kH} \tag{17}\]  
where \(s_{w}\) is the drawdown value of the well water level, m. The meaning of the other symbols is the same as above. In order to ensure the dewatering effect, the number of tube wells can be calculated using the following formula when tube wells are arranged at equal intervals for dewatering:  
\[n=(\,1.1\sim 1.2)\frac{Q}{q_{max}} \tag{18}\]  
where \(n\) is the number of wells and \(q_{max}\) is the maximum water yield of a well, m\({}^{3}\)/d. The maximum allowable pumping capacity of the pumping well is generally obtained from the pumping test. In the absence of a pumping test, it can be solved according to the following empirical formula:  
\[q_{max}=120\pi rl\sqrt[3]{K} \tag{19}\]  
where \(q_{max}(i)\) is the maximum pumping capacity of the \(i\)-th pumping well, m\({}^{3}\)/d; \(r\) is the screen radius of the pumping well, m; and \(l\) is the effective working length of the screen of the pumping well, m.  
Figure 4: Engineering geological longitudinal section and foundation pit location of canal head inverted siphon section XZ0+326–XZ0+500.