## 3 Field Experiments
### YA17 Pumping Test  
The pumping test was carried out in Pit A, where pumping well YA17 was used for pumping, and two wells (i.e., YA19 and H2) were utilized as observation wells. Figure 4 illustrates the relative spatial positions of the diaphragm walls, the drainage well, and the observation wells. Pumping well YA17 was situated at the widened section of Pit A, 6.9 m away from the nearest diaphragm wall. Observation well YA19 was also located in Pit A, 10.5 m away from YA17. The second observation well, H2, was placed outside the pit, 20.9 m away from YA17. Additionally, the depth of the filter of well YA17 was above the bottom of the diaphragm wall, thereby obstructing the dewatering effect within the pit. A three-step dewatering test was conducted through pumping well YA17, with the pumping time and pumping rate detailed in Table 3. Figure 5 shows the drawdown at varying depths of well YA17 in the three-step dewatering tests. For the first round of pumping (Figure 5a), the drawdowns at all monitoring points of the well significantly increased at the initial stage of pumping before stabilizing. The maximum drawdown, located near the bottom of the filter (i.e., at a depth of 43-46 m), reached 25 m. In the second-round pumping test (Figure 5b), the maximum drawdown was again located at a depth of 43-46 m, similar to the first-round pumping test. However, due to the reduced pump rate in the second-round pumping test, the drawdown at all monitoring points sharply decreased to zero initially before settling at a constant level. The maximum drawdown at the steady state of the second-round pumping test was approximately 9 m. Given that \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Pumping Well**} & \multirow{2}{*}{**Test Operation**} & \multirow{2}{*}{**Time (h)**} & \multicolumn{2}{c}{**Pump Rate**} \\ \cline{3-4}  & & & **m\({}^{3}\)/h** \\ \hline \multirow{4}{*}{YA17} & First round pumping & 19 & 35 \\  & Second round pumping & 4 & 11 \\ \cline{1-1}  & Water level recovery & 17 & — \\ \cline{1-1}  & Third round pumping & 12 & 31 \\ \cline{1-1}  & Water level recovery & 12 & — \\ \hline \multirow{4}{*}{YB8} & First round pumping & 18 & 60 \\ \cline{1-1}  & Second round pumping & 20 & 35 \\ \cline{1-1}  & Water level recovery & 11.5 & — \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Pumping test scheme. the pump rate of the third-round pumping test was close to that of the first dewatering step, the maximum drawdown in the third pumping test was the same as that of the first dewatering step. The results indicate that a nonuniform water drawdown distribution was observed near the filter, with the maximum drawdown located near the bottom of the filter. This was attributed to the 3D flow around the pumping well, which was caused by the deep pit with an unclosed diaphragm wall and a highly permeable layer. The drawdown and hydraulic gradient were influenced by the position of the pump. For a pumping well, the well bottom was close to the unclosed diaphragm wall, which formed an impermeable hydraulic boundary. The recharge was cut off in the side direction. For the aquifer, the unclosed diaphragm partially penetrated the aquifer, which reduced the side seepage cross-section area for horizontal flow, and the seepage cross-section was lowered. Groundwater has to change direction and flow vertically to enter the partially penetrated aquifer before horizontally flowing into the filter tube of a pumping well. The vertical hydraulic conductivity was smaller than the horizontal one, larger potential energy represented by the piezometric head was consumed by the lower vertical permeable characteristic. The pumping rate decreased because of the decreasing cross-section area under the same hydraulic gradient around the bottom of the diaphragm wall. The groundwater flow around the diaphragm wall is also increased by the amount of water. Figure 4: Spatial relative positions of the diaphragm walls, drainage well, and observation wells for the YA17 test: (**a**) Top view; (**b**) Section view.