## 6 Analysis Based on the Foundation Pit Project of Sanhuan South Road Station of Xuzhou Metro Line 3  
Located at the intersection between the Sanhuan South Road and Beijing Road, the Third Ring Road South Station of Xuzhou Metro Line 3 has an SN layout and is a transfer station of Metro Line 3 and Metro Line 4, as shown in Figure 15. The foundation pit is close to Mount Zhai. The terrain is gradually rising from north to south, and groundwater movement is intense. Caves are formed along the fissures and structural surfaces by long-term seepage of surface water in the site. The water pumping test shows that the permeability coefficient is about 3.5 m/d. The foundation pit is excavated to a depth of approximately 30 m, where limestone is mainly developed. The construction method of \"cut-bottom up\" is adopted for the main foundation pit of the station, and the retaining structure of \"retaining pile + horizontal inner supports (anchor cables)\" is adopted. A geological section of foundation pit cross section is shown in Figure 16.  
The western side slope of the pit is the bedding rock structural plane (C35 concrete pouring) with a \(\mathcal{O}\) 1000 mm @ 1500 mm cast-in situ bored pile (dip angle: 70\({}^{\circ}\)). The eastern side slope is the toppling rock structural plane with a \(\mathcal{O}\) 1000 mm @ 2000 mm cast-in situ bored pile. Concrete supports (C35 concrete pouring) are used as the first support; steel supports (diameter: 800 mm) are used as the second support; and the third steel support can be set locally. The mechanical parameters of rock mass are shown in Table 3. Structural plane parameters are shown in Table 2.  
Figure 16: Engineering geological map. Figure 15: Location diagram of Sanhuan South Road Station. The slopes of the toppling rock structure plane are monitored by two retaining piles on the east side of the pit under study, while four retaining piles are selected to monitor the slopes of the bedding rock structural plane on the west side, as shown in Figure 17. Six monitoring points for reinforcement meters are set on the soil-facing side of each retaining pile, and six concrete strainmeters are set on the soil backing side for data calibration, as shown in Figure 17. Meanwhile, reinforcement meters (soil backing side) and concrete strainmeters (soil facing side) are set on the crown beam, and 13 monitoring points are set correspondingly. A total of seven monitoring points for metal surface strainmeters are set on the steel waist beam, as shown in Figure 17.  
Figure 18 shows the horizontal displacement curve of the piles monitored on the bedding rock side and the toppling rock side of the foundation pit, as E1 and W2 were broken during construction and were not analyzed here. The E2 retaining pile is located on the side slopes on the toppling rock side. As seen from Figure 18a, the top of the retaining pile basically slopes toward the interior of the foundation pit until the foundation pit is excavated to \(-3\)F (\(-20\) m), which is the same as the trend as the pile displacement curve in Figure 11a. When the foundation pit is excavated to \(-30\) m, the mid- and lower-part of the pile body starts inclining toward the inside of the foundation pit, indicating that the lower rock mass of the foundation pit also starts inclining toward the inside of the foundation pit. Figure 17: Monitoring point of foundation pit. (**a**) monitoring point for inclination (unit: mm, scale: 1:200); (**b**) monitoring points for stress and strain meters of retaining piles (unit: mm, scale: 1:50); (**c**) monitoring points for stress and strain meters of support beam (unit: mm, scale: 1:200). Figure 16.0 _Cont._  
The W1, W3, and W4 retaining piles are located on the side slopes on the bedding rock side in Figure 18b-d. The displacement curve of the W1 pile shows that, as the excavation depth of the pit increases, the salient point of horizontal displacement gradually moves downward with the increase in excavation depth of the foundation pit.