## 4 3D Numerical Analysis
### Numerical Solution of Critical Slurry Weight  
The monitoring point selection process was critical to assessing the displacement behavior of the extraction trench wall. To achieve this, the depth of \(Z=62\) m in the peat soil layer was strategically chosen while setting the slurry weight at \(\gamma_{\mathrm{s}}=13.2\) kN/m\({}^{3}\). The results, as presented in Figure 14, showed that significant displacement was mainly concentrated at the center and the angle of the trench section. Based on this observation, a series of monitoring points were carefully selected, as illustrated in Figure 15. The displacement curves of the selected monitoring points, plotted against the depth, are presented in Figure 16, with positive values indicating displacement towards the inside of the trench and negative values indicating displacement towards the outside of the trench wall. Figure 14: Horizontal displacement cloud around the trench wall at the depth of \(Z=62\) m. (a) X-direction; (b) Y-direction. Figure 13: Hydrostatic slurry pressure diagram. Figure 15: Schematic diagram of the location of each measuring line. Each point near the surface position (Figure 16), within the 0-5 m depth range, experiences displacement towards the inner direction of the trench, owing to an insufficient increase in shallow mud pressure to counteract the surface overload, ultimately resulting in overall destabilization and collapse damage. In contrast, displacement in the out-of-trench direction occurs at each point beyond 10-20 m and 70 m into the rock. The mud pressure exhibits a linear increase for depths between 10 and 20 m, whereas the soil and water pressure increases at a slower pace. For depths less than or equal to 70 m, the surrounding rock exhibits a significant soil arch effect, and the soil and water pressure is minimal. Thus, the mud pressure exceeds the soil and water pressure, causing the displacement of each measurement point towards the outside of the trench. As the depth increased, the displacement of each measurement point towards the trench also increased, and it reached the maximum value at a depth range of 60-70 m, leading to a local instability collapse. This phenomenon can be attributed to two factors. Firstly, the meeting point of the soil and rock layers at this depth was prone to stress concentration, exacerbating the displacement towards the trench. Secondly, the physical and mechanical properties of the peat soil were inadequate, which resulted in the decreased stability of the diaphragm wall. The small natural density, high water content, large pore ratio, high compressibility, poor uniformity, and low bearing capacity of the peat soil all contributed to this instability. According to the data presented in Figure 17, the displacement values of each point follow a consistent pattern, with the middle of the inner side of A-A being the most vulnerable area to damage in the circular ground connection wall with a double-angle trench. Whether it is the general instability failure in the shallow part or the local instability failure at depths of 60-70 m, the order of displacement values is A-A > C-C > B-B > D-D > E-E > F-F > G-G. Therefore, the corresponding measuring points on the A-A, B-B, C-C, and D-D measurement lines at \(\text{Z}=\text{62 m}\) were selected for the further analysis of the local instability and damage points at depths of 60-70 m. As the slurry pressure decreases, the displacement change curve and change rate are as presented in Figure 17.  
The findings presented in Figure 18 reveal a noteworthy trend in the horizontal displacement of each measurement point as the slurry weight decreases. Notably, the increase in displacement can be classified into two distinctive stages, a linear stage I and an exponential stage II. During stage I, when the slurry weight is above 9.5 kN/m\({}^{3}\), each location displacement displays a linear growth trend in response to a decrease in slurry weight.