## 4 Analysis of Test Results
### Stress Analysis of Supporting Piles  
According to the knowledge of material mechanics, the formula for calculating the pile bending moment from strain is [21]:  
\[M=EI\frac{\varepsilon_{i1}-\varepsilon_{i2}}{B} \tag{1}\]  
where \(EI\) is the bending stiffness of the pile body, \(\varepsilon_{i1}\), \(\varepsilon_{i2}\) are the strain values on both sides of the \(i\) measuring point, and \(B\) is the pile diameter. It can be seen from the above formula that for the piles, their flexural stiffness \(EI\) and pile diameter \(B\) are fixed values, so the distribution of the bending moment of the piles' body is the same as the distribution of the strain difference. In order to analyze the change law of the internal force of the pile body intuitively, this paper uses strain instead of bending moment to analyze the strain of pile 2 under different excavation conditions and anchors tensioning conditions, and its distribution is shown in Figure 7. As the soil in front of the pile is excavated, the distribution of strain in the pile body shows itself to be nonlinear. When the pile is in working condition 1 (cantilever stage), the retaining side of the pile is under tension and the maximum value is at the top of the pile, and the strain distribution is in a shape which is small at both ends and large at the middle. As the pile moves from the cantilever state to the single pivot state, the pile bending moment changes from tension on the retaining side to tension on the hollow side. As the supporting pile enters the multi-supported state, the soil resistance in the passive zone gradually increases, the maximum bending moment, the anti-bending point of the pile are shifted downward, and the value of the maximum bending moment on the airside is significantly reduced. Unlike the pile in cantilever state, which is only subject to the soil force, the pile-anchor support structure is subject to the interaction of both soil unloading and anchor tensioning. The bending moment at the embedded end also changes from being characterized by tension on the retaining side in the initial working condition, to tension on the hollow side in the multi-pivot stage, and then returns to a state of tension on the retaining side after excavation is completed. This shows that the anchor is tensioned to form a synergistic effect with the pile and the soil, which effectively changes the force state of the supporting pile and avoids the ultimate force state of the supporting pile. Since the spatial position of the supporting pile has an influence on the bending moment of the pile, it is necessary to further analyze the bending moment law of the supporting pile at different positions. The strains of different supporting piles under the  
Figure 7: Strain distribution curve of pile 2. (**a**) Strain distribution of pile 2 under soil excavation conditions; (**b**) strain distribution of pile 2 under anchor tensioning conditions. final working condition are shown in Figure 8. It can be seen from the figure that the bending moment of pile 2 (intermediate pile) is larger than pile bending moment of pile 1 (side pile). Specifically, the embedded end bending moment is more obviously so. This is due to the boundary effect near the side-pile: the soil near the pile is more self-stabilizing, and the soil pressure acting on the pile is relatively small. This makes the pile bending moment smaller. In the actual situation on site, the supporting piles located at the corner of the pit have smaller pile bending moment than the supporting piles in the middle due to the existence of the shaded angle. The whole pile moment distribution is \"S\" type, and the reverse bending point is near the bottom of the pit. The supporting pile is under tension on the hollow side above the pit bottom and under tension on the retaining side below the pit bottom, and the maximum positive bending moment is greater than the maximum negative bending moment.