## 3 Results Analysis and Discussion
### Analysis of the Ground Surface Settlement  
The curve of ground settlement monitoring line No. 1 after the foundation pit excavation was selected, as depicted in Figure 18.  
After completing the foundation pit excavation, and the supporting structure moved towards the free face under the active earth pressure of the rear-side soil and the ground surface behind the supporting structure generated settlement. Figure 18 exhibits that the ground surface settlement curve around the foundation pit displays a \"spoon\" shape; the maximum ground surface settlement occurred at the 1.5 m position behind the supporting structure; the maximum ground surface settlement was \(-4.08\) mm, which is far smaller than the alarm value [31]. In the direction away from the foundation pit, ground surface settlement became increasingly small with increasing distance. The disturbing influence of the foundation pit excavation was when the soil became increasingly small. At the position 6 m away from the supporting structure, ground surface settlement decreased to \(-0.6\) mm, which was small. The ground surface settlement curve had a turning point. Thus, the main influence area of the ground surface settlement, caused by foundation pit excavation, was \(0.92\)\(H\). Analysis of Settlement Curve Shape  
According to the study results of Li [28], the influence area of the soil behind the foundation pit wall was divided as demonstrated in Figure 19, where \(L\) is the length of the foundation pit. Figure 18: Settlement curve of the ground surface. In Figure 20, \(X_{d}\) is the \(x\)-coordinate of the maximum settlement point, \(X_{z}\) is the \(x\)-coordinate of the turning point of the settlement curve, and \(X_{0}\) is the \(x\)-coordinate of the zero-settlement point in the settlement curve; furthermore, \(S_{z}\) is the ground surface settlement value of the turning point of the settlement curve, \(S_{0}\) is the ground surface settlement value near the supporting structure, and \(S_{m}\) is the maximum ground surface settlement value. An analysis showed that the two settlement patterns had similar shapes. The maximum ground surface settlement and boundary of the influence area in the ground surface settlement curve were larger in Area A than in Area B. Both settlement curves presented a \"square root\" shape, different to the ground surface settlement curve shape caused by the foundation pit excavation under the support of recyclable large-diameter steel-pipe piles. Based on the ground surface settlement patterns proposed by [PERSON] [28], a broken line was used to draw the ground surface settlement curve; the result obtained through [PERSON]'s method [28] was compared to the measured result, as presented in Figure 21.  
Based on the observation of the curve plotted in Figure 21, when \(X\leq 1.5\) m, the settlement curve is a straight line; the curve proposed by [PERSON] [28] agrees with the field test result. When \(1.5\) m \(<X<6\) m, a considerable difference exists between the two curves. When \(6\) m \(\leq X\leq 10.5\) m, the two curves are identical. Therefore, it is necessary to further analyze the shape of the settlement curve generated during the excavation of a foundation pit under the support of recyclable large-diameter steel-pipe piles. Figure 19: Division of the influence area of the soil behind the foundation pit. Figure 20: Ground surface settlement curve. Through further analysis of the settlement curve, it was found that the piecewise function can be used to describe the shape of the settlement curve. The function expressions are shown in Equations (4) and (5). \[y=ax+b\ (x\leq X_{d}), \tag{4}\]  
\[y=\frac{aX_{d}+b}{\left[1+\frac{(x-X_{d})^{c}}{d}\right]}\ (x\textgreater X_{d}), \tag{5}\]  
where  
\(y\)--ground surface settlement value;  
\(x\)--horizontal coordinate of the ground surface settlement monitoring point;  
\(a,b,c,d\)--parameters;  
\(X_{d}\)--horizontal coordinate of the maximum ground surface settlement monitoring point.