## 4 Analysis of Calculation Results
### Surface Settlement  
As shown in Figure 7, the deformation of the retaining structure is caused by the unloading effect of soil excavation, resulting in the settlement of the soil behind the retaining structure. Soil inside the foundation pit has uplift deformation, because its consolidation stress is greater than the self-weight stress due to the excavation of the upper soil. The maximum surface settlement is 14.67 mm, and the maximum uplift deformation of soil inside the foundation pit is 5.15 mm. Those deformations are both within the safety control range, which has a high degree of redundancy with the control limit value. The numerical simulation results show that the control effect of the PARB support system is reliable.  
Figure 8a,b shows the curve of surface settlement at the top of slope and the curve of maximum surface settlement with excavation steps. It shows that the following information:  
(1) When excavating to the bottom of the foundation pit, the field monitoring data show that the maximum surface settlement is about 15.2 mm, while the numerical simulation result is about 14.7 mm, with an error of about 4%. The position of the maximum surface settlement is about 12 m away from the pile top, while the numerical simulation result is about 16 m away from the pile top. The heterogeneous distribution of the strata may be the main reason for the difference in the maximum settlement position. Both the field monitoring data and the numerical simulation results show that the surface settlement curve presents in the shape of a \"V\". The distribution laws of both results are the same, and the calculation error is small, which verifies the correctness of this numerical simulation model.  
(2) When excavating the soil to 3 m of depth (Step 1), the surface settlement increases when the distance from the monitoring point to the retaining structure decreases. When excavating to deep soil (Step 2 and Step 3), the surface settlement curve presents in the shape of a \"V\". The surface settlement value first increases and then decreases. The distance from the maximum settlement point of the surface to the retaining structure is about one-half of the excavation depth. From the settlement distribution, the main influence range of the settlement trough is concentrated within the range of the excavation depth, and the surface settlement is not obvious beyond this range. Therefore, it is suggested to monitor the development of surface settlement within the range of the excavation depth.  
(3) Using the PAB support system, the maximum surface settlement reaches 22 mm when excavating to the bottom of the foundation pit, exceeding the limit value specified  
Figure 8: Curves of surface settlement. (**a**) Curve of surface settlement. (**b**) Change of maximum settlement.  
Figure 7: Cloud atlas of surface settlement.  
for construction. Therefore, it cannot meet the construction requirements, while the PARB support system is significantly improved and decreased the maximum surface settlement by 33%. The integrity of the anchor cable, pile and waist beam becomes stronger by the ribbed beam, and the resistance to deformation caused by unloading is more obvious. Ribbed beams also strengthen the bending stiffness of the pile and increase its ability to resist deformation.  
(4) Surface settlement increases slowly with an increase in excavation depth. Surface settlement caused by excavation of upper soil mass increases faster than that of lower soil mass, because of the redistribution of stress by upper soil excavation. The surface settlement law caused by the support of PARB and PAB is consistent.  
(5) Surface settlement increases with the increase in excavation depth. The deformation of the retaining structure and the loss of strata during excavation are the main reasons for surface settlement. Those two parameters are positively correlated with the excavation depth.