# Displacement Analyses of Main Structure of Parallel Pit Excavation and Analysis of Countermeasures
## 2 Project Overview
### Monitoring Results and Discussion  
In order to obtain the excavation-induced deformation data of the steel pipe structs in time, we master the deformation characteristics of the steel pipe column. Twenty surface (DB) vertical deformation monitoring points were firstly selected in the road surface of Area B, as shown in Figure 5, numbered as DB157-DB176. The steel pipe column in Area B is the main vertical bearing structure, and it has strict requirements for horizontal deformation as an axially pressurized structure. Excessive differential deformation results in the additional bending moment or eccentric load on the steel pile structs. According to the relevant specification of the \"Technical specification for the retaining and protecting of building foundation excavation\" (DB11/489-2016), the safety level of the pit side wall in this area is  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Name**} & \multirow{2}{*}{**Structure**} & \multicolumn{1}{c}{\begin{tabular}{c} **Young’s Modulus** \\ **(GFa)** \\ \end{tabular} } & \multicolumn{1}{c}{\begin{tabular}{c} **Diameter** \\ **(m)** \\ \end{tabular} } & \multicolumn{1}{c}{\begin{tabular}{c} **Length** \\ **(m)** \\ \end{tabular} } & \multicolumn{1}{c}{\begin{tabular}{c} **Design Tension** \\ **(kN)** \\ \end{tabular} } & \multicolumn{1}{c}{\begin{tabular}{c} **Length of the Free** \\ **(m)** \\ \end{tabular} } \\ \hline First pre-stressed anchor & \
\begin{tabular}{c} Cable element \\ Second pre-stressed anchor \\ Third pre-stressed anchor \\ Fourth pre-stressed anchor \\ \end{tabular} &
\begin{tabular}{c} C level 1. The structural deformation should meet the following provisions: the maximum horizontal displacement control value of the support structure is 0.1% H; the specified value of the vertical deformation of the steel pipe column structure is 10 mm; the vertical differential settlement is 10 mm; and the maximum horizontal deformation allowed is 8 mm.  
Figure 6 shows the vertical displacement of the pavement monitoring points in Area B during the excavation of soil above 13.5 m BGS in Areas A and C. The surface deformation trends of the south and north sides of Area B are the same, both showing a typical bulging-type profile. The maximum vertical deformation of 16 mm on the south side and 20 mm on the north side is mainly due to the rebound of the excavation, and excavation in Area C is faster than in Area A. In addition, it can also be seen that the east side of the B area's vertical deformation is much larger than the west side, mainly because the steel pipe structs of the west side arrangement is regular, with a spacing of only 9 m, and the steel pipe structs of the east side spacing is larger, about 18 m; the steel pipe column arrangement shape is irregular; the steel pipe structs' spacing is different so that the excavation-induced stress release on the steel pipe structs' impact of the east side is larger than the west side.  
From the above, it can be seen that the unsynchronized excavation of the soil in Area A and Area C makes the steel pipe structs produce differential deformation. In addition, the differential deformation of the steel pipe structs may cause cracking at the nodes of the beam, column, and slab, which affects the service performance of the main structure in the cover excavation area. Therefore, based on the spatial effect of the pit excavation, it is necessary to study the effect of different excavation schemes of the pit in Area A and Area C on the deformation of the steel pipe structs in Area B through a three-dimensional numerical simulation model to select the most optimal excavation scheme.  
Figure 5: Layout of monitoring points in Area B.  
Figure 6: Measured values of the ground surface in Area B: (**a**) north side of Area B; and (**b**) south side of Area B.