# Displacement Analyses of Main Structure of Parallel Pit Excavation and Analysis of Countermeasures
## 3 Parallel Pit Excavation Scheme
### Discussion  
#### 4.3.1 The Additional Deformation Rate  
In short, the closer the distance from the excavation side, the greater the horizontal deformation of the steel pipe column. The asymmetric stress distribution caused by the unsynchronized excavation makes the steel pipe column structure produce additional deformation. To evaluate the degree of impact of the different excavation schemes on the steel pipe structs, the additional deformation rate \(\eta\) was introduced to quantify the additional deformation generated by the asymmetric excavation on the structure. The maximum horizontal deformation of the D-walls and steel pipe structs during asymmetric excavation is \(\delta_{\mathit{gh}}\), and \(\delta_{\mathit{gh}}\) is the maximum horizontal deformation of the D-walls and steel pipe structs during synchronous excavation. Equation (4) is as follows:  
\[\eta=\frac{\delta_{\mathit{qh}}}{\delta_{\mathit{sh}}} \tag{4}\]  
According to the above Equation (4), the additional deformation rate of the structure for different excavation cases is as shown in Table 3:  
Figure 18: Case 4 staggered excavation, steel pipe column structure horizontal deformation.  
Combining the data in the Table 3, it can be concluded that:  
1. In Case 2, after the excavation of Area A is completed, the value \(\eta\) of the D-walls is 1.5; with the completion of the excavation of Area C in Case 2, the lateral soil pressure decreases, and the value \(\eta\) of the D-walls decreases to 1.1. For Case 3, the excavation of the Areas A and C is staggered, and the staggered distance is only 3 m, and the value \(\eta\) of the D-walls is similar to that of simultaneous excavation, which is 1.02. In Case 4, compared with the above options, after the middle 1 slab cast, the horizontal direction of the steel pipe column is constrained by the top slab and the middle slab, and the value of \(\eta\) of the D-wall is much smaller than that of the synchronous excavation, at only 0.72.
2. The additional deformation rate of the steel pipe structs in Case 2 > Case 3 > Case 4, which is more similar to the deformation trend of the steel pipe structs for the different excavation options. Different from the above deformation is the fact that the additional deformation rate of Struct 2 and Struct 3 is much larger than that of Struct 1 and Struct 4, mainly because the soil unloading caused less disturbance to Struct 2 and Struct 3, which causes the synchronous excavation to have a smaller \(\delta_{sh}\), as the value \(\eta\) of the steel pipe structs gets larger.
3. The value \(\eta\) in Case 3 is more consistent with Case 1, and the smaller excavation spacing in Areas A and C has no obvious effect on the steel pipe structs. The value \(\eta\) in Case 4 is less than 1. The greater the stiffness of the main structure in Area B, the smaller the additional deformation.  
According to the above analysis, the additional deformation rate of the steel pipe column is mainly related to the pit excavation's staggered spacing and the overall stiffness of the main structure. The spacing between Areas A and C is controlled at about 3 m, so the impact of the asymmetric additional stress on the steel pipe column is limited; by applying a middle 1 slab, the overall stiffness of the main structure in the cover excavation area is increased, which effectively reduces the additional deformation rate generated by the asymmetric excavation of the pit.