# Displacement Analyses of Main Structure of Parallel Pit Excavation and Analysis of Countermeasures
## 3 Parallel Pit Excavation Scheme  
The excavation plan of the parallel pit group faced the following problems: (1) the deformation control standard of the steel pipe structs in Area B is strict, while the impact of the excavation of pits in Area A and C on the main structure of Area B is unknown; (2) the steel pipe structs below the top slab has no restraining effect, whether the deformation of the steel pipe structs can be controlled more effectively by applying the middle floor slab; and (3) the construction progress of Area A and C is difficult to co-ordinate, causing additional deformation of the pre-existing steel pipe structs exceeding the specified value. In order to solve the problems, four different excavation schemes were proposed to study the excavation-induced deformations on the steel pipe structs, as shown in Figures 7-10. Combined with the location of the anchor cables in the D-walls in Area A, excavation was carried out in four layers with excavation elevations of 13.0 m, 10.5 m, 8.0 m, and 5.5 m, as shown in Figure 3.  
Figure 8: Case 2: Roof slab finished in Area B, sequential excavation in Area A and C.  
Figure 7: Case 1: Roof slab finished in Area B, simultaneous excavation in Area A and C.  
Case 1: The top slab of Area B was applied, and Area A and C were excavated simultaneously, as shown in Figure 7; Area A and C are excavated simultaneously to 13.5 mBGS, 10.5 m BGS, 8 m BGS, and 5.5 m BGS. Case 2: The top slab of Area B was applied, and Area A and C were excavated sequentially, as in Figure 8; Area A was excavated to the bottom of the pit first, and then Area C was excavated to the bottom of the pit. Case 3: The top slab of Area B was applied, and Area A and C were excavated in staggered steps, as shown in Figure 9. Area A was excavated to 10.5 m BGS first, then Area A and C were excavated in staggered steps to 8.0 m BGS and 10.5 m BGS, respectively, and excavated in staggered steps until completion. Case 4: The middle 1 slab of Area B was applied, and the excavation of Area A and C was staggered, as shown in Figure 10; Area A and C were excavated to 10.5 m BGS first, and the middle 1 slab in Zone B cast; then, Area A and C were excavated to 8.0 m BGS and 10.5 m BGS, respectively, and the excavation was staggered until completion. the beam; and the D-walls are simulated with the plate. The concrete structure is simulated with elastic, and the soil is simulated with the Plastic-Hardening (PH) constitutive model, as shown in Figure 11.  
To consider the shear-hardening and compression-hardening properties of the soil and to reflect the properties of the soil in the range of small strains [20; 21] where the shear modulus decreases with increasing shear strain, the Plastic-Hardening (PH) model is a shear- and volumetric-hardening constitutive model for the simulation of soil behavior. [PERSON] [22] proposed the basis of [PERSON]'s [23] framework of hardening plasticity. The main features of the PH model are (1) plastic strain in mobilizing friction; (2) plastic strain in main compression; and (3) elastic unloading/reloading compared to virgin loading. The model can be immediately calibrated through lab testing or field tests. It is effective in numerous applications, including excavation, settlement studies, and issues involving soil and structure interaction. Due to the soil stiffness modulus being strain-dependent, which decreases nonlinearly with the increase in shear strain, the excavation process of the foundation pit shows the opposite deformation trend of pit bottom uplift and surface settlement, using the plastic-hardening model with small-strain stiffness to simulate the unloading-reloading paths [24]. The other two parameters need to be defined: stiffness-0-reference (\(E_{0}^{ref}\)) and strain-70 (\(\gamma_{70}\)).