## 2 Project Profile  
### Engineering Background  
Changsha international financial center is the highest building in Hunan Province of China, with the maximum height of 452 m. The excavation depth of the foundation pit is 31.80 m, and the area of the foundation pit is 76,700 m\({}^{2}\), which is a deep and large foundation pit. There is a 17-floor high-rise building on the east side of the north sidewall and a 24-floor high-rise building on the west side of the north sidewall. The location of those two existing high-rise buildings is shown in Figure 1. The south side wall is about 488 m long, and there is a 34-floor high-rise building on the west side of the south side wall. The safety level of the side wall of the foundation pit is Level I and the structural importance coefficient is 1.1. The shape of the foundation pit is basically rectangular. The lengths of the east, south, west and north side walls of the foundation pit are about 168, 488, 136 and 546 m, respectively, as shown in Figure 1.  
The stratum around the foundation pit is composed of artificial fill, muddy silty clay, silty fine sand, medium-coarse sand and other stratum with poor self-stability and low impermeability. At the same time, a densely populated city is near the foundation pit. The existing buildings, roads and underground pipes are close to the foundation pit. The stratum of the foundation pit has poor stability, and if the retaining structure of foundation pit is unstable, severe disasters may occur. The internal support or reverse construction method is generally adopted for deep foundation pit in city centers. However, due to the irregular shape of the foundation pit, the use of internal support may have great risks and high cost, which cannot meet the construction requirements. Through the comparison of technology, economy, environmental protection and so on, the support system of this foundation pit is the pile and anchor support system. The reverse construction method of central island is adopted in two phases. The specific construction scheme is as follows:  
Phase 1 of project (Figure 2a): The pile-anchor support system is adopted in the area without high-rise buildings around the foundation pit. Piles are constructed for the foundation pit near the three high-rise buildings, and the soil mass is retained by the support system of sloping and soil nailing wall to ensure the safety of the three high-rise buildings. Phase 2 of project (Figure 2b): After Phase 1 of project is completed and the main structure forms a \"central island\", the foundation pit near the three high-rise buildings will be excavated and supported. Based on the piles constructed in Phase 1, the foundation pit support construction and main structure construction are established by the semi-reverse construction method or internal support construction technology. The construction process of the foundation pit is shown in Figure 2.  
The deformation of the retaining structure is the main reflection of the deformation of the surrounding strata, pipelines, roads and buildings [14; 15]. It is particularly important to clarify the relationship between the displacement of the retaining structure and the excavation depth. Therefore, during the excavation of foundation pit, the main monitoring  
Figure 1: Schematic diagram of foundation pit. objects are the lateral and vertical deformation of the slope top, the surface subsidence and the lateral deformation of the pile. (1) Horizontal and vertical displacement monitoring of slope top: The monitoring points can be arranged at the top of the wall with a spacing of 15 m.  
(2) Ground surface subsidence monitoring: The monitoring points can be arranged on the side close to the foundation pit. The monitoring points are suspended on both sides of the road where they will not affect the traffic and are convenient for preservation with a spacing of 25 m. Obtaining more information on road settlement, the road observation points are staggered from the pipeline settlement monitoring points on both sides of the road.