# Displacement Analyses of Main Structure of Parallel Pit Excavation and Analysis of Countermeasures
## 1 Introduction  
Urban transportation hubs are integrated with comprehensive development projects combining transportation, commercial, office, and entertainment development, which can intensively utilize underground space resources and facilitate passengers' access to stations and interchanges. The various underground facilities in the transportation hubs are characterized by a three-dimensional arrangement and centralized arrangement, resulting in large-scale hub projects, a deep excavation depth, a complex surrounding environment, numerous individual units, and different duration requirements for each unit. Due to the difficulty of co-ordinating the construction schedule of the pit group, the designers and contractors were not only faced with the pressure of not interfering with ground traffic but also with the difficulties of the tight construction schedule and mutual interference between excavations of parallel pits. Therefore, it is very necessary to choose a reasonable construction method and support structure [1, 2, 3, 4, 5, 6]. Due to the simple construction, short construction period, and low cost, most of the excavation of foundation pits with less strict deformation control use the bottom-up (BU) excavation method. During excavation, it is necessary to temporarily close the municipal road crossing the pit, which affects traffic passage; given this, in order to reduce the impact of pit excavation on traffic, the pit below the urban road is usually excavated by the top-down (TD) excavation method. Compared with the BU excavation method [7, 8], the TD method has the following advantages: (1) the excavation of the pit occupies the city road fora short period, and the road traffic can be resumed after the top slab is completed; (2) the main structural system is stiff, and the slab, steel pipe structs, and pile together form a stiffer structural system, which can reduce the impact of additional stress on the structure caused by the different excavation progress between the pit groups; and (3) there are fewer temporary supports to avoid waste. In addition, the TD method also faces the following disadvantages: (1) the limited construction space under the roof slab and slow excavation progress of the soil; (2) strict requirements for the accuracy of the steel pipe structs--the deviation of the verticality of the steel pipe structs during construction cannot exceed L/500 (L is the length of the steel pipe column); and (3) the additional deformation of the existing steel pipe structs caused by the construction of the adjacent foundation pit, which affects the performance of the main structure. It can be seen that the structural stiffness is greater when excavating by the TD method, while the construction is simple and the excavation efficiency is higher when using the BU method. At present, most of the excavations only use the BU method or TD method to study the deformation characteristics of the support structure caused by the excavation of the foundation pit. [PERSON] et al. [9] proposed eight types of foundation support and reinforcement schemes in order to reduce the impact of shield tunneling on the supporting system in the foundation pits with the BU method, and studied the deformation characteristics of the supporting system during the shield underpass process. [PERSON] [7; 10] proposed an innovative deep foundation excavation method using a semi-cover excavation method for problems such as a complex environment around the foundation pit. [PERSON] et al. [11] used numerical simulation and field monitoring to study the effect of excavation on the supporting system in four pits with the BU method. [PERSON] et al. [12] studied the deformation characteristics of a multi-stage supporting system during the excavation of multi-stage foundation pits with the BU method. In contrast, the aforementioned studied cases only focus on the influence of different excavation schemes on the supporting system under one construction method.