# Numerical Study of Dig Sequence Effects during Large-Scale Excavation
## 1 Introduction  
Metro construction can be an efficient way to relieve the transportation stresses that are produced by fast-expanding cities and growing populations. To address the ever-increasing traffic pressure, the construction of metro stations needs to address challenges in different features related to construction depth (large excavation depth), construction scale (large scale), proximity (close to important buildings), and difficult conditions (complex geological environment and working conditions) [1; 2; 3]. These factors further lead to long-duration construction and high construction risk during the foundation pit excavation process. If the foundation pit collapses, it will cause large economic losses and casualties. At present, foundation pit excavation methods can be mainly divided into two categories: unprotected [4] and protected excavation [5]. Due to the increasing difficulty of metro foundation pit construction, support is necessary to ensure the safety of the project. Therefore, more and more metro support excavation methods and theories have been proposed, namely, the open cut method [6] and the undercutting method [7]; the common support structures include composite soil nailing walls [8], pile row supports [9], diaphragm walls (DWs) [10], artificial freezing [11], prestressed fish-belly beam supports [12], trench-belly beam supports [13], soil mixing walls [14], and center island excavation [15]. DWs have received extensive research interest and have been widely applied because they may significantly lower the likelihood of safety mishaps during deep foundation pit projects, making it the most significant and crucial support structure in foundation pit construction. [PERSON] et al. [16] designed a test model incorporating a scalable internal support system and investigated the coordination of deformation characteristics between the internal support system, the ground-connected wall, and the soil behind the wall. [PERSON] et al. [17] conducted a measurement analysis of a long and narrow deep foundation pit in a deepsoft ground layer in Guangzhou, China, using a ground-connected wall with internal support as the enclosure system; they elucidated the traits of the enclosure system of long and narrow deep foundation pits in soft ground. [PERSON] et al. [18] numerically simulated a symmetrical foundation pit in Suzhou, China, using finite element software to investigate the deformation mechanism of the enclosing structure and the surrounding soil during the excavation of a soft soil foundation pit. They performed sensitivity analysis and finally verified the reliability of the model using field-measured data. [PERSON] et al. [19] proposed a method of using isolation piles and diaphragm walls for joint support to protect buildings near a foundation pit; they used numerical simulation software to analyze the deformation law of a building adjacent to the foundation pit under the combined support throughout the excavation process, as well as the deformation characteristics of the isolation pile diaphragm wall. [PERSON] et al. [20] proposed a Bayesian-network-based deflection risk analysis model for underground diaphragm walls, which more accurately accounts for the dynamic characteristics of hydrogeological design updates and the construction variables during construction progress based on subway construction site data. The proposed method was compared with traditional methods such as fault tree analysis. [PERSON] et al. [21] combined many excavation examples in the soft soil area in Suzhou, China, and investigated the excavation performance indices of diaphragm walls such as uplift and lateral deformation, the vertical deformation of the ground surface, the vertical deformation of the surrounding buildings, and earth pressure. [PERSON] et al.