# Research on Construction Sequencing and Deformation Control for Foundation Pit Groups
## 1 Introduction  
With accelerating urbanization intensifying land scarcity, underground space development has transitioned from localized structures to metropolitan-scale networks. Contemporary foundation pit projects now routinely encompass multi-level complexes exceeding40 m depth--including large-section metro interchanges, integrated commercial hubs, and underground logistics systems--reflecting an industry-wide shift toward megascale engineering. This paradigm evolution demands innovative solutions for deep excavation challenges [1; 2; 3]. Large shopping malls, underground complexes, and transportation hubs have become increasingly common and complex, particularly in densely populated metropolitan areas. These projects often involve the simultaneous or overlapping construction of multiple adjacent foundation pits, rather than single pits. When affected by surrounding developments and integrated construction methods, combined foundation pit groups, including irregular interconnected and nested pits, emerge [4]. As shown in Figure 1, based on their functional use and spatial relationships, these groups can be categorized into three main types:  
* Adjacent pits: Separated by a soil wall or structure. * Segmented pits: A single large pit divided into smaller sections constructed sequentially. * Interconnected pits: Pits sharing a common boundary or connected structure. Studying the mutual influences and stress-deformation principles during their excavation is a complex issue. This paper takes the Phase II underground space project in the central area of Jiangbei New District, Nanjing, as the case study. The project involves ultra-deep construction pits, defined as excavations exceeding 20 m in depth according to the Chinese Technical Code for Retaining and Protection of Building Foundation Excavations. This depth threshold represents scenarios requiring specialized engineering solutions due to compounded challenges from groundwater pressure, soil-structure interaction, and construction risks. It focuses on the construction sequencing challenges of large-scale, multi-combination foundation pit groups, specifically addressing the following:  
1. Deep-to-shallow: Excavating deeper sections prior to shallower sections. 2. Shallow-to-deep: Excavating shallower sections prior to deeper sections. 3. Synchronous: Simultaneous excavation of adjacent/segmented sections. This study systematically investigates stress redistribution and deformation mechanisms under these sequences and varying pit spacings (B/He ratios). Additionally, it addresses key technical issues, such as the stress and deformation characteristics of retaining structures under sequential or simultaneous excavation of foundation pit groups. In recent years, scholars worldwide have employed various methods to study the deformation characteristics of foundation pit excavation and its impact on surrounding structures [5; 6; 7]. In existing structure impact and deformation control, [PERSON] et al. [8], via the Shenyang Metro Line 1 case, used numerical simulation and on-site monitoring to prove that new long foundation pit construction causes existing metro structure displacement, and cross-sections need focused monitoring of support axial force and soil settlement. [PERSON] et al. [9], through the Nanjing tunnel project numerical simulation, found that new cut-and-cover tunnels can cause heaving of existing underlying tunnels, and phased ex  
Figure 1: Foundation pit groups with different layout configurations. cavation can reduce heave by 30-50%. [PERSON] et al. [10], by combining numerical simulation with field investigation, showed that when tunnels undercross existing stations, existing pile foundations with tube curtain support can effectively restrain station settlement, but multiple control measures like surface grouting are needed. [PERSON] et al.