Performance of an Irregular Top-Down Excavation with Synchronous Construction of Superstructure and Substructure: Case Study and Numerical Exploration

This study investigates a 24-m-deep irregular excavation in Shenzhen, China, which is surrounded by dense buildings and constructed by the top-down method with synchronous construction of the superstructure and substructure. Based on field instrumentation data, this paper uses finite-element software PLAXIS to simulate the entire construction process and verifies the precision of the numerical model. The numerical analysis reveals the three-dimensional deformation characteristics of the retaining wall, the vertical deformation patterns of the supporting system, and the axial force redistribution in the slabs, thereby providing a theoretical basis for optimization design. The following major findings are obtained: (1) adjacent buildings, especially high-rise buildings, increase retaining wall deflections as a surcharge, and the construction of the superstructure has a negligible effect on retaining wall deformation; (2) the irregular excavation geometry leads to an asymmetrical wall deflection distribution, with larger deflections near external corners because of a smaller boundary constraint. The large internal corner causes a double-peak distribution of wall deflection; (3) the overload generated by superstructure construction leads to a decrease in the rising rate of the column and retaining wall and even causes settlement; (4) the stress concentration phenomenon is observed at the excavation openings, and structural design recommendations such as local strengthening around the edges of openings are provided; (5) the excavation-induced building damage potential is assessed as negligible, confirmed by the absence of cracks in practice.