Xinwang Zhang

Ultra-deep excavations in soft soil pose major challenges for deformation control. Based on field monitoring of a 38.3 m deep five-story metro excavation in Tianjin, this study systematically investigates the spatiotemporal deformation of the diaphragm wall, columns, and surrounding environment. Key innovations include the proposal of an extended ground settlement influence model and the quantification of stage-wise deformation development ratios. The maximum lateral wall displacement is about 40 mm, ranging from 0.028% H_e to 0.184% H_e (average 0.087% H_e), outperforming comparable bottom-up excavations in Shanghai. Wall top vertical displacement varies from −0.23% H_emax to 0.04% H_emax, and column rebound averages 3.5 mm, is significantly lower than that of excavations using the bottom-up method. The extended settlement model shows that the maximum settlement occurs at H_e/3 from the wall, the primary influence zone extends to 3H_e, and the secondary zone reaches 5H_e. Building settlement strongly depends on distance and foundation type, with raft foundations settling much more than pile-raft foundations. Stage-by-stage analysis reveals that, immediately after the completion of diaphragm wall construction, the settlement already exceeded 60% of the final maximum ground settlement. Furthermore, the deformation on the long side developed at a faster rate than that on the short side. These findings provide quantitative benchmarks for designing ultra-deep excavations in soft soil.

With the increasing development in urban underground spaces towards greater depth, scale, and complexity, the prediction and control of deformations in deep excavation engineering have become critical challenges in geotechnical engineering. This study investigates the ultra-deep excavation of the Tianjin Metro Line 8 Liulitai Station, analyzing the deformation characteristics of the retaining structure during top-down construction in soft soil based on field monitoring data. The results reveal a typical “bulging” pattern in the horizontal displacement of the diaphragm wall, which accumulates progressively with excavation depth. To enhance deformation prediction accuracy, a self-developed beam-plate finite element method (BPFEM) platform, implemented in Python (version 3.11.9), is introduced. The platform integrates code-specified analytical methods and the incremental approach to simulate the internal forces and deformations of the support system with high precision. By incorporating a dual-parameter back-analysis technique—adjusting both the horizontal subgrade reaction modulus and active earth pressure—the numerical model achieves significantly improved agreement with monitoring data. The proposed method demonstrates strong predictive capability, with a maximum error of only 4.4% in subsequent construction stages, confirming its feasibility and reliability for deformation forecasting in top-down deep excavations. The BPFEM framework and parameter inversion strategy presented herein provide an effective technical basis for intelligent prediction and dynamic control in deep excavation projects under complex geological conditions.