Numerical simulation of horizontal displacement at the top of support piles for ultra-deep foundation pits in silty formations

The difficulty in predicting the horizontal displacement of the support pile top in ultra-deep foundation pits within muddy formations, combined with insufficient consideration of parameter discretization characteristics in existing methods, motivates this study. Taking the Songtao Street Station project of Suzhou Metro Line 8 as a case study, this paper classifies the soil layers and implements corresponding support technologies. First, based on the engineering geological and hydrological conditions, the discrete values of earth pressure, the discrete combination of support structure thickness, and the discrete gradient of the lateral earth pressure coefficient are selected as core discrete variables. Second, a numerical model is constructed using FLAC3D software, and the soil-structure interaction is simplified via the elastic foundation beam method. The deflection differential equation of the support structure is derived to verify the accuracy of the elastic modulus conversion formula and the earth pressure calculation method. Subsequently, the influence of key parameters, such as the lateral earth pressure coefficient and the cohesion of silty clay, on displacement is analyzed. Finally, the reliability of the model is verified using on-site monitoring data from 12 monitoring points throughout the entire construction period. The results indicate that the displacement error between numerical simulation and actual measurement is ≤ 3.3%. When the lateral earth pressure coefficient is 1.0, the horizontal displacement of the pile top is minimized. The safety factor of uniformly thick shotcrete support is 1.8–2.9 times higher than that of non-uniform schemes. Significant creep characteristics are observed during the excavation and sealing of ultra-deep foundation pits in muddy formations. This study provides a quantitative basis for the discrete selection of support parameters for ultra-deep foundation pits in similar silty formations and improves the accuracy of displacement prediction.