Field performance of inclined H-beam strength composite piles as internal bracing in a deep soft soil excavation

This study investigates a novel combined support system for deep soft-soil excavations, addressing the critical need for deformation control in urban environments where ground anchors are often prohibited. The proposed system integrates 45° inclined H-beam strength composite piles (SCPs) with triaxial deep-mixing columns (TDCs). A comprehensive full-scale field investigation was conducted on a 6.15–7.10 m deep excavation situated in Nanjing mucky silty clay, specifically chosen due to its proximity to sensitive metro tunnels and dense underground utilities. Field load tests performed on three inclined SCPs demonstrated ultimate vertical capacities ranging from 1,350 kN to 2,090 kN, with corresponding total settlements limited to 14.07 mm to 15.17 mm. Regression analysis, employing

a power-function model (s = aQ^b), extrapolated ultimate capacities to 2,299 kN to 3,682 kN, indicating that existing technical specifications (JGJ/T 327) offer reliable yet conservative estimates, typically within 62%–84% of observed capacities. Continuous field monitoring revealed that the maximum lateral wall deflection was restricted to 6.36 mm, representing less than 0.1% of the excavation depth (H), which significantly surpasses the performance of conventional cantilever systems. These findings validate a structural system transformation from a quasi-cantilever to a quasi-simply supported configuration, facilitated by the inclined bracing. This research provides a practical benchmark and quantified performance data for advanced excavation support.

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