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

The design of internal bracing is critical for controlling deformations in soft-soil foundation pits, especially in congested urban areas where ground anchors are prohibited. This study investigates a novel combined support system consisting of 45° inclined H-beam strength composite piles (SCPs) and triaxial deep-mixing columns (TDCs). A full-scale field investigation was conducted in a 6.15–7.10 m deep excavation in Nanjing mucky silty clay, adjacent to sensitive metro tunnels and dense underground utilities. Field load tests on three inclined SCPs demonstrated ultimate vertical capacities of 1,350–2,090 kN, with total settlements limited to 14.07–15.17 mm. Regression analysis using a power-function model ( s = aQ b ) predicted extrapolated ultimate capacities of 2,299–3,682 kN, indicating that current technical specifications (JGJ/T 327) provide reliable but conservative estimates (approximately 62%–84%). Continuous field monitoring revealed that the maximum lateral wall deflection was confined to 6.36 mm (less than 0.1% H ), significantly superior to conventional cantilever systems. These results validate a structural system transformation from a quasi-cantilever to a quasi-simply supported configuration enabled by the inclined bracing. This study provides a practical benchmark and quantified performance data for the application of inclined SCPs as a low-carbon, high-efficiency alternative to traditional internal bracing in complex soft-soil environments.