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.