Combined support system and calculation model for deep foundation pits in fill soil areas

In central and southern China, deep and loose unconsolidated soil means that current conventional support methods cannot simultaneously address the needs to minimize environmental disturbance and accommodate site limitations when constructing a deep foundation pit. In this study, a combined support system consisting of prestressed composite pile-anchors and miniature steel pipe piles was accordingly evaluated. Based on the elastic reaction method considering nonlinear interaction, a calculation model was developed to obtain the horizontal displacement of the supporting structure considering both the non-limit earth pressure theory and interaction between the piles and soil. The relationship between the lateral displacement of the supporting structure and ground settlement was established by referring to the Japanese standard. Finally, finite element analysis models were established to compare the proposed combined support method with conventional support methods. The effects of the different support methods were also compared with measured data collected from a case study and the deformation rule was summarized. The results show that the theoretical calculation agrees well with the numerical simulation results, verifying the developed calculation model and analysis method. The combined support scheme is shown to be more effective at controlling the deformation in and around the foundation pit excavation, and the forces on the supporting structure are more reasonable. This study thus provides a theoretical basis and reference for the design and analysis of similar deep foundation pit engineering projects.