Study on Pile-Soil Interaction Mechanism and Failure Modes of CFG Rigid Pile Composite Foundation in the Fluid-Plastic Soft Soil of High-Speed Railway

In order to solve existing engineering problems, such as variable and dynamic slope stability issues of railway embankments, limited soil bearing capacity, and post-construction settlement of foundations in fluid-plastic soft soil, etc., the Cement-Fly Ash-Gravel (CFG) rigid pile composite foundation has been used as the main foundation design method. However, due to the lower shear strength and the thixotropy of fluid-plastic soft soil, and the lack of research on pile-soil interaction mechanisms especially for the pile-soil modulus ratio, these will influence the effectiveness of CFG rigid pile foundation design. Therefore, it is crucial and essential to study the anti-shear contribution and the failure mode of CFG rigid piles in the foundation treatment of fluid-plastic soft soil. This paper starts with the finite element analysis of the structural mechanism of single rigid piles in the fluid-plastic soft soil under different working conditions. The results show that the stiffness of the pile body itself, the pile-soil modulus ratio, the shear strength of soil, and the angle between pile and the sliding arc tangent of soil are the main influencing factors of the apparent shear strength (defined as the maximum anti-shear force the pile can provide). Furthermore, with an increase of the angle between the pile and the shear plane, the failure modes of piles will gradually transition from the tensile bending failure to compression bending failure. When the pile-soil modulus ratio is greater than 4000:2, the CFG pile and other rigid piles are not suitable for fluid-plastic soft soil foundation. Considering both safety and economy factors, drainage consolidation treatment should be used to improve the shear strength of fluid-plastic soft soil foundation, then follow with the CFG rigid pile composite foundation treatment. Based on research achievements and some investigations from new-built railway construction projects, these combined treatment methods can significantly improve the effectiveness of foundation treatment and increase the slope stability of railway embankments.