Abstract:

The incremental method based on the beam finite element method is commonly used for calculating foundation pit enclosure structures. However, there is a lack of research on the dynamic decrease of soil spring stiffness in the passive zone during excavation, which leads to soil resistance softening. This paper introduces two stiffness variation coefficients to quantify the dynamic change of soil spring parameters in the passive zone and establishes an incremental method for calculating foundation pit enclosure structures based on this process. For nonlinear foundation models, a refined incremental method is proposed for numerical calculations. Three basic soil spring models (linear elastic, ideal elastic-perfectly plastic, and hyperbolic) are used to obtain the softening p-y curves in the passive zone. A comparison of these models shows that the variation curve of the equivalent horizontal resistance coefficient obtained using the hyperbolic soil spring model aligns better with actual observations. An engineering example demonstrates that the calculation results based on the hyperbolic soil spring model correspond better to measured wall displacements at different excavation stages. Parameter analysis highlights the significant influence of the stiffness variation coefficient related to the ultimate resistance p_u of the soil in the passive zone. Therefore, when using the proposed method, careful attention should be given to calculating the ultimate resistance p_u and the influence of excavation on it in the passive area should be considered.

Key words: foundation pit enclosure structure, incremental method, foundation pit passive zone, nonlinear soil spring, softening p-y curve