Abstract: This study analyzes the dynamic impedances of pile group foundations in unsaturated ground using a numerical approach that couples the boundary element method (BEM) and finite element method (FEM). The governing equations for the pile element are derived using beam equations, establishing a numerical model of the pile group using FEM. The governing equations for the unsaturated ground in the BEM are derived using the weighted residual method and Laplacian transform, based on the governing equations of the unsaturated poroviscoelastic medium. The fundamental Green solutions for the unsaturated medium are obtained using the properties of the adjoint matrix. The coupled numerical model, comprising the BEM model for the unsaturated ground and the FEM model for the pile group foundation, is obtained from force balance conditions and displacement compatibility at the interfaces between the piles and soil. The study investigates the effects of soil saturation degree, pile length, pile spacing, pile number, and ground properties on the dynamic impedances of group piles. The research findings indicate that an increase in saturation degree reduces the stiffness of the pile group foundation and the damping coefficient at low frequencies. Additionally, both an increase in saturation degree and an increase in pile number enhance the interaction between the pile and soil. The presence of an underlying bedrock in layered ground increases the pile-soil interaction and significantly influences the horizontal impedance of pile group.

Key words: unsaturated medium, group piles foundation, dynamic impedance, coupled FE+BE method, layered ground