Longitudinal dynamic response of strength composite piles embedded in viscoelastic unsaturated soils

The strength composite pile, as a novel pile foundation, holds significant practical significance in dynamic response analysis. This study, based on elastic dynamic theory and porous media model, investigated the longitudinal dynamic response of the strength composite pile in fractional viscoelastic unsaturated ground through theoretical derivation and parametric analysis. The research took into account the unique structure of the strength composite pile and the flow-independent viscosity of the soil skeleton. Firstly, the longitudinal vibration equation of the strength composite pile was established through mechanical equilibrium. The dynamic behavior of the soil around the pile was described using the existing governing equations for unsaturated soils, where the fractional standard linear solid (FSLS) model was utilized to characterize the frequency-dependent viscosity of the soil skeleton. Subsequently, the analytical solution of the dynamic impedance at pile head was deduced through a rigorous theoretical derivation. Finally, the study delved into the influence of pile and soil parameters on the dynamic impedance at the pile head through numerical calculations, parameter analysis, and mechanism discussion. The results reveal that an increase in both the cross-sectional proportion of cement-soil pile and the pile length enhances the dynamic impedance at pile head. An increase in the fractional order and the strain relaxation time, along with a reduction in the stress relaxation time, all improve the dynamic impedance at pile head. Additionally, increasing the soil saturation or decreasing its intrinsic permeability also elevates the dynamic impedance at pile head.