Abstract: A longitudinal dynamic model for largediameter pipe piles is developed by integrating the constitutive behavior of horizontally isotropic materials with the viscoelastic motion equation for soil. To account for stratification effects and groundwater fluctuations, the surrounding soil is idealized as two layers: a horizontally isotropic single-phase medium and a horizontally isotropic saturated medium. The soil-plug response is represented using a Voigt model with added mass, while the lateral inertial effects of large-diameter piles are captured through the Rayleigh-Love rod formulation. Building on Novak’s thin-layer theory, saturated porous-medium theory, and existing analytical solutions for transversely isotropic single-phase soils, a coupled vibration equation is established for the pile-soil system comprising the pile (including soil plug), the single-phase soil layer, and the saturated soil layer. A closed-form theoretical solution for the pile-head dynamic response is subsequently derived. The analytical solution is implemented in MATLAB to perform pile-soil parameter analysis. Parametric studies reveal the governing influence of soil layering, saturation, viscoelastic properties, and pile geometry on the longitudinal vibration characteristics of large-diameter pipe piles. The findings provide a quantitative basis for evaluating dynamic pile behavior and offer guidance for engineering design involving layered and partially saturated ground conditions.

Key words: Large Diameter Pipe Pile, Horizontally Isotropic, Dynamic Response, Longitudinal Vibrations