Vertical vibration characteristics of an offshore end-bearing pile embedded in saturated soils

Abstract

Pile foundations have increasingly been used extensively for the foundations of offshore engineering structures such as cross-sea bridges, offshore wind turbines and gas infrastructure. Meanwhile, pile integrity testing (PIT) is an important method for evaluating the safety of the pile foundation. In this study, the vertical vibration characteristics of an offshore end-bearing pile embedded in saturated soils is studied by a semi-analytical method. The upper water is assumed to be an ideal compressible fluid and the lower soil is assumed as saturated soil and satisfies Biot's theory of poroelasticity. The dynamic governing equations of the ideal compressible fluid and the saturated soil governing equations are decoupled by introducing potential functions and Helmholtz decomposition using the Laplace transform. Analytical solution for the offshore end-bearing pile vertical vibration is obtained in the frequency domain. The semi-analytical solution for an offshore pile excited by a half-sine wave in the time domain is then obtained by means of inverse Fourier transform. By downscaling the pile-water-soil model to a pile-soil model, the accuracy of the proposed solution was verified. A parametric study is conducted to investigate the influences of water depth, pile radius, soil shear modulus, soil porosity, and soil permeability on the vertical dynamic responses of offshore pile, including complex impedance, the admittance of velocity, and time domain velocity response. The results indicates that the water depth, pile size and soil properties have considerable influence on the response of offshore pile vertical vibration.