Propagation characteristics of flexural waves in piles and the transient response at the pile-top under lateral excitation

As a complement to the longitudinal wave-based method, the flexural wave-based method has been used to evaluate the pile integrity. This paper reveals dispersion and attenuation characteristics of F-(flexural) waves in piles and emphasizes the importance of capturing the axial velocity response for evaluating the pile integrity. The characteristics of F-wave propagation in piles and the transient dynamic response at the pile-top in the time domain are investigated to provide guidance for pile integrity testing. The pile is simplified as an elastic TM (Timoshenko) beam and the surrounding soil is simplified as a Winkler foundation. The dispersion and attenuation relations of the first propagating F-wave in the free and embedded piles are analyzed. The transient axial and lateral velocity responses of intact and defective (necking, bulging and mud clamping) piles in the time domain are obtained using the FDM (finite difference method) when the pile is subjected to a transient lateral excitation afterwards. The effects of pile and soil properties on the transient F-wave propagation and the dynamic response of the pile-top are investigated through a comprehensive parametric study. The finite difference solution based on the pile-soil model established in this paper is validated through comparisons with the 3D finite difference solution and experimental results. It is found that the stiffness of the surrounding soil has a great influence on the dispersion and attenuation characteristics of low-frequency F-wave. Shorter pulse durations or larger pile diameters can result in stronger reflections at the pile-tip; however, they can also enhance the thickness-shear mode and lead to the appearance of high-frequency interference. Reflections from the pile-tip or defects are more pronounced in axial velocity than in lateral velocity, so it is recommended that axial velocity should be collected in addition to lateral velocity during pile testing using the F-wave-based method. A method for eliminating the high-frequency interference using the difference between axial velocity responses on both sides of the pile-top is proposed, based on 3D simulation results. The TM model cannot adequately characterize the 3D effects of F-wave propagation in piles.