Screw piles are uniquely-shaped concrete piles with screw threads that have been widely used in various fields, including construction, structural design, and geotechnical engineering. Research on the dynamic characteristics of screw piles under vertical loads is limited compared with that investigating traditional circular piles. This report describes an analytical solution that has been developed to investigate the dynamic features of a screw pile under a longitudinal load while considering the cushion cap effect. The Laplace transform and Potential functions are applied to decouple the three-dimensional wave equations of the soil. The dynamic response of the screw pile is deduced using a modified impedance transfer function method. Finally, the cushion cap displacement and velocity in the frequency domain are determined by combining the initial conditions. The analytical solutions are compared with field-measured curves to validate the developed method. The results indicate that the soil around the pile can be regarded as a threedimensional continuous medium to simulate the radiation-damping effect as the wave propagates outward. The cushion cap reduces the screw pile damage caused by resonance, particularly in the low-frequency range. Considering the effects of vibrational loads, a screw pile should employ a large lightweight cushion cap, i.e., with the largest reasonable dimensions and with concrete materials that are as light as possible. The results of this study provide a theoretical basis for designing a dynamic foundation of a screw pile.
