Investigation of Dynamic Responses of a Helical Pile in Clay Using Vibration Tests

Dynamic behaviors of helical piles are of significant interest because such piles can offer an efficient alternative to conventional piling systems to resist static and dynamic loads. However, dynamic properties of helical piles with larger slenderness ratios have not been fully investigated, such as natural frequencies and damping ratios. To investigate dynamic behaviors of soil-helical pile interaction, full-scale vibration tests were performed on a 4.3 m long and double-helix helical pile using a shaker. Three accelerometers were installed on top of the pile, and strain gauges were placed on pile surface at different depths. The shaker was installed on the pile cap. The pile was subjected to harmonic (quadratic) loading of different force intensities acting within a frequency range from 9.8 to 19.5 Hz that covered the resonant frequencies of the tested soil–pile system. Based on the displacement amplitude of the pile at different excitation frequencies during the vibrating tests, the natural frequency of the pile is 15.3 Hz. Experimental p–y curves were developed to estimate the dynamic soil–pile interaction. The equivalent damping ratios vary from 0 to 1 along the pile length and were obtained using the energy dissipation method. The results indicate that the damping ratios decreased with the number of vibration cycles in the upper portion of the pile and increased with the number of vibration cycles in the lower section of the pile due to the soil–pile gap development extending from the soil surface along the upper part of the helical pile.