Influence of Pile Geometry on the Dynamic Response of an Under-Reamed Pile-Paper

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The study investigated how under-reamed pile geometry affects dynamic response. Three 5 m-deep, 0.2 m-diameter piles were installed in silty sand: a straight pile, a pile with one bulb, and a pile with two bulbs. Researchers applied sinusoidal vertical and horizontal forces at different excitation levels and measured pile-cap accelerations to estimate resonant frequencies, vibration amplitudes, soil–pile stiffness, and damping.<br /><br />For both vibration directions, resonant frequency generally decreased as excitation force and induced strain increased. The authors concluded that, at comparable force levels, the number of bulbs had little influence on resonant frequency or maximum vibration amplitude, suggesting that the full pile length may not be significantly mobilized during these tests. However, the reported horizontal-test measurements show some differences among piles.<br /><br />Vertical stiffness was higher before resonance than after resonance and decreased as strain increased. In horizontal vibration, rotational stiffness also decreased with increasing strain. Damping increased with excitation level, particularly in horizontal tests, and increased marginally in vertical tests.<br /><br />The findings indicate that soil–pile response is nonlinear and depends on vibration level. The authors note that stiffness values before and after resonance may be relevant to the design of machines operating at different frequencies.

Keywords

under-reamed piles

dynamic response

resonant frequency

vibration amplitude

soil-pile stiffness

damping

vertical vibration

horizontal vibration

silty sand

nonlinear soil behavior