Determination of dynamic interaction factors for closely spaced pile groups using spectral element methodology considering soil–pile interaction and weak zone effects

This study explores the dynamic response of closely spaced pile groups under horizontal vibrations, emphasizing soil–pile interaction and dynamic interaction factors. A detailed framework is developed to evaluate pile-to-pile interactions, incorporating the propagation of primary and secondary waves. Vibrations from a source pile propagate through the soil, interact with adjacent piles, and generate secondary waves that influence the system’s overall dynamic response. To account for soil property variations caused by pile driving, the soil medium is modeled with an inner field (weak zone) and an outer field. The weak zone, characterized by reduced stiffness, significantly affects soil–pile interaction. Averaging the impedance contributions from both zones enables accurate modeling of the soil between piles, with the spectral element methodology employed to compute group pile impedances. Results show that for a 2 × 2 pile group, increasing the shear modulus ratio ($\frac{G_i}{G_o}$) from 0.3 to 1 leads to a 16% rise in stiffness and a 10% increase in damping, while larger 3 × 3 groups exhibit greater damping at lower frequencies. Increasing boundary zone thickness reduces stiffness by up to 20% and raises damping by 40%. Weak zone damping further lowers effective stiffness by up to 20%. These findings provide critical insights into frequency-dependent effects, offering guidance for optimizing pile-supported foundation designs.