Research on the Acceleration Response Spectra of Piles in Saturated Sand Based on a Modified Finn Model

Existing studies on pile response in saturated sites mostly neglect the real-time degradation of soil modulus with pore pressure accumulation， making it difficult to accurately capture the nonlinear variations in the pile-soil system during earthquakes. To address this， a modified Finn model incorporating the dynamic degradation of modulus was introduced， and a three-dimensional numerical model of pile-soil interaction in saturated sand was developed. Validated against shaking table test results， the model was used to systematically analyze the effects of pile diameter， pile spacing， and pile length on the acceleration response spectra of piles. The results showed that the acceleration response spectrum at the pile tip exhibited a bimodal pattern， whereas that at the pile head displayed a unimodal pattern， with the spectral peak value gradually increasing along the pile height. The response spectrum value of corner piles was approximately 25% higher than that of center piles. Increasing the pile diameter and pile length shifted the main peak of the response spectrum toward higher frequencies and significantly reduced the spectral values in the low-frequency range （2-3 Hz）， with reductions of 40%-72%， thereby effectively suppressing the risk of low-frequency resonance in the foundation. Compared with pile diameter， pile length and pile spacing exerted relatively smaller influences on the acceleration response spectrum.