Applicability of principle of superposition in obtaining horizontal impedance functions of pile groups under soil nonlinearity

The effects of soil nonlinearity on the pile-to-pile horizontal interaction factors and horizontal impedance functions of piles through model testing are investigated. Separate experiments with a solitary pile and closely spaced 3×3-pile groups with spacing to diameter ratio of 2.5 embedded in cohesionless soil, are considered. Lateral harmonic pile-head loadings in the form of accelerations with amplitudes of 0.2, 0.5, 1 and 2 m/s2 for a wide range of frequencies are reported. Results obtained from model soil-pile systems encased in a laminar shear box show that local nonlinearity and resonant behavior of soil have a significant impact on both the interaction factors and the impedance functions. Utilizing the measured values of horizontal impedance functions of a single pile and the horizontal pile-to-pile interaction factors, Poulos' superposition method is assessed in obtaining the horizontal impedance functions of the pile group. Comparisons between computed and experimentally measured values show very good agreement, suggesting that the principle of superposition is applicable under nonlinear conditions.