Dynamic response of pile foundations has been studied for many years with different models ranging from distributed Winkler springs to advanced analytical solutions. Special features in the dynamic impedances of pile foundations, which are mostly related to the group effect or pile-soil-pile interaction, have inspired researchers to develop special computational tools for capturing these features. In parallel, a number of research has been carried out to investigate whether these features are realistic, rather than the result of the idealized models used for the pile and the soil domain. For example, one issue is to what extent the number of piles and the pile configuration affect the large peaks in the impedance functions. Another question is if these impedance characteristics change by the soil inhomogeneity in the vertical and horizontal directions. This paper attempts to answer some of these questions in the range of nearly elastic soil response for a small pile group and a large group consisting of about 300 piles. Two profiles with shear moduli representing typical sand (parabolic with depth), and typical clay (linear with depth) will be considered together with a homogenous soil profile for reference and comparison of the results. The presented results will be the foundation impedances as functions of frequency in the horizontal and vertical directions together with representative pile-soil-pile interaction diagrams for different pile spacing.
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