Simplified analytical models are developed for the lateral harmonic respons
e of single piles and pile groups in layered soil. File-to-soil interplay i
s represented by a dynamic Winkler formulation based on frequency-dependent
springs and dashpots. For pile-to-pile interaction, the wave field origina
ting from each oscillating ("source") pile and the diffraction of this fiel
d by the adjacent ("receiver") piles are considered. The response of single
piles and pile pairs is evaluated both numerically (through a transfer-mat
rix formulation) and analytically (introducing an efficient virtual-work ap
proximation). Closed-form solutions are obtained: (1) for the impedance of
single piles; (2) for the dynamic interaction factors between two piles; an
d (3) for the "additional" internal forces ("distress") developing in group
ed piles because of pile-to-pile interaction, a phenomenon frequently ignor
ed in current methods of analysis. Both swaying and rocking vibrational mod
es are considered. The effect of pile length and soil layering on the imped
ances and interaction factors is studied. The predictions of the model are
in agreement with earlier results, while its simplicity offers a versatile
alternative to rigorous solutions.