Kinematic seismic response of single piles and pile groups

Results of a numerical study are presented on the kinematic response of groups of vertical floating piles connected through rigid massless caps and subjected to vertically propagating harmonic S-waves. Pile-soil and pile-pile interaction effects are modeled rigorously. Parametric results for the “effective seismic pile-cap motion,” normalized by the “free-field” ground-surface motion, are displayed in dimensionless form for a number of typical pile-group configurations, in three idealized soil profiles: a homogeneous half-space, a half-space with modulus proportional to depth, and a two-layered stratum. It is shown that, whereas the influence of the nature of the soil profile is profound at all frequencies, the effects of pile-group configuration, number of piles in the group, and relative spacing between piles are usually insignificant for lateral displacements, but quite important for pile-cap rotations. Pile-head “fixity” conditions and the pile/soil modulus ratio are found to affect appreciably and in a similar way the seismic response of a single pile and of a pile group.