Kinematic pile bending during earthquakes: analysis and field measurements

The passage of seismic waves through the soil surrounding a pile imposes lateral displacements and curvatures on the pile, thereby generating 'kinematic' bending moments even in the absence of a superstructure. These moments are concentrated in the vicinity of interfaces of alternating soft and stiff soil layers and, in the case of restrained-head piles, at the pile head. The scope of this paper is threefold: (a) to critically review some existing design methods for kinematic pile loading; (b) to develop new analytical results for piles in homogeneous and layered soils; (c) to present a case study in which theoretical predictions are tested against field measurements. To this end, an approximate beam-on-dynamic-Winkler-foundation (BDWF) model is implemented, specifically developed for the seismic response of piles in layered soil, Both fixed- and free-head piles, and different boundary conditions at the pile toe, are considered. It is shown that the magnitude of kinematic moments depends mainly on the stiffness contrast between the soil layers, the pile-soil stiffness contrast, the excitation frequency, and the number of excitation cycles. A unique case history involving the instrumented pile foundation of a multistorey building in Japan is presented, Time histories of bending and axial strains recorded at six locations along two piles are successfully compared with results computed from simple formulae and methods presented in the paper.