Reduction of seismic loading on structures induced by piles in inhomogeneous soil

The kinematic response of flexible piles in inhomogeneous soil is explored under harmonic and seismic excitation. The system under investigation consists of a long fixed-head pile embedded in viscoelastic soil with stiffness varying continuously with depth. A generalized power-law function is employed to describe the variable soil stiffness. The problem is treated numerically by means of a Beam-on-Dynamic-Winkler-Foundation (BDWF) model based on a layer transfer-matrix (Haskel-Thompson) formulation. This study aims at: (a) investigating numerical and modeling aspects related to Winkler analyses of soil-pile kinematic interaction in a non-homogeneous soil; (b) exploring soil-pile kinematic interaction as affected by varying subsoil conditions under harmonic oscillations; (c) providing a novel closed-form expression for the kinematic interaction factor of pile-head over free-field response as a function of a single dimensionless frequency controlling the physical phenomenon and (d) elucidating the beneficial role of piles in the reduction of seismic loading imposed on pile-supported structures in terms of spectral acceleration for different pile-soil configurations and earthquake motions. Results show that: (a) large-diameter piles in soils with very low stiffness at shallow depths may substantially reduce the seismic acceleration imposed on structures and (b) the role of pile diameter becomes less important for strongly inhomogeneous soils. The latter indicates that even small-diameter piles may induce substantial filtering of ground motion if embedded in soft clays, which may be of importance in pile design practice.