Foundation motion filtered by piles: Effect of soil Inhomogeneity

The effect of soil inhomogeneity on kinematic response of single flexural elastic piles to vertically-propagating seismic SH waves is explored. The system under consideration consists of a fixed-head long pile embedded in a viscoelastic soil layer underlain by a rigid bedrock; soil stiffness is assumed to increase linearly or constant. Both harmonic and real earthquake motions are employed to investigate soil-pile kinematic interaction in frequency and time domain. Pile response in inhomogcncous soil is analysed in terms of kinematic interaction coefficients relating pile-head to free-field soil lateral motion and compared to its homogeneous counterpart. The problem is tackled numerically by means of both rigorous elastodynamic Finite-Element analyses and Beam-on-Dynamic-Winkler-Foundation (BDWF) formulations. The role of model parameters such as pile diameter, rate at which soil stiffness increases with depth and average shear wave velocity VSi30 referring to soil type C or D according to ECS is elucidated. Results indicate that: (a) the horizontal displacement of fixed-head piles under harmonic excitation is essentially governed by a single dimensionless frequency parameter based on an average Winkler wavenumber incorporating pile-to-soil stiffness ratio, pile slenderness and soil inhomogeneity and (b) piles-induced filtering effect tends to increase by increasing the degree of soil inhomogeneity and pile diameter, revealing a substantially reduced seismic demand on the superstructure compared to that pertaining to die free-field motion. The above filtering action although neglected in seismic codes may of importance in pile design practice.