Soil-Pile-Bridge Seismic Interaction: Kinematic and Inertial Effects. Part I: Soft Soil

Abstract

A substructuring method has been implemented for the seismic analysis of bridge piers founded on vertical piles and pile groups in multi-layered soil. The method reproduces semi-analytically both the kinematic and inertial soil-structure interaction, in a simple realistic way. Vertical S-wave propagation and the pile-to-pile interplay are treated with sufficient rigor, within the realm of equivalent-linear soil behaviour, while a variety of support conditions of the bridge deck on the pier can be studied with the method. Analyses are performed in both frequency and time domains, with the excitation specified at the surface of the outcropping ('elastic') rock. A parameter study explores the role of soil-structure interaction by elucidating, for typical bridge piers founded on soft soil, the key phenomena and parameters associated with the interplay between seismic excitation, soil profile, pile-foundation, and superstructure. Results illustrate the potential errors from ignoring: (i) the radiation damping generated from the oscillating piles, and (ii) the rotational component of motion at the head of the single pile or the pile-group cap. Results are obtained for accelerations of bridge deck and foundation points, as well as for bending moments along the piles.

Seismic response of bridge pier on rigid caisson foundation in soil stratum

“…The method of analysis (Mylonakis et al, 1997) is schematically illustrated in Fig 5. As previously mentioned, the mass of the superstructure is concentrated at the top of the pier, and the pier is modeled as a beam. The soilcaisson interaction is reproduced through the dynamic impedances computed in the previous subsection.…”

Experimental p-y loops for estimating seismic soil-pile interaction

“…Notwithstanding the sharp soil stiffness contrast at the level of the clay-sand interface (z = 6 m), the peak bending moment occurs close to the pilehead. This should be attributed to inertial loading transmitted directly to the single pile foundation as a shear force and bending moment during superstructure oscillations (Mylonakis et al 1997;Wilson 1998). Note also that the depth where the maximum bending moment occurs increases with increasing peak base acceleration.…”

Rotational excitation of bridges supported on pile groups in soft or liquefiable soil deposits

“…In principle, kinematic interaction accounts for the scattering of the incoming waves by the foundation as the result of its inability to conform to the displacement profile imposed by the vibration of the soil. In such a case, the displacement of the pile head (or the cap of the pile group) U 11 ð0Þ is related to that of the free-field surface U ff ð0Þ through the kinematic interaction factor C, which is a measure of the effect of the foundation presence on dynamic soil response [15]:…”

“…) is close to zero [14], hence naturally, the potential effect of rotational excitation is negligible [15]. Recent has revealed cases [14] where the rotation of the pile group, might have a considerable effect on the seismic demand of the superstructure, particularly in cases of inhomogeneous soil formations.…”