Takashi Tazoh

LINEAR AND NONLINEAR VALLEY AMPLIFICATION EFFECTS ON SEISMIC GROUND MOTION

ABSTRACT Records and analyses have shown that, apart from soil stratigraphy, the geomorphic conditions (such as those characterising an alluvial valley) tend to modify the amplitude, the frequency content, the duration, and the spatial variability of seismic ground shaking. As most of the related records and studies to date refer to weak motions (and thereby to linear soil response), the question that has been raised is whether and by how much the unavoidably nonlinear soil behaviour during strong shaking may reduce the unavoidable “valley amplification” effects. The paper aims at shedding some light on this important issue by analysing numerically the effects of the sub-surface geomorphic conditions of a valley on its ground surface seismic motion, with emphasis on the influence of soil nonlinearity. Two-dimensional linear and equivalent-linear ground response analyses are performed to study an alluvial valley in Japan, the behaviour of which had been monitored during many earthquakes in the early 1980s. Then, using the geometry of this valley as a basis, a parametric investigation is performed on the effects of potential soil nonlinearity arising from the increased intensity of base excitation and/or decreased “plasticity” index* of the clayey soil material. It is shown that strong soil nonlinearity may depress the amplitude of the multiply-reflected and, especially of the horizontally propagating Rayleigh waves, leading to substantially lower valley amplification.

Prediction of the measured response of a scaled soil-pile-superstructure system

Abstract The validity of the Winkler foundation model is investigated by predicting the experimentally measured displacement transfer functions and strain spectra of a single pile embedded in a sandbox and supporting a single-degree-of-freedom superstructure. The foundation-superstructure system is a scale model and was subjected to shake table excitations. The distributed springs and dashpots of the Winkler foundation model are frequency dependent and the calibrated model predicts satisfactorily the displacement transfer function at different depths for both fixed- and free-tip pile conditions. On the other hand, the pile-axial-strains are substantially underestimated when expressed in terms of the second derivative of the computed elastic line of the pile. It is shown that a much more dependable prediction is achieved when pile-axial-strains are expressed in terms of the inertial forces acting along the pile-superstructure system.

Pile-Group Response to Large Soil Displacements and Liquefaction: Centrifuge Experiments versus a Physically Simplified Analysis

The paper presents a physically simplified method for computing displacements and structural forces on piles under conditions of lateral spreading triggered by the large seaward displacement of a harbor quay wall. The method avoids the empirical selection of stiffness- reduction factors and the associated use of p-y curves that current state-of-the-art methods use. Instead, the three-dimensional (3D) highly nonlinear problem is approximated in two steps, both involving two-dimensional (2D) plane-strain analyses. The first step involves a vertical (representative)sliceinwhichthepilegrouphasbeenomittedandthat,shakenatitsbase,givesthepermanentdeformationofthequaywalland of the liquefiable soil. It is an effective stress analysis. In the second step, a horizontal (representative) slice taken from the middle of the liquefiable zone is subjected to an outward quay wall displacement; the goal is to evaluate the reduction of the pile displacement over the free- field one and the ensuing pile group distress. The pile resistance to ground deformation depends heavily on the constraints imposed by the superstructure, as well on the exact stiffness of the soil layers. Thus, the interplay between soil piles-quay wall under soil flow conditions is capturedinaphysicallymeaningfulway.Thepredictionscomparewellwithresultsfromtwocentrifugetests.DOI:10.1061/(ASCE)GT.1943- 5606.0000759.