George Gazetas

Analysis of machine foundation vibrations: State of the art

Abstract The paper reviews the state-of-the-art of analysing the dynamic response of foundations subjected to machine-type loadings. Following a brief outline of the historical developments in the field, the concepts associated with the definition, physical interpretation and use of the dynamic impedance functions of foundations are elucidated and the available analytical/numerical methods for their evaluation are discussed. Groups of crucial dimensionless problem parameters related to the soil profile and the foundation geometry are identified and their effects on the response are studied. Results are presented in the form of simple formulae and dimensionless graphs for both the static and dynamic parts of impedances, pertaining to surface and embedded foundations having circular, strip, rectangular or arbitrary plan shape and supported by three types of idealized soil profiles: the halfspace, the stratum-over-bedrock and the layer-over-halfspace. Consideration is given to the effects of inhomogeneity, anisotropy and non-linearity of soil. The various results are synthesized in a case study referring to the response of two rigid massive foundations, and practical recommendations are made on how to inexpensively predict the response of foundations supported by actual soil deposits.

SEISMIC SOIL-STRUCTURE INTERACTION: BENEFICIAL OR DETRIMENTAL?

The role of soil-structure interaction (SSI) in the seismic response of structures is reex-plored using recorded motions and theoretical considerations. Firstly, the way current seismic provisions treat SSI effects is briefly discussed. The idealised design spectra of the codes along with the increased fundamental period and effective damping due to SSI lead invariably to reduced forces in the structure. Reality, however, often differs from this view. It is shown that, in certain seismic and soil environments, an increase in the fundamental natural period of a moderately flexible structure due to SSI may have a detrimental effect on the imposed seismic demand. Secondly, a widely used structural model for assessing SSI effects on inelastic bridge piers is examined. Using theoretical arguments and rigorous numerical analyses it is shown that indiscriminate use of ductility concepts and geometric relations may lead to erroneous conclusions in the assessment of seismic performance. Numerical examples are presented which highlight critical issues of the problem.

SOIL—PILE—BRIDGE SEISMIC INTERACTION: KINEMATIC AND INERTIAL EFFECTS. PART I: SOFT SOIL

SUMMARY 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 suƒcient 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 earth dams: some recent developments

Abstract The paper focuses on theoretical methods for estimating the dynamic response of earth dams to earthquake ground excitation. Following an outline of the historical developments in this field, the basic concepts/models for response analysis are introduced and their salient features, advantages and limitations are elucidated. The major phenomena associated with, and factors influencing, the response are identified and studied. Particular emphasis is accorded to inhomogeneity due to dependence of soil stiffness on confining pressure, nonrectangular canyon geometry, and nonlinear-inelastic soil behaviour. Several new formulations that have evolved over the last five years are outlined and characteristic results provide considerable insight into the problem. The simplicity of some of these formulations is underlined and attempts are made to compare their predictions with measurements from full-scale, natural and man-made, forced vibration tests. The basic validity as well as the limitations of the proposed analysis methods is demonstrated and topics of needed further research are suggested.

Effects of Local Soil Conditions on the Topographic Aggravation of Seismic Motion: Parametric Investigation and Recorded Field Evidence from the 1999 Athens Earthquake

During the 1999 Athens earthquake, the town of Adames, located on the eastern side of the Kifissos river canyon, experienced unexpectedly heavy damage. Despite the particular geometry of the slope that caused significant motion amplification, topography effects alone cannot explain the uneven damage distribution within a 300-m zone parallel to the canyon’s crest, which is characterized by a rather uniform structural quality. In this article, we illustrate the important role of soil stratigraphy and material heterogeneity on the topographic aggravation of surface ground motion. For this purpose, we first conduct an extensive time-domain parametric study using idealized stratified profiles and Gaussian stochastic fields to characterize the spatial distribution of soil properties, and using Ricker wavelets to describe the seismic input motion; the results show that both topography and local soil conditions significantly affect the spatial variability of seismic motion. We next perform elastic two-dimensional wave propagation analyses based on available local geotechnical and seismological data and validate our results by comparison with aftershock recordings.

Slope Stabilizing Piles and Pile-Groups: Parametric Study and Design Insights

This paper uses a hybrid method for analysis and design of slope stabilizing piles that was developed in a preceding paper by the writers. The aim of this paper is to derive insights about the factors influencing the response of piles and pile-groups. Axis-to-axis pile spacing (S), thickness of stable soil mass (Hu), depth (Le) of pile embedment, pile diameter (D), and pile group configuration are the parameters addressed in the study. It is shown that S ¼ 4D is the most cost-effective pile spacing, because it is the largest spacing that can still generate soil arching between the piles. Soil inhomogeneity (in terms of shear stiffness) was found to be unimportant, because the response is primarily affected by the strength of the unstable soil layer. For relatively small pile embedments, pile response is dominated by rigid-body rotation without substantial flexural distortion: the short pile mode of failure. In these cases, the structural capacity of the pile cannot be exploited, and the design will not be economical. The critical embedment depth to achieve fixity conditions at the base of the pile is found to range from 0:7Hu to 1:5Hu, depending on the relative strength of the unstable ground compared to that of the stable ground (i.e., the soil below the sliding plane). An example of dimensionless design charts is presented for piles embedded in rock. Results are presented for two characteristic slenderness ratios and several pile spacings. Single piles are concluded to be generally inadequate for stabilizing deep land- slides, although capped pile-groups invoking framing action may offer an efficient solution. DOI: 10.1061/(ASCE)GT.1943-5606.0000479.

Topography and Soil Effects in the MS 5.9 Parnitha (Athens) Earthquake: The Case of Adámes

Large concentration of damage to residential and industrial buildings occurred in regions near the banks of the Kifisos river canyon during the 7-September-1999 Parnitha (Athens) Earthquake. One such region, which experienced unexpectedly heavy damage, was the small community of Adámes, which borders the canyon near its deepest point. To explore whether in addition to structural factors the particular topographic relief and/or the actual soil profile contributed to the observed concentration and non-uniform distribution of damage within a 300 m zone from the edge of the canyon cliff, wave propagation analyses are conducted in one and two dimensions. Finite-element and spectral-element formulations are used to this end. To avoid spurious wave reflections at the artificial boundaries, ourtwo-dimensional (2-D) finite-element analyses utilize Bielaks Effective Seismic Excitation method. Soil layering and stiffnesses are determined from 10 SPT-boreholes and 4 crosshole tests. Ricker wavelets and six realistic accelerograms are used as excitation; two of the latter are selected from the literature and four are obtained on the basis of the four strongest motions of the earthquake, recorded in central Athens.The results show that the 2-D topography effects are substantial only within 50 meters from the canyon ridge. These effects materialize only in the presence of the relatively soft soil layers that exist in the profile at a shallow depth. The so-called Topographic Aggravation Factor (TAF), defined as the 2-D over 1-D Fourier spectral ratio, varies around 1.4 over a broad frequency band which covers the significant excitation frequencies. At the location of four collapsed buildings, about 250 m from the edge, 2-D (topography) effects are negligible, but the specific soil profiles amplify one-dimensionally all six ground base excitations to spectral acceleration levels that correlate well with the observed intensity of damage, at least in a qualitative sense.

Effects of Near-Fault Ground Shaking on Sliding Systems

A numerical study is presented for a rigid block supported through a frictional contact surface on a horizontal or an inclined plane, and subjected to horizontal or slope-parallel excitation. The latter is described with idealized pulses and near-fault seismic records strongly influenced by forward-directivity or fling-step effects (from Northridge, Kobe, Kocaeli, Chi-Chi, Aegion). In addition to the well known dependence of the resulting block slippage on variables such as the peak base velocity, the peak base acceleration, and the critical acceleration ratio, our study has consistently and repeatedly revealed a profound sensitivity of both maximum and residual slippage: (1) on the sequence and even the details of the pulses contained in the excitation and (2) on the direction (+ or - ) in which the shaking of the inclined plane is imposed. By contrast, the slippage is not affected to any measurable degree by even the strongest vertical components of the accelerograms. Moreover, the slippage from a specific record may often be poorly correlated with its Arias intensity. These findings may contradict some of the prevailing beliefs that emanate from statistical correlation studies. The upper-bound sliding displacements from near-fault excitations may substantially exceed the values obtained from some of the currently available design charts.

Hybrid Method for Analysis and Design of Slope Stabilizing Piles

Piles are extensively used as a means of slope stabilization. Despite the rapid advances in computing and software power, the design of such piles may still include a high degree of conservatism, stemming from the use of simplified, easy-to-apply methodologies. This paper develops a hybrid method for designing slope-stabilizing piles, combining the accuracy of rigorous three-dimensional (3D) finite- element (FE) simulation with the simplicity of widely accepted analytical techniques. It consists of two steps: (1) evaluation of the lateral resisting force (RF) needed to increase the safety factor of the precarious slope to the desired value, and (2) estimation of the optimum pile configuration that offers the required RF for a prescribed deformation level. The first step utilizes the results of conventional slope-stability analysis. A novel approach is proposed for the second step. This consists of decoupling the slope geometry from the computation of pile lateral capacity, which allows numerical simulation of only a limited region of soil around the piles. A comprehensive validation is presented against published experimental, field, and theoretical results from fully coupled 3D nonlinear FE analyses. The proposed method provides a useful, computationally efficient tool for parametric analyses and design of slope-stabilizing piles. DOI: 10.1061/(ASCE)GT.1943-5606 .0000546.

Dynamic response of pile groups with different configurations

A general methodology is outlined for a complete seismic soil-pile-foundationstructure interaction analysis. A Beam-on-Dynamic-Winkler-Foundation (BDWF) simplified model and a Greens-function-based rigorous method are utilized in determining the dynamic response of single piles and pile groups. The simplified model is validated through comparisons with the rigorous method. A comprehensive parameter study is then performed on the effect of pile group configuration on the dynamic impedances of pile foundations. Insight is gained into the nature of dynamic pile-soil-pile interaction. The results presented herein may be used in practice as a guide in obtaining the dynamic stiffness and damping of foundations with a large number of piles.