Importance of Soil Anisotropy on Foundation Displacement Functions

Consideration is given to the effect of soil cross-anisotropy on the dynamic displacement functions of a rigid strip surface foundation subjected to vertical or horizontal forces and moments that vary harmonically with time and are distributed uniformly across the longitudinal axis so that plane strain conditions prevail. The results are obtained using an analytical-numerical formulation which models realistically the rough soil-foundation interface, properly accounts for phenomena associated with propagation of waves emanating from the foundation and considers linear hysteretic material damping in the soil. Particular emphasis is accorded to the sensitivity of the calculated frequency-dependent foundation displacements to the assumed values of the anisotropic soil constants, appropriate for drained loading conditions

Inconclusive Case Histories in Earthquake Geotechnics From Christchurch

It is argued that there is still a need for further exploratory research to unravel the ultimate causes of unresolved case histories (especially those involving failures) in geotechnical earthquake engineering. Three specific examples motivated from the records of the four major seismic episodes that shook the city of Christchurch, New Zealand, in 2010 and 2011 are presented in detail. Peculiarities in these records call for an investigation of a number of plausible seismological and geotechnical contributing factors including source mechanics, forward-rupture directivity, 1Dsoil amplification, soil liquefaction, 2D basin amplification, and topographic aggravation

Lessons from Revisiting Three Cases of Slope Failure

Three old failures of geotechnical projects are analyzed, where some original design uncertainties were lately discovered. The first case concerns the failure during construction of a slope at the portal area of a tunnel in Greece, due to unexpected excessive water pressures. The failure was restricted to the upper weathered zone of flysh formation. Due to difficulties in performing laboratory tests with such hard soil material necessitated the estimation of shear strength parameters on the basis of the rockmass classification systems. The next case deals with an extensive failure of an open pit mine slope in Greece. The ground consists of alternations of lignite layers with very stiff clay. Despite the detailed investigation on the peculiar nature of shear strength, the unavoidable scatter of the values in combination with the applied low safety factors and the development of excessive pore pressures, lead to this failure. The last case refers to the well known failure in Kimola Canal and addresses the probable failure mode and the proper analysis method. The analyses of the slope stability in formation of post glacial clays had been carried out with φ=0 and the use of undrained shear strength. The failure surface was developed upwards to a great distance and it could interpreted on the basis of the effective stresses analyses, taking into consideration the influence of the shear strength anisotropy

Aspects of Seismic Analysis and Design of Rockfill Dams

Theoretical methods for estimating the dynamic response and predicting the performance of modern rockfill dams subjected to strong earthquake shaking are reviewed. The focus is on methods accounting for nonlinear material behavior, for 3-Dimensional canyon geometry, and asynchronous base excitation. It is shown that both strong nonlinearities and lack of coherence in the seismic excitation tend to reduce the magnitude of the deleterious whip-lash effect computed for tall dams built in rigid-wall narrow canyons. Particular emphasis is accorded to Concrete-Faced Rockfill dams and a case study involving an actually designed dam in a narrow canyon points to some potential problems and suggests some desirable modifications. In the light of theoretical results the paper concludes with a discussion on design rules and defensive measures that would lead to robust design schemes of Earth-Core and Concrete Faced Rockfill dams

Offshore Caissons on Porous Saturated Soil

While currently available methods of dynamic soil-foundation interaction idealize the soil as a continuum, this paper presents a general theory to obtain the dynamic response of offshore caissons resting on a saturated or nearly saturated poroelastic medium. The model, based on Biot\u27s theory, considers the compressibility of both solid and fluid phase and assumes that the fluid flow is governed by Darcy\u27s Law for an isotropic medium. Results are presented as plots of normalized amplitudes of displacement load or rotation-moment ratios for a rigid strip founded on a dense coarse sand. The results demonstrate that fluid compressibility, which is primarily a function of the degree of saturation, has an important effect on rocking motions. Soil permeability appears to have a rather minor effect on the response

Seismic Design Chart for Anchored Bulkheads

Evaluation of numerous case histories reveals that the seismic performance of anchored sheet pile quaywalls depends primarily on the anchoring system. Current pseudo-static procedures often lead to deficient anchoring, whose excessive displacements or failure trigger excessive permanent seaward displacement at the top of the bulkhead, accompanied by cracking and settlement behind the anchor. The results of the case histories lead to a Seismic Design Chart to be used in conjunction with the pseudo static procedure. The Chart delineates between acceptable and unacceptable degrees of damage, depending on the values of two dimensionless parameters that are functions of the material and geometric characteristics of the bulkhead, and the intensity of seismic shaking. Soil softening/degradation due to development of pore water-pressures is indirectly accounted for in the proposed method; however, the engineer must ensure that no liquefaction-flow failure of cohesion less soils will occur in the backfill or the foundation

Local Soil Effects and Liquefaction in the 1978 Thessaloniki Earthquakes

The geotechnical aspects of three earthquakes that struck the city of Thessaloniki, Greece, on May 25, June 20 and July 4, 1978 are presented. General background information on the observed damage, seismic history and geology of the area is followed by detailed description of soil profiles, structural characteristics and accelerograms of ground motions recorded at three sites. Acceleration spectra are then examined and compared in order to assess the degree to which local and regional geology and soil-structure interaction affected the recorded motion. Finally, the possibility of liquefaction having taken place in a 6 m-thick saturated loose layer of silty sand, under the monumental \u27White Tower\u27, is investigated. Conclusions are drawn in the light of the current state-of-art of assessing liquefaction potential of soils

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 Adàmes, 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

Seismic Response of Inelastic Pile Foundations: A New Performance Based Design Philosophy

While seismic codes do not allow plastic deformation of piles, the Kobe earthquake has shown that limited structural yielding and cracking of piles may not be always detrimental. This paper focuses on the influence of soil compliance, pile-to-pile interaction, intensity of seismic excitation, pile diameter, above–ground height of the pile, location of plastic hinges (above or below ground development), on the seismic response of pile supported bridge structures. Evaluation of the bridge pier behaviour is achieved through key performance measure indices, as is: the displacement (global) and curvature (local) ductility demands and the maximum drift ratio. It is shown that the ductility demand of a bridge pier decreases with both (a) increasing soil compliance, and (b) below-ground location of plastic hinges development. By exploiting the results, a new performance based design method is developed that allows for soil and pile yielding instead of over-designing the foundation to behave nearly elastically and forcing the potentially developed plastic hinges to occur in the pier (as with conventional capacity design)