Achilleas G. Papadimitriou

Plasticity model for sand under small and large cyclic strains: a multiaxial formulation

Abstract This paper presents the multiaxial formulation of a plasticity model for sand under cyclic shearing. The model adopts a kinematic hardening circular cone as the yield surface and three non-circular conical surfaces corresponding to the deviatoric stress ratios at phase transformation, peak strength and critical state. The shape of the non-circular surfaces is formulated in accordance with the experimentally established failure criteria, while their size is related to the value of the state parameter ψ . To simulate cyclic response under small and large shear strain amplitudes without a change in model parameters, it was found necessary to introduce: (a) a non-linear hysteretic (Ramberg–Osgood type) formulation for the strain rate of elastic states and (b) an empirical index of the effect of fabric evolution during shearing which scales the plastic modulus. This index is estimated in terms of a macroscopic second-order fabric tensor, which develops as a function of the plastic volumetric strain increment and the loading direction in the deviatoric plane. Comparison of simulations to pertinent data from 27 resonant column, cyclic triaxial and cyclic direct simple shear tests provide a measure for the overall accuracy of the model.

A critical state interpretation for the cyclic liquefaction resistance of silty sands

Contrary to many laboratory investigations, common empirical correlations from in situ tests consider that the increase in the percentage of fines leads to an increase of the cyclic liquefaction resistance of sands. This paper draws upon the integrated Critical State Soil Mechanics framework in order to study this seemingly not univocal effect. Firstly the effect of fines on the Critical State Line (CSL) is studied through a statistical analysis of a large data set of published monotonic triaxial tests. The results show that increasing the content of non-plastic fines practically leads to a clockwise rotation of the CSL in (e ‐l np ) space. The implication of this effect on cyclic liquefaction resistance is subsequently evaluated with the aid of a properly calibrated critical state elasto-plastic constitutive model, as well as a large number of published experimental results and in situ empirical correlations. Both sets of data show clearly that a fines content, less than about 30% by weight, may prove beneficial at relatively small effective stresses ( p0 , 50 ‐ 70 kPa), such as the in situ stresses prevailing in most liquefaction case studies, and detrimental at larger confining stresses, i.e. the stresses usually considered in laboratory tests. To the extent of these findings, a correction factor is proposed for the practical evaluation of liquefaction resistance in terms of the fines content and the mean effective confining stress. q 2002 Elsevier Science Ltd. All rights reserved.

Optimizing the Seismic Early Warning System for the Tohoku Shinkansen

The Tohoku Shinkansen is a high-speed passenger train that connects Tokyo to the northern city of Morioka. The line is protected by a seismic early warning system, which includes two sets of accelerometers: one set is deployed along the line (wayside system), while the other comprises eight accelerometers placed along the eastern coast of Honshu (coastal system). The coastal system is designed to protect the train against earthquakes with origin in the highly active offshore subduction zone. It causes trains to automatically stop when the ground acceleration at one of the stations exceeds a preset limit. At the time of the study, the system was causing frequent train delays, while providing an unclear level of seismic protection. We have found that, with the current early warning system, the rate of earthquake-induced derailments along the entire 500 km line is about 2–3 per 100 years. By changing various system parameters, one can lower this rate to about 1.5 derailments per 100 years while reducing the rate of false alarms and unnecessary delays by a factor of about 40. These benefits arise mainly by operating the wayside system on spectral acceleration rather than peak ground acceleration.

Proof of Incompleteness of Critical State Theory in Granular Mechanics and Its Remedy

AbstractAccording to classical critical state theory (CST) of granular mechanics, two conditions on the stress ratio and void ratio are satisfied when reaching and maintaining a critical state (CS)...

Failure Mechanisms of Landfills under Dynamic Loading

Seismic stability of above-ground landfills is an issue of extreme engineering interest, and the proper implementation of the corresponding analyses requires dealing with several uncertainties related mainly to the special features of such large-scale geostructures. This study aims at providing an insight into the role of these parameters on the development of the potential failure modes that may take place during a seismic event. Additionally, the different failure modes are compared with respect to their seismic instability potential. Furthermore, an insight is provided on phenomena related to the simultaneous generation of more than one failure surfaces within the landfill. This is examined by developing a simple model based on the Newmarks sliding-block approach. Results indicate that the circular surfaces may provide a more critical mode of failure than the base sliding.

FLIQ: Experimental Verification of Shallow Foundation Performance Under Earthquake-Induced Liquefaction

The seismic performance of a square footing, resting on a liquefiable sand layer, with a non-liquefiable clay crust, is examined herein, with the aid of three centrifuge experiments. Emphasis is given on the seismic settlements of the foundation, while it is for the first time attempted to measure its (degraded) post-shaking bearing capacity, with the aid of a hydraulic piston, specially programmed to push the footing to failure, immediately after the end of shaking and before the dissipation of excess pore pressures. Aimed to examine the effect of clay crust thickness H on foundation performance, the experiments were performed for H = 2/3B, B and 5/3B, with B being the width of the footing. Following a brief presentation of the testing configuration, soil properties and excitation characteristics, the experimental results are presented and evaluated through comparison with relevant numerical and analytical predictions. Thus, the beneficial effect of the clay crust thickness H is quantitatively substantiated and the existence of a “critical clay crust thickness”, beyond which subsoil liquefaction does not deter foundation performance, is experimentally verified.