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Title: Numerical modelling of earthquake induced liquefaction under irregular and multi-directional loading
Author: Tsaparli, Vasiliki
ISNI:       0000 0004 8504 401X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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This PhD thesis details the numerical investigation of earthquake-induced liquefaction using state-of-the-art research tools implemented in the in-house Imperial College Finite Element Program (ICFEP). The study draws particular emphasis on the role of the multi-directional and irregular nature of the seismic motion on liquefaction, aiming to enable better risk characterisation in engineering practice. The first part focusses on the role of the vertical seismic motion on sand liquefaction, which is largely neglected in design standards. The major contribution relates to a novel concept of liquefaction triggering due to the vertical ground motion, the impact of which was catastrophic during the 2010-2011 Canterbury Earthquake Sequence in New Zealand. Energy principles are adopted in P-wave propagation to aid the interpretation of the physical mechanism. The performance of two time-integration schemes are also compared to provide guidance on numerical geotechnical earthquake engineering problems involving compressional waves. The second part concerns suggested modifications to the constitutive model for sand to mitigate limitations identified in the first part in terms of the simulated undrained cyclic strength. The modified formulation is presented and calibrated based on published element testing and its performance is thoroughly assessed. The third part investigates the applicability of the Palmgren-Miner hypothesis in liquefaction evaluation through the equivalent number of stress cycles concept. The latter is directly applicable to the magnitude scaling factors used in liquefaction assessments, but also to the laboratory evaluated cyclic strength. A procedure to test this numerically is outlined. Based on the conclusions, guidelines for improved use are discussed. The final part of this thesis models a case study. The Mw 6.2 22nd February 2011 Christchurch seismic event in New Zealand was chosen for the analyses, as it presents a well-documented case. A thorough discussion on the inferred geotechnical parameters and on the calibration of the constitutive models is made. The selection of representative input ground motions is also presented in detail. The numerical predictions are compared against the monitoring and field data, but also against the predictions of the simplified liquefaction procedure.
Supervisor: Kontoe, Stavroula ; Taborda, David Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral