Numerical modelling of earthquake induced liquefaction
Soil liquefaction of saturated sandy soils is one of the major hazards related to seismic activity. Numerous cases of significant damage to buildings, bridges, roads, pipelines and ports resulting from the failure of the underlying soils as a result of liquefaction have been observed in many seismic events: recent destructive earthquakes, e.g. Kobe (1995), Chi-Chi, Taiwan and Kocaeli, Turkey (1999), and Christchurch, New Zealand (2010 and 2011) are prime examples of the potential for earthquake-induced liquefaction to cause significant damage and disruption to civil infrastructure. Furthermore, despite the well-known phenomenon of densification of a sand deposit following liquefaction, the recent earthquakes in Christchurch, New Zealand (2010 and 2011) were accompanied by the occurrence of re-liquefaction of the same soil deposits.
Despite the abovementioned significance of the liquefaction hazard, current design procedures are based on crude empiricism and there are no widely accepted guidelines to design appropriate mitigation measures.
The research project will employ the in-house Finite Element code ICFEP (Potts & Zdravkovic, 1999) of the Geotechnics group at Imperial College to simulate the liquefaction phenomenon. The code is equipped with state-of-the-art time integration schemes (Kontoe et al., 2008) and advanced boundary conditions for dynamic analysis (Kontoe et al., 2009). Moreover, a bounding plasticity constitutive model has been implemented, which can numerically reproduce most of the significant features of soil behaviour under cyclic loading and realistically simulate liquefaction (Taborda, 2011). The figure below displays the model surfaces in the deviatoric plane.
This model is based on the Papadimitriou & Bouckovalas (2002) modified version of the original two-surface model proposed by Manzari & Dafalias (1997), combining Critical State soil mechanics with the Bounding Surface Plasticity theory. However, several alterations have been incorporated to improve various aspects of its capabilities.
The ability of the model to realistically reproduce basic dynamic soil characteristics, such as stiffness degradation and damping ratio curves, as well as its ability to reproduce the impact of the occurrence of liquefaction in soil deposits has been investigated (Taborda, 2011). However, given the lack of monitoring data in the past, the model has only been used in boundary value problems simulating extensive centrifuge testing carried out as part of the large collaborative VELACS research project (Arulanandan & Scott, 1993). Furthermore, only the phenomenon of cyclic mobility has been investigated in boundary value problems.
The main aspects of this PhD thesis will concern the following:
(i) Calibration of the numerical models using real case studies. As real stratigraphies can be very complicated, with layers of clays in between sand deposits, a simpler cyclic non-linear constitutive model, which cannot reproduce the liquefaction mechanism under cyclic loading, but can adequately incorporate the basic aspects of dynamic soil behaviour, i.e. stiffness degradation and damping due to hysteresis, in finite element analysis, will also be used and calibrated to model non-sandy strata.
(ii) Assessment of the models through available field observations.
(iii) The investigation of the phenomenon of liquefaction, looking at particular aspects of multi-directional cyclic loading, as well as the occurrence of re-liquefaction.
(iv) The last phase of the proposed work will involve the numerical investigation of liquefaction mitigation techniques, focusing on sand-tyre mixtures (SRM). Recent shaking table and centrifuge tests have showed that these are more resistant to liquefaction than clean sands and have an excellent energy absorption capability which reduces ground shaking. Furthermore the use of recycled tyres for liquefaction mitigation is a sustainable alternative to traditional mitigation approaches (e.g. deep cement mixing, grouting) which are energy demanding. While there is experimental evidence of the promising features of SRM there is a need for a systematic numerical investigation of its use in boundary value problems involving real structures (e.g. quay walls).
- Arulanandan, K. & Scott, R.F. (Eds.) (1993) Verification of numerical procedures for the analysis of soil liquefaction problems, Proceedings of the International Conference, 17-20 Oct. 1993, Davis, California. Rotterdam, The Netherlands, Balkema.
- Kontoe S., Zdravkovic L., Potts D.M. (2008) An assessment of time integration schemes for dynamic geotechnical problems. Computers and Geotechnics, 35 (2), 253-264.
- Kontoe S., Zdravkovic L., Potts D.M. (2009). An assessment of the domain reduction method as an advanced boundary condition and some pitfalls in the use of conventional absorbing boundaries.International Journal for Numerical and Analytical Methods in Geomechanics, 33, 309-330.
- Manzari, M.T. & Dafalias, Y.F. (1997). A critical state two-surface plasticity model for sands. Geotechnique, 47 (2), 255-272
- Papadimitriou, A. G. & Bouckovalas, G. D. (2002). Plasticity model for sand under small and large cyclic strains: A multiaxial formulation. Soil Dynamics and Earthquake Engineering, 22 (3), 191-204.
- Potts, D.M. & Zdravkovic, L. (1999) Finite Element Analysis in Geotechnical Engineering: Theory. London, Thomas Telford Publishing.
- Potts, D.M. & Zdravkovic, L. (1999) Finite Element Analysis in Geotechnical Engineering: Application. London, Thomas Telford Publishing.
- Taborda, D.M.G. (2011). Development of constitutive models for application in soil dynamics. Ph.D. thesis. Imperial College London, London.
PhD Candidate - Geotechnics
Department of Civil & Environmental Engineering
Imperial College London SW7 2AZ