Seminar - Fernando Alonso-Marroquin - A continuum model for buckling of force chains and shear band formation in granular materials

Wednesday 22 September 2010, 4.00 pm - 5.00 pm
Civil Engineering Lecture Theatre 1

Dr Fernando Alonso-Marroquin
School of Civil Engineering
University of Sydney

Abstract:
Predicting the onset of instability in granular soils is still a challenging problem in geomechanics. Typically these instabilities appear in forms of shear bands where the deformation localises. Examples of shear bands are fault zones in earthquakes or slip planes in landslides. They exhibit intricate geometries and, in some cases, not-persistent behaviour. Still continuum mechanics is not ready to predict shear band instabilities, because there is no a well-established “Navier-Stokes” type of equations for granular materials. With this aim I present our preliminary work on the development of a micromechanical constitutive model to captures kinematics of shear bands. Our development is inspired in Discrete Element Modelling and photo-elastic experiments, which show that the stress changes is governed by well known dynamics of force chains, which are filamentary structures where the stress propagates. The formation of shear band is characterised by buckling of force chains, and the localization of these bucking modes leads to the formation of shear bands. Our model consists of two continuum model interconnecting via the couple stress. The first one is a microscopic continuum model which accounts deformation and stress distribution inside individual grains. This high-resolution model allows ab initio calculation of couple stresses produced by rolling resistance. The couple stress is plugged into a mesoscopic, high-order constitutive model, which explicitly accounts the bucking of force chains. The proposed mesoscopic continuum model harmonises two previous frameworks, the Cosserat theory and high-order continuum models, which have been traditionally used to remove of the mesh-dependency in the continuum modelling of geostructures.


Short Biography
I enjoy working in numerical simulations, but I always try that my mind is not replaced by the CPU of the computer. That is why sometimes I prefer analytical calculations

After developed an analytical model for the electronic structure of the fullerenes molecules as part of the Diploma thesis, I escaped from the Quantum Physics and come back to the fascinating world of Newtonian Mechanics. In 1998 I moved to Madrid to study Mathematical Engineering. I learned about asymptotic and numerical methods for engineering problems. In 2004 I finished my doctoral thesis in Stuttgart on the study of ratcheting and incremental response of granular soils, using particle-based simulations. In 2004-2005 I was EU postdoctoral Fellow. I participated in the European DIGA project "Degradation and Instabilities of Geomateriales Applied to Hazard Mitigation" in Athens. Motivated in the understanding of natural failures such as earthquakes and landslides, we worked in the connection between micromechanics and the continuum theories. I arrived to Australia In 2005 and I worked in The University of Queensland as Computational Scientist, Australian Postdoctoral Fellow and Affiliate Academic. I founded the group MoSCoS (Modelling and Simulation of Complex Systems) in the University of Queensland, where we developed a generic model to simulate particulate systems with complex morphologies. Currently I am lecturing Computational Geomechanics in the University of Sydney.