Modelling of Granular Materials

Abstract: The aim of this doctoral study is to develop and implement a material model for granular materials. The course of this work was concentrated upon a non-associated plasticity model with yield surface and the flow potential expressed in terms of the mean stress p and the third stress invariants I3. The plastic work hardening in the model depends upon both plastic volumetric and deviatoric strain increments in order to model dilatancy before the ultimate state. The calibration procedure that captures the principal features of the stress-strain and axial to volume strain relations is developed and all six material parameters in the model are determined from the data of one standard triaxial test. An efficient implicit integration algorithm, for granular material models including I3-plasticity, is developed and implemented as a user defined material behaviour in a commercial finite element code. The implemented algorithm is utilized to study the difference in strength and stress distribution between rigid and flexible footings resting on a surface of sand and the differences between two and three dimensional simulations of strip and square footings. The material behaviour for a two-component soil system consisting of sand and gravel particles is studied micromechanically and then the global response is investigated. The dependance of the strain localization phenomenon on the particle form and the effects of dilantcy are studied. For many geotechnical purposes in practice the soil foundations contain not only soil but also water. In order to include the influence of flowing water on soil, the material model for dry soil is incorporated in a binary mixture model. The mixture model is formulated by eliminating the supply terms due to interactions and hence these terms do not need to be constituted. The model is implemented and incorporated into the finite element code and simulations of footing resting on water saturated sand have been performed.

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