Micromechanical investigation of a 2-D granular material with respect to structure evolution and loading paths

The paper examines the evolution of microstructure and contact forces in a two-dimensional analogue granular material consisting of photoelastic disks under various loading paths. Several stress (strain) paths together with stress (strain) probes are performed in the context of biaxial element tests, and the resulting strain (stress) response envelopes are determined. This is a more objective way of evaluating the stress-strain behaviour of granular soils with reference to anisotropy and dilatancy. One of the findings of this investigation is the elucidation of the microstructural changes accompanying the instability behaviour of granular materials when all effective stresses nullify such as in static liquefaction. It is found that strong force chains develop in a dilating specimen, and that a flow type of failure is due to the buckling of these force chains. Instability is also more formally analyzed within the framework of Hill’s second order work. It is interesting to note that experimental results obtained in a ‘coarse’ two-dimensional material (200-400 particles of mean diameter of 5-7 mm) are very consistent with the theory of plasticity augmented with microstructural information through a fabric tensor. This confirms the pivotal importance of microstructure in any constitutive modelling endeavour.