Experimental investigation of instabilities of granular materials in relation to dilatancy and fabric issues

The paper examines an experimental evaluation of the intertwined effects of microstructure (fabric) and dilatancy on the stability and strength of granular materials. The basic premise is that a granular material such as sand has at the outset a certain potential to dilate by an amount commensurate with the nature of the applied external loads (stress or strain imposed) and how conducive they are to fabric changes. As such, the deformation of dense packings of two-dimensional photoelastic disks of various cross-sections is investigated along so-called proportional strain paths that correspond to various dilation rates. 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 such a 'coarse' two-dimensional material (200-400 particles with mean diameter of 5-7 mm) are very consistent with simulation results from a micromechanically based plasticity model reported in a previous publication. This confirms the pivotal importance of microstructure in any constitutive modelling endeavour.