Heterogeneity and soil element testing

Studies of strains within granular assemblies using digital image correlation have shown that as the resolution of observation becomes finer so the detail of the local variations of strains becomes more complex. The scale of this observed complexity is typically of the order of a few (5-10?) particles. The notion of a localised shear band of uniform shear or volumetric strain is not easily supported. In addition, studies reveal somewhat regular patterns of heterogeneity within samples which have been subjected to uniform external boundary deformations. A heterogeneous state is the natural state for granular materials. These observations have implications for the scales that should be adopted for testing of granular materials either in single-element or in physical model tests. Testing of a single element of a size that just accommodates a single region of 'localisation' will inevitably impose constraints on the particles in this region. On the other hand, one can envisage a representative element that is uniformly heterogeneous. Such an element would need to have dimensions that are large in comparison with the dimensions of the building blocks of the patterns of heterogeneity. These would define a length scale that is two orders of magnitude greater than typical particle sizes.