Multi-scale Study of Grain Crushing in Granular Soils

Grain crushing is a phenomenon occurring at the grain-scale along areas of stress or strain concentration where the stress imposed exceeds particle strength. The grain fragmentation determines fine generation and a local densification of the material, which may induce contractions, settlements, decrease of hydraulic conductivity, etc. This work reports the results of a wide experimental program aimed to link the macroscopic behavior of granular materials subjected to oedometric compressions to grain crushing accounting for the micro-structural features of the single grains, e.g. size and shape. The grain scale was assessed by the use of the X-ray tomography facilities to characterize the material and quantify what is occurring along deformation. Tests performed included one-dimensional compression tests on dry samples till high pressure (~50 MPa) in order to observe significant amount of crushing. The material adopted is the Light Expanded Clay Aggregate (LECA), an artificial granular material characterized by light, porous and crushable grains. The compressibility curves has been linked to the grain crushing phenomena upon loading. The beginning of comminution is ruled by the strength of the single particle and by the initial assortment of size and shape. As the micro-structure influences the grain crushing and the material response, vice versa the grain crushing influences particles size and shape. The collected experimental evidence is finally used to develop a simple constitutive model able to predict the evolution of material porosity with loading.