Experimental Observations on the Critical State of Granular Materials

A wide class of constitutive models adopts Critical State as a reference concept to capture the combined dependency of the stress-strain response of granular materials on initial density and stress level. A controversial issue is however represented by the experimental determination of Critical State Locus (CSL), since steadiness of stresses and volume is asymptotically approached at very large strains where sample distortion and strain localization make determination of state variables largely inaccurate. These difficulties are herein discussed based on the results of triaxial tests performed on sands compacted at different initial void ratios and sheared at different mean effective stresses. The comparison between triaxial compression and extension shows that soil state tends to different loci. Finally, a collection of triaxial test results obtained on a large variety of materials including uniform steel beads, two artificial sandy compounds and three gravels are compared with those retrieved on literature to investigate the dependency of Critical State Locus on the grading of materials. This comparison shows a fundamental role of particles size heterogeneity in determining denser and more resistant critical states.