Particle shape factors and fractal dimension after large shear strains in carbonate sand

Abstract

Changes of grain shape and breakage under compression and shearing can affect the mechanical properties of granular soils. In this work, a parametric study was performed through ring-shear tests on carbonate sand, under σn = 600 kPa and v = 6 mm/min, to evaluate the evolution of particle breakage, fractal dimension and particle shape with the shear strain. Breakage mostly occurred for γ ≤ 4000%, with a consequent increase in fractal dimension. Under larger strains, only negligible breakage occurred, while the particles became more rounded, and the fractal dimension increased only slightly. Interestingly, the shape factors also tend towards similar values for particles of any size larger than 0·063 mm after large shearing. This indicates that these particles tend towards a self-similar form – and hence scale invariance – when the grading reaches a fractal particle size distribution. Some tests were also performed on soils with different median size and coefficient of uniformity, which showed that larger particles and poorly graded soils are more prone to breakage, and are closer to a fractal distribution after shearing. Conversely, the final particle shape did not seem to be affected by grading and size significantly.