Characterisation of the ultimate particle size distribution of uniform and gap-graded soils

The ultimate particle size distribution of uniform and gap-graded soils is examined on specimens of carbonate sand that were subjected to large strains in a ring shear apparatus. The gap-graded soils were seen to retain a memory of their initial grading even at large strains. The particle size distributions were plotted in double logarithmic graphs either by mass or by number computed assuming different shapes. It was not possible to find linear subsets of the data, and since the samples were found experimentally to have converged to an ultimate grading, this suggests that the initial bimodal distribution prevented reaching an ultimate fractal distribution. Plots of the probability density functions of the particle sizes before and after shearing show the evolution of the gap-graded soils from a bimodal to a multi-modal distribution. This is accompanied by an evolution of the shape of the particles, visible in microphotographs and projections of the grains before and after test.