Grain shape effect on the anisotropic behaviour of silt–sand mixtures

Abstract

The anisotropy of sand–silt mixtures has not been fully studied in terms of the effects of different grain shapes and silt content. This study investigated the effects on anisotropy in three different sands with various silt contents and grain shapes. The tests performed in this research include 90 hollow cylinder torsion shear tests conducted on three types of sand: Hamadan sand with high angularity, Firoozkuh sand with medium angularity and Tehran sand with low angularity. The samples had silt contents of 0, 15, 30 and 50 per cent. To define the shape criteria in this study, two ratios were defined. The first was roundness, which can be obtained from the ratio of the radius of the corners of the grains to the largest circle that encompasses the entire grain; this ratio is denoted by rr. The second criterion was sphericity, which can be obtained by dividing the smallest pictorial width by the minimum graphical length; this ratio is denoted by rs. Under similar initial effective confining pressures, sand with high roundness and sphericity showed contractive behaviours. An increase in silt content, as well as in the ratios rr and rs, decreased the sample strength. Moreover, increasing these two ratios caused the stress–strain curves to be close to one another.