Collapse of loose sand with the addition of fines: the role of particle shape

This paper describes a fundamental study to explore the role of particle shape in the shear behaviour of mixtures of sand and fine particles (fines) through macro-scale and grain-scale laboratory experiments together with interpretations in the framework of critical state soil mechanics and conceptual micromechanics models. Two non-plastic fines of distinct shape (angular crushed silica fines and rounded glass beads) were added to two uniform quartz sands (Toyoura sand and Fujian sand) to produce four binary mixtures. Laboratory quantification of particle shape and size was conducted for both fines and sands, and a series of undrained triaxial tests was performed to investigate the overall response of the mixed soils at different percentages of fines. One of the significant findings is that the critical state friction angle of a mixed soil is affected not only by the shape of coarse particles but also by the shape of fine particles, and this shape effect is coupled with fines content. When a small amount of crushed silica fines was added to either Toyoura sand or Fujian sand, the critical state friction angle increased slightly with fines content; however, when a small amount of glass beads was added to either sand, the critical state friction angle decreased markedly with an increase in fines content. A new index, termed combined roundness, is proposed to account for the coupled effects of particle shape and fines content. The study also provides evidence showing that the undrained shear behaviour and collapsibility of a mixed soil are closely related to the shape of its constituent particles: a mixed soil containing rounded fines tends to exhibit higher susceptibility to collapse than a mixed soil containing angular fines of the same percentage, and this tendency will become more evident if the base sand is also composed of rounded particles. In addition, the study provides the first experimental evidence showing that the quasi-steady state marks a transition from a metastable to a stable microstructure. It is postulated that in a binary mixture composed of fine and coarse particles that are both rounded, the particles favour rolling, thus yielding a microstructure that is unstable, whereas in a binary mixture composed of fine and coarse particles that are both angular, the particles favour sliding rather than rolling, thus leading to a much more stable structure and response.