Investigation of the mechanical behavior of granular material : effects of particle size distribution and particle shape

Abstract

Granular materials are ubiquitous in nature and are the most commonly-manipulated materials in engineering practice and industrial applications. Despite tremendous research effort, the mechanical behavior of granular materials which results from particle-level interactions is not yet fully-understood at different strain levels. The primary objective of this research is to investigate the effects of particle size distribution and particle shape on the mechanical behavior of granular materials. At large strain levels, there is abundant experimental evidence that when sheared under undrained conditions, loose granular materials will collapse rapidly with a very low residual strength. This flow-type behavior, known as static/flow liquefaction in soil mechanics, can cause devastating consequences in geotechnical engineering. Considerable effort has been made in the past to understand static liquefaction, mainly by means of well-controlled laboratory tests on clean sands. However, the characteristics of most natural sands encountered in practical engineering are more complex in grading and morphology: for example, the natural sand may be a mixture of uniform sands and certain amount of fines (referred to as silty sands); the morphology of constituent grains may be spherical, rounded, blocky, elongated or platy. Owing to the discrete nature of granular materials, a particulate approach known as the discrete element method is employed. Specifically-designed laboratory experiments are carried out along with the modelling under the framework of critical state soil mechanics. Particular effort will be made to discuss the applicability of different state variables (e.g. void ratio, state parameters, skeleton void ratio and its modified versions) and to establish the quantitative relationships between the critical state parameters and the particle properties. DEM simulation and laboratory test indicate that a high sensitivity of the critical state parameters to the variation of particle properties (e.g. fines content, gradation, particle shape). A systematic analysis is made to investigate the effect of particle properties on small strain mechanism by exploiting the discrete element simulation, the laboratory tests and the analytical models. The discrepancy in stress exponent between the experimental data and the theoretical prediction are attributed to the following two factors: 1. the discrepancy between the Hertz-Mindlin contact law and the real contact condition (e.g. a rough surface contact condition); 2. the fabric change due to increasing confining pressure, which causes an additional increase in stiffness. It is shown that the stress exponent increases with increasing surface roughness for random packing specimen, while the stress exponent for face-center-cubic packing composed of mono-size spheres is 1/3, in line with the prediction of Hertz-Mindlin contact law. The stress exponent generally increases with increasing the dimensional tolerances of sphere, which indicates that the microstructure affects the stress exponent. Furthermore, the linkage between the macro responses and the micro scale properties (e.g. coordination number, distribution of contact normal force) is also addressed.