An Experimental Investigation of the Micromechanical Contact Behaviour of Soils


Student thesis: Doctoral Thesis

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Awarding Institution
Award date16 Mar 2017



The mechanical behaviour of granular materials at the macro-scale depends on a combination of different factors, such as the density and the fabric of the assembly, the shape and roughness of particles and the presence of bonds between them. Historically, the behaviour of soils has been studied using the means of Continuum Mechanics, which enabled the formulation of the fundamental theory of Critical State Soil Mechanics, while in more recent years the behaviour of granular materials has been studied also using discrete mechanics theory, thanks to the gain in popularity of the Discrete Element Method. Therefore, micro-scale approaches have been developed in order to study the interactions between grains and the way these govern the behaviour of a whole assembly at macro-scale.

The present work has intended to investigate the contact response between two discrete soil elements (in particular sands, but also others, such as clay scales) in order to describe it in terms of compressibility and shearing behaviour. This was achieved by means of laboratory tests carried out using a custom-made Inter-Particle Loading Apparatus designed and built at the City University of Hong Kong. A first version of the apparatus was previously designed in order to apply combinations of forces and displacements along two directions. During this work, the Inter-Particle Loading Apparatus was upgraded in order to achieve more testing combinations along three orthogonal directions and a higher accuracy of results by means of a compliance reduction and equipment updates.

Different materials were characterised and tested, which were four natural sands (Leighton Buzzard quartz sand, biogenic carbonate sand, completely decomposed granite and Eglin sand), one artificial sand (crushed limestone), two scaly clays (Santa Croce di Magliano clay and Pisciolo clay) and two artificial granular materials (ceramic balls and chrome steel balls). Also, a cemented Leighton Buzzard sand was tested after preparing samples by means of a custom-made procedure in order to study the behaviour of two sand particles artificially bonded using Portland cement.

The Inter-Particle Loading Apparatus has enabled the investigation of the behaviour of these materials in both compression (except the cemented LBS) and shearing. Different load levels and environmental conditions were used during the tests in order to assess the influence of these variables on the contact behaviour of soils. Also, high quality tangential stiffness results have been plotted for these materials and the coefficients of inter-particle friction have been determined. Finally, comparisons between the experimental results and the most common theoretical models used in the theory of contact mechanics have been discussed for both compression and shearing.

The test results typically show a non-linear elastic response at the contact between pairs of particles in compression. The environmental conditions seem not to play a significant rôle in the shearing contact behaviour of particles, while the tangential stiffness generally increases for higher confining normal forces. The angles of inter-particle friction are lower for those materials stiff and characterised by a low surface roughness and they are also significantly lower than the typical angles of shearing resistance determined for the same materials at the macro-scale.