This paper describes an experimental study examining the influence of the mechanical and geometrical properties of the constituent grains on the overall material response of cohesionless granular materials. Glass ballotini were used as an analogue soil; their relatively simple geometry allowed the influence of particle shape and inter-particle friction to be examined independently. Techniques were developed to control the surface roughness of the ballotini to facilitate a parametric study. The particle shape was also varied by crushing the ballotini. At the micro-scale, the particle characterisation included accurate measurements of inter-particle friction, contact stiffness, particle surface roughness and particle shape. At the macro-scale the sensitivity of overall material response to changes in surface roughness and geometry was characterised using triaxial tests and oedometer tests on smooth spherical ballotini, roughened ballotini and crushed angular ballotini. Compression tests indicated that the initial load deformation response at particle–particle contact points is significantly softer than previously believed. Optical interferometry of particles after single particle–particle shearing tests confirmed that plastic strains occurred at the contact point, which were related to plastic yield. A Hertzian response was only seen at higher contact loads. A clear relationship between the inter-particle friction and the particle surface roughness was found. However, the macro-scale experiments indicated that while the material response may be slightly dependent on the surface roughness and friction, the influence of particle shape is very much more significant.

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A new apparatus is described that measures interparticle friction between sand-sized grains over relatively large displacements and also under immersion in a fluid. Its relatively simple design allows the key calibrations to be checked by statics. An analysis of the geometry of simple spherical particle contacts and the forces at those contacts revealed that there are strict constraints on the permissible stiffness of the interparticle friction apparatus to avoid stick-slip behaviour. Tests on ball bearings gave highly repeatable data, while others on glass ballotini revealed a significant effect of ambient humidity on the data obtained. The interparticle friction was found to increase with the roughness of the ballotini. Immersion in water increased the interparticle friction slightly for both the ballotini and quartz sand particles, while immersion in oil reduced the friction considerably for the quartz sand, especially at higher contact force levels

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Single-particle compression tests, in which an individual sand grain is vertically compressed between two rigid horizontal platens, are often used in particle-scale soil mechanics studies. They are useful index tests to examine the susceptibility of a given sand to particle breakage; they provide information for calibration of particulate discrete-element models that capture crushing; and they can give information on size–strength relationships. The test is conceptually simple, but the response of an irregular particle in these compression tests is not straightforward. During compression the particle can rotate. Both horizontal and vertical forces are induced at the particle–platen contacts, and so there may be frictional sliding at the contact points at the same time as, or prior to, compression of the bulk particle. Asperities can yield, changing the particle geometry. The variation in the response mechanism during compression leads to a load–deformation response that is not always easy to interpret. This paper describes two relatively simple analytical studies of an irregular particle in a particle compression test. The susceptibility of the particle to rotation under the applied compressive force is shown to depend on the particle geometry and the particle–platen friction. The rotation of the particle is shown to induce a kinematic degradation or reduction in the effective stiffness of the system, and the system stiffness depends on the particle size. Frictional sliding at the contact points will also cause a reduction in stiffness. These observations may have implications not only for the test itself, but also for the response of irregular particles participating in the strong force chains in stressed granular materials.

It is well established that particle geometry is a key parameter influencing the response of a granular material as well as its packing density. There are many challenges associated with obtaining data for full 3D quantification of the geometry (shape) of large numbers of particles. Consequently we typically assess particle shape by considering two dimensional images of the particles. This paper proposes a technique for developing a three dimensional description of particle geometry by combining sets of two dimensional images with multiple orientations. The paper establishes a relationship between the two‐dimensional measure of particle geometry circularity (as defined by ISO standard 9276‐6∕2006) and the three‐dimensional measure sphericity as defined by Wadell. An analytical comparison between these measures is achieved by considering a set of scalene ellipsoids (with three differing principal diameters). Each ellipsoid was systematically rotated to obtain a set of 2D projections and the circularity of each projection was quantified. The results indicate that there is a close relationship between the mean circularity and Wadell’s sphericity. The results of this analysis have implications for modern image analysis based technologies that can automatically assess the shape of large numbers of particles. Data for real coarse sand size particles obtained using the QicPic apparatus (Sympatec) are reviewed in light of the findings of the analytical parametric analysis

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