Effect of Particle Shape on the Mechanical Behavior of Natural Sands

A database of the mechanical behavior of 25 natural sands was compiled from the existing literature. Particle shape and size analysis, obtained by dynamic imaging analysis, for each material in the database has subsequently been linked to its mechanical properties; selected sands were also subject to interferometry study for particle surface roughness measurements. This paper reviews the effect of the particle shape properties of these sands on their Critical State and stiffness parameters, introducing a new parameter to optimize the correlations

Quantitative Evaluation of Particle Morphology

In geomechanics research shape is most often qualitatively assessed. Various definitions on how to quantify shape have been applied in the literature. This paper assesses the feasibility of applying these definitions to digital images of sand grains. Firstly the way in which size can be calculated from these digital images is discussed, then the sphericity and convexity definitions proposed by Sympatec (2008) are considered. These definitions of sphericity and convexity (SQP, Cx) are relatively easy to apply. By relating SQP and Cx to the qualitative measures of particle shape that are most often used in geotechnical sample description, we argue that there is significant scope to introduce these measures to engineering practice (Altuhafi et al., 2012). We show that distributions of convexity and sphericity obtained in 2D and 3D analyses differ

Effects of particle breakage and stress reversal on the behaviour of sand around displacement piles

The stresses acting in the soil mass adjacent to the tips and shafts of displacement piles during installation and loading in medium-dense sand have been simulated in triaxial stress path tests on Fontainebleau NE34 sand. The very high normal and shear stresses recorded in calibration chamber model pile tests involving the same sand were first reproduced in high-pressure triaxial tests, so changing the sand's physical properties markedly. The behaviour of the mutated sand was then examined in second, lower stress, stages of the same experiments, demonstrating important changes in the sand's mechanical behaviour, including a significant increase in the angle of shearing resistance and a relocation of the sand's critical state line in the e−log p′ plane. Image analysis confirmed changes in the sand particles' micro-characteristics. The particles' size distributions altered and grain surface roughness increased markedly, while particle sphericity was only mildly affected. Similar surface roughness changes were noted between the particulate characteristics of specimens examined after the triaxial laboratory tests and those sampled from around the shafts of the calibration chamber model piles

A hypothesis on the relative roles of crushing and abrasion in the mechanical genesis of a glacial sediment

Relatively new techniques of optical microscopy and interferometry have been used combined with fractal and modal analyses to study possible deformation mechanisms in glacial sediment from Langjökull, Iceland. Previous studies have shown that the Langjökull till has reached an ultimate grading during deposition and transport. The results presented here add information on the deformation mechanisms that it may have suffered until and after reaching this ultimate grading. The modal and fractal analyses suggest high mechanical weathering. This is supported by the shape and texture of particles determined using the different imaging techniques of laser scanning, optical microscopy combined with three-dimensional image construction, interferometry and scanning electron microscopy. The larger particles have highly weathered surfaces, indicating severe abrasion to their surface during transport. The finer particles show less abrasion stamp and higher angularity. They also bear evidence of relatively fresh fractures. This seems to indicate that as the sediment became better distributed during transport, breakage moved from the larger to the smaller particles, with the larger grains only suffering abrasion. This supports the hypothesis of fractal breakage in this type of soil. © 2011 Elsevier B.V.link_to_subscribed_fulltex

The mechanics of subglacial sediment: An example of new "transitional" behaviour

A series of isotropic compression tests and drained and undrained triaxial compression tests have been performed on glacial sediment from Iceland. Langjökull sediment, which is well graded, is thought to have reached a critical grading during deposition and transportation. Multiple parallel normal compression lines (NCLs) were found, but a unique critical state line (CSL) could be identified. This is unlike other so-called "transitional" soils, whose grading varies between reasonably well graded to gap graded, which tend to have distinct NCLs and critical state lines depending on the specimen density. It is thought that in the case of the Langjökull sediment studied, its particular strain history that involved incessant shearing during deposition accounts for the difference in behaviour. This provides the interesting case of a soil that has been crushed to a critical grading in situ, which depends on the mineralogy of the grains, which was then sampled and tested. Despite the unique grading, samples with a range of different void ratios can be prepared and the combination of grading and density seems to set a fabric that cannot be changed by compression, resulting in multiple parallel NCLs. At the critical state, however, the fabric has been destroyed and the CSL is unique.published_or_final_versio

Particle breakage in glacial sediments

In most areas of glacial deposits, soils were deposited by ice sheet expansion or contraction, and produced by deformation and mixing of pre-existing soft sediments. The resulting soil (till) is well graded. In some cases the remixing and shearing of sediments by glaciers has caused a redistribution of particle sizes by abrasion and crushing. The breakage capacity of glacial sediments recovered from the foreshore of two different glaciers in Iceland, Langjokull (West) and Breidamerkurjokull (South-East), is presented. The initial gradings of the sediments are distinct, the Langjokull sediment being better graded than the Breidamerkurjokull sediment. Results from ring shear tests performed on both kinds of sediments show that the Breidamerkurjokull sediment still has capacity for breakage while particles from the Langjokull sediment do not seem to break even when the soil is sheared to very large strains. It is suggested that the latter sediment has reached a critical grading while being deposited. © 2006 Taylor & Francis Group.link_to_subscribed_fulltex