A series of ring shear tests was conducted to investigate the development of particle breakage with shear strain for a carbonate sand. It was found that at very large displacements the soil reached a stable grading, but that the final grading was dependent on both the applied normal stress and the initial grading. The particle breakage caused a volumetric compression, which again ceased only when the stable grading had been attained, emphasising that critical states as observed at much smaller strains in triaxial tests are not rigorously defined. Despite the severe degradation of the soil particles the mobilised angle of shearing resistance was found not to change significantly.Nous avons mené une série d'essais de cisaillement annulaire afin d'enquêter sur le développement des cassures de particules avec une déformation de cisaillement pour un sable carbonate. Nous avons trouvé qu'avec de très importants déplacements, le sol parvenait à une granulométrie stable mais que la granulométrie finale dépendait de la contrainte normale appliquée et de la granulométrie initiale. La cassure des particules provoque une compression volumétrique qui, encore une fois, ne cesse que lorsque la stabilité granulométrique est atteinte, soulignant le fait que les états critiques tels que ceux qui sont observés avec des déformations bien plus petites dans les essais triaxiaux ne sont pas rigoureusement définis. Nous avons trouvé que malgré la dégradation sévère des particules de sol, l'angle mobilisé de résistance au cisaillement ne changeait pas de manière significative.

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.

It is commonly accepted that the onset of particle breakage in sands during compression marks the start of yielding. Although particle breakage is strongly associated with the tensile strength of a single soil grain, initial density and initial sample grading have great influence on the probability of particle breakage. This paper examines the effect of initial grading and density on the sample behaviour during one-dimensional compression for three sands with distinct mineralogies. It was found that a unique normal compression line is the outcome of a large amount of breakage in poorly graded samples and that by changing the initial grading to a better graded sample a significant reduction in particle breakage is observed, until for very well-graded samples no significant particle breakage can be measured. At this point a difficulty in defining a unique normal compression line for the sample was found and a transitional behaviour was identified. In addition to particle size, the paper examines the changes to some important particle characteristics such as particle shape and particle surface roughness in an attempt to relate the soil behaviour to the nature of the microscopic particle damage during yielding.

The behaviour of artificially cemented carbonate sand was investigated in triaxial tests at confining pressures of up to 9 MPa. The results show that an important effect of cementing is a reduction in specific volume resulting from the increase in fines content This influences both the stress-strain behaviour and the peak strength at strains beyond those required to fracture the cement bonding. Comparisons between the behaviour of cemented and uncemented soils should, therefore, be carried out on samples with the same gradings. For cemented samples it is possible to identify a yield curve outside the state boundary surface of the uncemented soil. A framework for the behaviour has been defined which depends on the relative magnitudes of the confining pressure and cement bond strength. The behaviour of a natural cal-carenite agreed well with this framework which is also likely to be applicable to other cemented soils. Le comportement d'un sable carbonate artificiellement cimenté a été étudié à l'aide d'essais tri-axiaux à pression de confinement pouvant atteindre 9 MPa. Les résultats obtenus montrent que la cimentation se traduit par une diminution du volume spécifique due à une augmentation du nombre de particules fines. Ceci modifie le comportement contrainte-déformation ainsi que la résistance ultime pour des déformations supérieures à celles provoquant la rupture des liaisons du ciment. L'étude comparative du comportement de sols cimentés et de sols non-cimentés a donc dû être réalisée sur des échantillons de mâme granulo-métrie. Pour des échantillons cimentés, une courbe de rupture située en dehors de l'enveloppe limite d'un sol non-cimenté a été mise en évidence. Une classification de comportement a été élaborée en fonction des amplitudes relatives de la pression de confinement et de la résistance des liaisons du ciment. Le comportement d'une calcarénite naturelle est tout à fait en accord avec cette classification qui apparaît applicable à d'autres sols cimentés.

Particle breakage is of fundamental importance for understanding the mechanical behaviour of sands and is relevant to many geotechnical engineering problems. In order to gain new insights into the mechanism of breakage of individual sand particles under single-particle compression, this study combines mechanical tests with three-dimensional X-ray micro-computed tomography (μCT) performed 'in situ', that is, during loading. A novel mini-loading apparatus was developed to perform in-situ compression tests within a laboratory nanofocus X-ray CT. The tests were performed on eight particles, four Leighton Buzzard sand (LBS) particles and four highly decomposed granite (HDG) particles, to study their different fracture mechanisms. A series of image processing and analysing techniques was utilised to obtain both qualitative and quantitative results. The most important factors in determining the fracture patterns of the LBS and HDG particles were found to be particle morphology and initial microstructure, respectively. Versatile fracture patterns deviating from simple vertical splitting were observed, particularly in HDG particles. The change of morphology parameters during loading was found to depend on the fracture mechanisms and material properties, independently of their initial values. The fragments of both the LBS and HDG particles satisfy the fractal distribution, which indicates that the fragmentation is scale invariant. Different energy dissipation mechanisms were found. The energy dissipation by friction gradually prevails against the energy dissipated in generating new surfaces.

The mechanical behaviour of a typical biogenic carbonate sand was investigated using triaxial testing at standard and also at high pressures up to 8 MPa. The test results show that despite the particle breakage which is characteristic of these soils, their behaviour is still qualitatively similar to that observed for more commonly encountered soils, and is consistent with the principle features of critical state soil mechanics. The higher stresses used in these tests were an order of magnitude greater than are generally used in soils testing, and the values of some individual soil parameters were found to be outside the range of normal experience. Micrographic and gradings analyses were used to examine the influence of isotropic and shear stresses on particle breakage. Triaxial test results illustrate the influence of this breakage on the mechanical behaviour of the soil. Le comportement mécanique d'un sable carbonaté biogénique typique a été étudié par des essais triaxiaux à pression normale et aussi à des pressions élevées jusqu'a 8 MPa. Cet article présente les résultats des essais indiquant que malgré la rupture des particules qui est caractéristique pour ces sols, leur comportement reste du point de vue de qualité semblable à celui observé dans le cas de sols plus fréquemment rencontrés et s'accorde avec les caractéristiques principales de la mécanique des sols dans l'etat critique. Les contraintes les plus élevées employées au cours de ces essais étaient supérieures d'un ordre de grandeur à celles génera-lenient utilisées dans les essais effectués sur les sols et on a trouvé que les valeurs de quelques paramètres individuels du sol se trouvaient hors des valeurs normalement enregistrées. Des analyses micrographiques et granulométriques pour étudier l'influence exercée par les contraintes isotropes et de cisaillement sur la rupture des particules. Les résultats des essais triaxiaux illustrent l'influence de cette rupture sur le comportement mécanique su sol.

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