This paper presents a study of the micromechanical behaviour of crushable soils. For a single grain loaded diametrically between flat platens, data are presented for the tensile strengths of particles of different size and mineralogy. These data are shown to be consistent with Weibull statistics of brittle fracture. Triaxial tests on different soils of equal relative density show that the dilatational component of internal angle of friction reduces logarithmically with mean effective stress normalized by grain tensile strength. The tensile strength of grains is also shown to govern normal compression. For a sample of uniform grains under uniaxial compression, the yield stress is related to the average grain tensile strength. If particles fracture such that the smallest particles are in geometrically self-similar configurations under increasing geotoscopic stress, with a constant probability of fracture, a fractal geometry evolves with the successive fracture of the smallest grains, in agreement with the available data. A new work equation predicts that the evolution of a fractal geometry gives rise to a linear compression line when voids ratio is plotted against the logarithm of geotoscopic stress, in agreement with published data. Cet exposé présente une étude du comportement micro mécanique des sols concassables. Pour un seul grain chargé de manière diamétrale entre des platines plates, nous présentons les données de résistance à la rupture des particules de différentes dimensions et minéralogies. Nous montrons que ces données correspondent aux statistiques de Weibull sur la rupture de fragilité. Les essais triaxiaux sur divers sols de meême densité relative montrent que le composant de dilatation de ľangle interne de friction baisse de manière logarithmique en mê"me temps que la contrainte effective moyenne normalisée par la r6sistance à la rupture du grain. Nous montrons aussi que la résistance à la rupture des grains gouverne la compression normale. Pour un échantillon de grains uniformes sous compression uniaxiale, la limite 61astique est liée à la rupture moyenne des grains. Si la rupture des particules est telle que les plus petites particules sont dans des configurations à similitude géomé trique intrinsèque sous un effort geotoscopique de plus en plus grand, avec probabilité constante de rupture, une géométrie fractale apparaît avec la rupture successive des grains les plus fins, en accord avec les données disponibles. Une nouvelle équation de travail prédit que ľévolution de la géometrie fractale provoqueľPapparition ďune ligne de compression linéaire quand le taux de pores est représenté sous forme de courbe par rapport au logarithme de ľeffort geotoscopique, en accord avec les données pub-liées.

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Traditional railway foundations or substructures have become increasingly overloaded in recent years, owing to the introduction of faster and heavier trains. A lack of substructure re-engineering has resulted in maintenance cycles becoming more frequent and increasingly expensive. Two significant problems arising from increasing axle loads are differential track settlement and ballast degradation. One potential method of enhancing the substructure is to manipulate the level of ballast confinement. To investigate this possibility, a series of highfrequency cyclic triaxial tests has been conducted to examine the effects of confining pressure and deviator stress magnitude on ballast deformation (permanent and resilient) and degradation. Experimental results indicate that, for each deviator stress considered, an 'optimum' range of confining pressures exists such that degradation is minimised. This range was found to vary from 15-65 kPa for a maximum deviator stress of 230 kPa to 50-140 kPa when deviatoric stresses increase to 750 kPa. Ballast specimens tested at low confining pressures indicative of current in situ conditions were characterised by excessive axial deformations, volumetric dilation, and an unacceptable degree of degradation associated mainly with angular corner breakage. The results suggest that in situ lateral pressures should be increased to counteract the axle loads of heavier trains, and practical methods of achieving increased confinement are suggested.

Discrete-element modelling has been used to investigate the micro mechanics of one-dimensional compression. One-dimensional compression is modelled in three dimensions using an oedometer and a large number of particles, and without the use of agglomerates. The fracture of a particle is governed by the octahedral shear stress within the particle due to the multiple contacts and a Weibull distribution of strengths. Different fracture mechanisms are considered, and the influence of the distribution of fragments produced for each fracture on the global particle size distribution and the slope of the normal compression line is investigated. Using the discrete-element method, compression is related to the evolution of a fractal distribution of particles. The compression index is found to be solely a function of the strengths of the particles as a function of size.

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A discrete element model has been developed for geogrid-reinforced ballast. A model for unreinforced ballast has first been developed and evaluated using simulations of large-scale triaxial experiments and comparing with available data. A model for the geogrid has also been developed by bonding many small balls together to form the desired geometry and evaluated by simulating standard tests. The discrete element method (DEM) has then been used to model the interaction between ballast and geogrid by simulating pull-out tests and comparing with experimental data. The DEM simulations have been shown to predict well the peak mobilised resistance and the displacement necessary to mobilise peak pull-out force. The effect of the ratio of the geogrid aperture size to ballast particle diameter on pull-out resistance has also been investigated. The zone of influence of the geogrid has also been examined by simulating larger-scale pull-out tests in addition to cyclic triaxial tests. The method holds much promise as a tool for investigating aggregate–geogrid composite systems with a view to choosing appropriate geometries to optimise performance.