A model that describes the small-strain behaviour of soils is derived using micromechanics theory. The model allows examination of the effects of fabric anisotropy, stress conditions and contact characteristics on the small-strain modulus of soils. The closed-form solutions of the small-strain modulus are presented for the case of an isotropic fabric assembly under isotropic stress conditions. A fabric tensor is used to model the fabric anisotropy, and the small-strain modulus for the case of cross-anisotropic fabric assembly under general stress conditions is numerically calculated. The effect of the contact condition between soil particles is examined by incorporating three different contact laws (the linear elastic, Hertz±Mindlin and rough-surface contact models) into the model. It was found that the numerical results using the rough-surface contact model compare well with the published experimental data, and that the model provides microscopic insight into the small-strain behaviour of soils observed in the laboratory.
The effects of initial soil fabric on the shear behaviour of granular materials are investigated by employing distinct element method (DEM) numerical analysis. Soil specimens are represented by an assembly of non-uniform sized spherical particles. DEM specimens that have different initial contact normal distributions were prepared at two different densities (loose and dense) and then isotropically consolidated triaxial tests were simulated with the following conditions: (a) different drainage conditions (undrained/drained), (b) different modes of shearing (compression/extension), and (c) different directions of shearing (vertical/horizontal). The numerical analysis results are compared qualitatively with the observed experimental data and the effects of initial soil fabric on resulting soil behaviours are discussed. The discussions include the effects of specimen reconstitution methods, effects of large preshearing, and anisotropic characteristics in drained and undrained conditions. The effects of initial soil fabric on the quasi-steady state are also investigated. The numerical analysis results can systematically explain that the observed experimental behaviours of sand are due principally to the effects of initial soil fabric. The outcome provides insights into the observed phenomena in microscopic view.
An experimental study of the cross-anisotropic elastic parameters was performed on two heavily overconsolidated clays: London Clay and Gault Clay. The study utilised a special triaxial testing apparatus that incorporates local strain measuring system and elastic wave velocity measurement system. Isotropically consolidated drained compression tests and shear wave velocity measurements were undertaken on both vertically and horizontally cut specimens. The experiments were performed under in situ isotropic stresses to study the inherent anisotropic characteristics of the clays in terms of the elastic stiffness. The deformation characteristics during triaxial compression were carefully investigated to obtain various elastic moduli and Poisson ratios.
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