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Behaviour of a collapsible structured unsaturated fill material

El Mountassir, Grainne (2011) Behaviour of a collapsible structured unsaturated fill material. PhD thesis, University Of Strathclyde.

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Abstract

Many geotechnical engineering problems are related to the behaviour of unsaturated soils. Despite this many studies on unsaturated soils have used artificially created soil mixtures investigated primarily to validate constitutive models, or expansive clays researched for their proposed use in geological disposal applications. This thesis investigates a fill material from a typical engineering application: flood embankments located along the Bengawan Solo River in East Java, Indonesia. Conventional load and soak oedometer tests carried out on this fill, highlighted the collapsible nature of this material under as-constructed conditions. A microstructural study conducted to investigate the fabric changes occurring due to loading and wetting, indicated that the undisturbed material may already have undergone the process of volumetric collapse in-situ. This research argues that volumetric collapse upon wetting may contribute to flood embankment instabilities, although the mechanism is often overlooked in favour of more spectacular failure modes. The isotropic compression and shearing behaviour of the Bengawan Solo fill was also investigated under saturated, suction controlled and constant water content conditions. Isotropic compression results indicated that both yield stress and post-yield compressibility increased with increasing suction. The angle of friction and cohesion determined from triaxial tests were both found to increase with increasing suction. An irregular compaction curve was found to exist at low compactive energy levels. The nature of the soil aggregates (created on mixing with water) was found to be responsible for the compacted structure created at low dry densities. This research isolated the influence of compacted fabric on volumetric behaviour by conducting suction controlled oedometer tests on specimens with different soil structures but similar conditions of moisture content and dry density. Results from suction controlled tests suggest that the influence of compacted fabric alone (separate from initial state conditions of moisture content, dry density, mean net stress and suction) is small, and therefore in terms of constitutive modelling it is sufficient to assume one set of model parameters for a given soil (of specific particle size distribution and mineralogy) regardless of compaction conditions.