Ventini, Roberta (2020) Mechanical behaviour of rockfill with different degrees of saturation. [Tesi di dottorato]

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Coarse-grained soils are used in many civil engineering structures, such as land reclaiming, rockfills embankments and dams. In order to ensure an appropriate performance of these structures in working conditions, it is required to understand the main factors affecting the mechanical behaviour of granular materials. Looking for answers to these problems, the thesis, of a distinctly experimental nature, is aimed at the analysis of the main state variables influencing the stress-strain response of coarse-grained soils. After an in-depth examination of the scientific literature, the thesis describes the equipment and experimental procedures used, with emphasis on a "large" triaxial cell for specimens of an unconventional size (diameter equal to 200 mm and height of 410 mm). This prototype is capable to investigate on the behaviour of saturated and partially saturated granular materials, as well as their mechanical response on transition from unsaturated to the saturated state. The contribution of unsaturated soil mechanics is required for a proper understanding of the basic mechanisms of soil deformation that occur in a wide variety of engineering situations. In fact, the environmentally induced change in water content is generally the triggering mechanism of soil deformations in many situations involving unsaturated soils. This is a well-known phenomenon which has received a continuous interest ever since the widespread use of rockfill in dams for which rapid “collapse” settlements have been associated with water impoundment. The extensive experimental program presented starts from the description of the procedures used for the preparation of compacted specimens of two gravels with different mineralogy. The aim is to highlight how the characteristics of individual particles and the packing conditions affect the mechanical behaviour. For both materials, the results of mechanical tests obtained using grain size distributions different by uniformity coefficient and maximum particle size are presented and critically discussed. For each of the materials resulting from the different mineralogy and the grain size distribution, the influence of several stress and hydro-volumetric state variables on their mechanical response is analysed. This analysis leads to the following two main topics: i) the change of the grain size distribution due to particle breakage, and ii) the relationship between the stress-strain response and particle breakage. Concerning this second topic, a micromechanical analysis has been performed to justify the observed changes of the grain-size distribution due to variations in the stress paths followed during the tests. Some of the tests performed are aimed at assessing the effect of the degree of saturation and the transition from partial to total saturation on the stress-strain behaviour of the tested rockfills. Once the factors affecting rockfill behaviour have been identified, a model able to predict the influence of the relevant factors on the performance of rockfill materials has been introduced, with specific insights into some aspects such as the effects of particle breakage and degree of saturation on their behaviour.

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