Development of Micromechanically-Based Models for Unsaturated Geomaterials

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Emerging geomechanics problems associated with infrastructure, energy and the environment present challenges to basic tenets of geotechnical engineering. For instance, unsaturated, thermal and coupled conditions demand analyses that go beyond traditional theories which are mostly phenomenological in nature. Unsaturation in the simplest case refers to the presence of more than one fluid, e.g. liquid and gas separated by interfaces within the pore space. This results in the behaviour of an unsaturated medium being vastly different from the dry or fully saturated case. In fact, unsaturation confers the medium with an apparent cohesion, a higher strength and stiffness that are a function of moisture and temperature. This gain in strength is evanescent under varying hydraulic and thermal loads such as climatic fluctuations or gassy conditions. As such, strength could be rapidly lost causing spontaneous collapse failures in geostructures.***The overall goal of this research is to pursue a more fundamentally scientific study of unsaturated conditions in geomaterials as multiphasic systems. This will be accomplished by exploring the various scales down to the particle and pore scales, including the water menisci. Our understanding of unsaturated media will be extended by recognizing new phenomena key to the improvement of design in engineering practice in terms of judgement, quality and safety. The adopted approach is multiscale, probing into the nature of the pressures exerted by the individual phases, including interfaces separating them, on particles that are in mutual contact through mechanical forces. An important aspect will be to elucidate the role of interfacial tension that exists within interfaces on multiphase flow, as well as the strength, deformation and failure characteristics of unsaturated geomaterials.***The proposed research mathematically examines force transport in a heterogeneous random granular assembly with particles connected by discrete liquid menisci as a representative elementary volume. As such, stress contributions from the various phases, including the interfaces, can be worked out through a homogenization scheme knowing the statistics of the grain packing and liquid bridges. This will elucidate many well-known processes driven by interfacial tension such as capillary or suction stresses as a function of saturation. In contrast to previous work, the analysis will encompass the spatial distribution of liquid menisci, fabric anisotropy of solid skeleton, temperature, and identify an effective stress that controls strength and deformations. Besides determining the various components of the stress tensor in an unsaturated medium, the associated strain tensor will also be derived following an energy approach.***Applications of this research include toxic waste disposals, contaminant transport in porous media, and CO2 injection in geological formations.**