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Unsaturated Hydro-Chemo-Mechanical Modelling Based on Modified Mixture Theory

Chen, Xiaohui

[Thesis]. Manchester, UK: The University of Manchester; 2010.

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Abstract

New unsaturated coupled models have been developed for fluid transport in deformable rock by using modified mixture theory rather than a fully mechanics-based approach. These models include the following: an unsaturated hydro-mechanical coupled model for both non-swelling and swelling materials, in which a new coupled formulation for hydration swelling rock has been included; and an unsaturated hydro-mechanical-chemo coupled model, incorporating a new coupled formulation including osmosis flow and an unsaturated version of Darcy’s law which has been extended by including osmosis effects.Modified mixture theory is mainly based on non-equilibrium thermodynamics. Helmholtz free energy is used to give the energy relationship between the fluids and solid and, by using the Gibbs-Duhem equation, the interactions between different fluids such as gas, water and chemical can be obtained. In this research, general coupled formulations for both large small and deformations have been obtained. For swelling rocks, the water between the clay platelets can be modeled by including the difference between the free energy of whole domain and that of the pore water plus the solid skeleton. By assuming small deformations, the final equations can be compared with those derived using the mechanics approach.The new coupled models have been tested by carrying out simple benchmark numerical simulations using finite elements. Problems analyzed include: (1) the consolidation of saturated swelling rocks in which the hydration swelling effects on consolidation have been analysed in detail; (2) the desaturation and resaturation of seasonally affected rocks around tunnels; (3) the desaturation stage for swelling rocks used in the containment of nuclear waste disposal; (4) chemical transport in very low permeability rock used for nuclear waste disposal, in which particular attention has been focused on osmosis flow and chemical consolidation. In summary, this thesis extends modified mixture theory and develops new coupled formulations which can be applied to deep nuclear waste disposal, including tunnelling, drilling and chemical transport in low permeability host rock.