As climate change escalates and the demands for energy resources increase, modern geotechnical engineering must tackle critical challenges to ensure sustainable development and enhance the resilience of infrastructure in society. The coupled chemo-hydro-mechanical processes in multiphase materials present significant challenges in geotechnical engineering, particularly for applications like carbon sequestration, geological disposal of nuclear waste, and hydraulic fracturing with reactive fluids, all of which involve highly heterogeneous and strongly anisotropic multiphysics environments. This dissertation introduces a multiphysical computational framework specifically designed to address the challenges associated with these unconventional applications. In this dissertation, we consider not only the local multiphysical coupling effects in the constitutive model but also the nonlocal effects arising from pore fluid flow, chemical species convection and diffusion, chemical reactions occurring in both solid and fluid constituents, and damage due to fluid pressure acting on fractures in the solid. We have integrated all these physical processes and developed a single unified model capable of handling the complex hydro-chemo-mechanical responses of geomaterials under varying geochemical conditions, confining pressures, and external loading scenarios. This computational framework offers a comprehensive simulation tool to investigate the long-term stability of geomaterials, which is determined by the intensity of chemical reactions under specific temperature and pressure conditions (assuming an isothermal condition in this dissertation), as well as the sustainability of geotechnical infrastructure in erosive environments driven by both mechanical and chemical processes. Three key aspects of engineering applications related to the effects of chemical reactions in geotechnical engineering are addressed. Firstly, we have integrated a complete calcite reaction system into poromechanics to couple pore geochemistry with poroelasticity theory. This integration is capable of predicting the geomechanical response essential for long-term stability analysis in \ch{CO2} sequestration engineering. Key features of this model include a multi-field finite element approach, local-equilibrium explicit geochemistry characterization of the calcite dissolution/precipitation reaction system, a robust algorithm for sequentially coupling pore geochemistry with poromechanics, and strategies to enhance the computational efficiency of solvers. Secondly, for applications involving acid working fluids in hydraulic fracturing, we have extended and adapted previous models within the phase field method framework. This extended integration effectively addresses the effects of chemically assisted fracturing in hydraulic fracturing operations. The key innovations of this model are the implementation of the phase field method to capture crack behaviors with poromechanics, the modeling of acid fluid transport in porous media and fractures, and its application to multiple mineral reaction systems. Thirdly, we have proposed a constitutive model that incorporates pore geochemistry and the pressure dissolution effect into internal variables, effectively capturing the chemical reactions contributing to softening in geomaterials. This model effectively illustrates and predicts chemically induced weathering or damage in granular porous media, such as sinkholes and subsidence. Derivations of a thermodynamically-based degradation index consider the influences of pore geochemistry and contact forces between grains and bonds. The model also proposes cross-scale relationships that consider reaction effects from individual particle sizes to particle aggregates. Furthermore, these relationships are incorporated into classical Cam-Clay-type models, along with the derivation of a consistent tangent modulus. / Dissertation / Doctor of Philosophy (PhD) / This thesis presents the comprehensive behaviors of geomaterials under mechanical, fluid, and chemical interactions, which result in displacement and cracking. Since there is no existing software or simulation tool that includes all the physical behaviors considered in this dissertation, the development and implementation of these physical mechanisms, followed by testing and analysis for engineering problems, constitutes the main contribution of this work. The newly developed simulation tool ranges from simulating the mechanical behavior of porous media saturated with water and reactive fluid to modeling the seepage of water/reactive fluid that triggers damage (cracks) in the porous media. This simulation tool can effectively analyze engineering problems that focus on the interactions between the working fluid and the host solid matrix under complex solution conditions. Examples include modeling carbon sequestration in saline aquifers and the storage of nuclear waste in subsurface repositories etc. The simulation tool proposed in this thesis incorporates rigorous mathematical derivations, efficient and accurate multiscale discretization techniques, robust non-iterative and iterative numerical coupling strategies, and thorough comparisons between numerical results and experimental/laboratory data. Simultaneously, it is important to recognize the model's limitations. Although the model assumes local equilibrium and interactions between physical mechanisms, it cannot fully capture all behaviors under these assumptions due to the restrictions in our understanding and potential constraints of numerical methods.

Existing numerical techniques for modeling saturated deformable porous media are based on homogenization techniques and thus are incapable of performing micro-mechanical investigations, such as the effect of micro-structure on the deformational characteristics of the media. In this research work, a numerical scheme is developed based on the parallelized hybrid lattice-Boltzmann finite-element method, that is capable of performing micro-mechanical investigations through direct numerical simulations. The method has been used to simulate compression of model saturated porous media made of spheres and cylinders in regular arrangements. Through these simulations it is found that in the limit of small Reynolds number, Capillary number and strain, the deformational behaviour of a real porous media can be recovered through model porous media when the parameters porosity, permeability and bulk compressive modulus are matched between the two media. This finding motivated research in using model porous geometries to represent more complex real porous geometries in order to perform investigations of deformation on the latter. An attempt has been made to apply this technique to the complex geometries of ªfeltº, (a fibrous mat used in paper industries). These investigations lead to new understanding on the effect of fiber diameter on the bulk properties of a fibrous media and subsequently on the deformational behaviour of the media. Further the method has been used to investigate the constitutive relationships in deformable porous media. Particularly the relationship between permeability and porosity during the deformation of the media is investigated. Results show the need of geometry specific investigations.