Multiphysical Modelling of Soils with a Focus on Microbially Induced Calcite Precipitation

In recent years, the complexity of geomechanical problems has greatly increased as a result of the demand for new and intricate types of applications. Experimental characterization and numerical prediction by means of multiphysical approaches are in that context crucial to yield a better understanding of the behaviour of soils by describing the contribution and interaction of the underlying physical processes. The principal objective of this PhD work is to address the multiphysical modelling of soils in a general framework covering two main issues. The first part of this work addresses the bio-chemo-hydro-mechanical processes involved in Microbially Induced Calcite Precipitation (MICP). Then, a model for the description of the visco-thermo-mechanical behaviour of soils is developed and the problem of nuclear waste disposal is numerically studied. Following the demand for understanding, controlling and predicting the soil response in these settings, a systematic reflexion is carried out throughout this work in order to identify the major processes and their couplings, and to incorporate them into a unified numerical tool. Microbially Induced Calcite Precipitation (MICP) has arisen as a natural alternative ground improvement technique which overcomes many of the limitations associated to traditional ground improvement methods. By temporarily regulating the concentration of bacteria and nutrients in a soil, a new engineering material can be generated through bacterially induced calcite cementation of the existent soil matrix. The issue of MICP is addressed through a comprehensive research study to better understand and describe the coupled phenomena of multispecies reactive grout transport in a saturated, deformable soil. An experimental study designed to assess the impact of different environmental factors and conditions on the propagation and kinetics of the MICP processes as well as on the bacterially enhanced mechanical properties is conducted. The propagation and kinetics of the processes involved in MICP are evaluated by column injection tests and the resulting material is evaluated by mechanical tests and soil structure studies. Based on the experimental results and the literature review, a unique predictive model of the behaviour of the porous media during MICP is presented. The general field equations describing the system are derived from the macroscopical balance equations and constitutive equations. The set of field equations is numerically discretised. Numerical examples are provided to validate the capabilities of the proposed model. An additional goal is the development of a specifically designed constitutive model for the mechanical behaviour of the MICP-treated soils based on the description and quantification of the observed processes under representative loading sequences. The equations are presented in a unified theoretical framework and the model is demonstrated to be able to tackle all specific features of the mechanical respon