Theoretical and numerical modeling of hydro-mechanical behavior of natural structuredgeomaterials in finite deformations

A number of important challenges emerging in modern geotechnical engineering, most of which related to sustainability issues, require a multiphysics approach considering the coupled balance equations of momentum, mass and energy. In considering these problems, several factors may play an important role. First of all, the behavior of natural geomaterials may be substantially affected by the presence of structure - intended as a combination of the effects of microstructural fabric and interparticle bonding - and its degradation with accumulated inelastic deformations. This last aspect of the material response is often responsible for the localization of the deformations into thin zones known as shear or compaction bands, depending on the nature of the deformation inside the band. In presence of such phenomena, the classical inelastic constitutive models based on the principle of local action cannot provide any indication about the thickness of the localized zone due to the lack of an internal length scale. A well-known consequence of this fact is that any attempt in solving numerically a boundary value problem by means of the classical, displacement-based, Finite Element Method is pathologically affected by mesh dependence of the numerical solution in the post-localization regime. Finally, a number of recent geotechnical engineering applications are characterized by significant changes in the soil mass geometry and very high deformation levels, which require the incorporation of geometric non-linearity in the mathematical formulation of the problem. Among them, we mention the evaluation of pile bearing capacity of offshore platforms; the modeling of subsidence phenomena associated to hydrocarbon extraction and sinkhole formation; the study of the effects of pile driving; the interpretation of cone penetration tests under undrained or partially drained conditions, and the modeling of slow slope deformations in presence of significant modifications of the slope geometry. The main objective of this thesis is to develop a multiphysics computational platform for solving fully non-linear coupled deformation and flow problems in natural structured geomaterials, capable of addressing all the aforementioned issues for the analysis of geotechnical applications of practical interest. To this end, a finite deformation, isotropic hardening, non-associative elastic-plastic constitutive model (the FDMilan model) has been developed to describe the mechanical behavior of a wide range of bonded natural geomaterials such as stiff overconsolidated clays, porous soft rocks or bio-improved soils. To deal with the occurrence of strain localization, the model has been equipped with a non-local version of the hardening laws. This approach is capable of regularizing the pathological mesh dependence occurring in the post-localization regime when adopting classical plasticity models. In order to define a suitable strategy for the numerical solution of practical problems of interest in the fully non-linear regime, the choice has been made to adopt the Particle Finite Element Method (PFEM), for its robustness, its efficiency and the similarity existing between PFEM and classical FEM in the formulation of the elements. The FDMilan model has therefore been implemented in the Particle Finite Element code G-PFEM, specifically developed for geomechanics applications within the computational platform Kratos Multiphysics. The G-PFEM code has then been applied to investigate the effects of soil destructuration and of the adopted internal length scale on the results of a series of simulated cone penetration tests with pore water pressure measurements (CPTu tests) on a natural structured clay soil. The results of the PFEM simulations show that the deformations around the piezocone are strongly affected by the soil characteristic length lc. For heavily structured soils, when lc is relatively small with respect to cone radius, the accumulated plastic deviatoric deformation field is characterized by clearly visible shear bands while, as lc increases, the detection of localized deformation regions becomes more difficult, if not impossible. This depends on the fact that, for large values of lc, the shear band width may be of comparable size to the cone radius. In all cases, the soil around the advancing piezocone is subjected to a very strong destructuration process, which leads to the complete loss of bond strength in a large region around the cone tip and shaft. As a consequence, the use of conventional cone factor values from the literature in the interpretation of the CPTu test performed in heavily structured soils could provide undrained strength values closer to the ultimate undrained shear strength. At the same time, the use of empirical correlation to estimate the yield stress in oedometric conditions would lead to a significant underestimation of the soil overconsolidation ratio.