Modellazione di mezzi porosi soggetti a condizioni di carico complesse

Many results from cyclic triaxial experiments indicate that porous media, such as clays, exhibit various long-term behaviours under different cyclic stress ratios (CSRs). These can be classified into three main categories, namely, cyclic shakedown, cyclic stable and cyclic failure. Modelling these soil deformation responses, along with pore pressure and other fundamental cyclic aspects, such as closed hysteresis cycles and degradation, is still an open challenge, and research to date is limited. In order to properly describe and capture these characteristics, an enhanced plasticity model, based on the bounding surface and stress distance concepts, is developed here. In detail, a new uniform interpolation function of the plastic modulus, suitable for all loading stages, is proposed, and a new damage factor associated with the plastic shear strain and the deformation type parameter, is also incorporated into the plastic modulus. Accordingly, cyclic shakedown and cyclic failure can be distinguished, and degradation is achieved. Closed hysteresis loops, typical of clays, are obtained through a radial mapping rule along with a moving projection centre, located by the stress reversal points. The model is further extended for the description of non-isotropic mechanical behavior of soil by incorporating both rotational and distortional hardening rules into the bounding surface framework. The model has been validated through both isotropically and anisotropically consolidated samples. Comparisons between numerical and experimental results confirm the goodness of the constitutive approach, capable of correctly capturing anisotropy and reproducing the key aspects of clays behaviour under complex loading conditions. Apart from the unique cyclic behavior of soil, another aspect of porous media, e.g., geomaterials, that distinguishes them from other materials is their particular fracture characteristic, since the existence of fluid pressure coupled with external load can contribute the growth of the fracture of solid phase while the fracture growth is only relevant to the effect of external load on purely solid mechanical property for traditional material. In this regard, the Natural Hydraulic Fractures (NHFs) in sandstone are studied in the next part of the thesis. Compared to Induced Hydraulic Fractures (IHFs), NHFs do not have an increasing but rather experience a decreasing pore pressure in the fracture zone. Numerous studies have been reported on the interaction between NHFs and IHFs, while rare research has addressed why the former exist and how they grow. This research about NHFs aims to provide a theoretical basis for their generation and intend to justify fracture criteria of porous geomaterials within the framework of Theory of Porous Media (TPM) embedded with the phase field method. For this purpose, permeability was first identified as a key parameter for the generation of tensile stresses that would cause fracture or damage within the sandstone via a basic biphasic model. Further, the influence of an important fracture material parameter, the energy release rate, Gc, on the growth of NHFs was studied. An estimated critical range of Gc > 110 N/mm that does not allow the growth of NHFs is given for reference for future studies. In addition, the growth of NHFs shows a strong dependence on permeability. With higher permeability the material is only damaged and produces no fractures at all, while NHFs tend to grow with lower permeability even though the external force of both cases remains the same. The study of NHFs provides relevant insights for engineering practice, e.g., measures to in- crease either the permeability or the energy release rate of the material in order to avoid excessive fracture growth within geomaterials. The TPM framework along with the phase field method can also serve as a powerful tool for engineering practice in general situ state.