The complexity of the mechanics of active clays and, more generally, of geomaterials with clay minerals, under saturated and unsaturated conditions, still provides relevant scientific challenges today. Equally relevant challenges are the understanding of the mechanics of unsaturated geomaterials under undrained conditions (and the consequent possibility to develop useful analytical tools) and the estimation of in-situ effective stress in geomaterials with a complex stress history (e.g., shales) (and the consequent possibility to develop appropriate estimation methodologies). This doctoral thesis aims to contribute to each of the above-mentioned scientific areas. In particular, in the context of the mechanical modeling of active clays and geomaterials with clay minerals, the following scientific questions are relevant: (i) what may be the most suitable definition of effective stress for describing the mechanical behavior of these geomaterials when variations in pore water pressure - even in the negative range - and variations in pore water chemical composition are expected to occur; (ii) what are the most suitable stress variables to be defined contextually, how they can be computed and what are the constitutive stress-strain laws for describing their evolution during a generic stress path. This thesis exploited thermodynamics and geochemistry principles to develop an effective stress concept that suitably considers the interaction between clay particles and water. The proposed generalized effective stress concept makes it possible to account for the mechanical effects of variation in total stress, pore water pressure (even in the negative range), and chemical composition of the pore water. Existing experimental results in the literature were used to highlight the interpretative benefits resulting from adopting the proposed effective stress concept. An elasto-plastic model for describing the soil water retention curve, distinguishing capillarity, and adsorption water retention mechanisms, was also presented. In the context of the mechanics of unsaturated geomaterials subjected to change of total stresses in undrained conditions, the following scientific gap can be highlighted before this work: non-existence of an effective stress-based approach employing an effective stress model consistent with the state of saturation of the geomaterial of interest. In light of this, the advantages of adopting the generalized effective stress were adopted to modify the so-called Skempton pore pressure coefficients. A direct transition between saturated and unsaturated states and a lower number of necessary parameters are some of the advantages that distinguish the approach proposed from the current one for unsaturated media. Concerning the estimation of the in-situ effective stress in geomaterials with complex stress history (e.g., shales) before this work, the theoretical applicability of an approach using after-extraction pore water pressure measurements was limited due to the following aspects: (i) the isotropic pore water pressure coefficient can be lower than one; (ii) temperature variations may have an impact on the pore water pressure changes induced during the extraction process, particularly when referring to high depths. The proposed theoretical formulation accounted for these aspects. An estimation strategy was formulated.