The existence of structuration in natural clays and shales is believed to change their stiffness, yielding, dilatancy and strength; the components that are widely accepted to unite with those of the reconstituted parent soil upon large straining. However, some experimental results show that this unification may not occur in either isotropic/one-dimensional compression characteristics and/or critical state friction angle. From the constitutive modeling point of view the former can be captured with many existing models while the latter has been barely introduced in constitutive models of natural geomaterials. Based on the experimental observation on the Colorado shale, the present study aims at introducing the structured critical state friction angle in the constitutive model of Nakai et al (2011). A new internal variable is introduced to capture irreversible degradation of the structured critical state line in the stress space. Model simulations of lab experimental tests on Colorado shale are presented in order to show the improved predictive capabilities of the new model.
Natural clays and shales generally develop a complex structure as a result of physicochemical and mechanical processes during their sedimentation and postsedimentation history. The term structure, therefore, refers innately to the combined effects of fabric and interparticle bonding caused by cementation, aging or, more generally, digenesis, . However, following the suggestion of Leroueil and Vaughan  the term "structured" soil have been widely used in the literature to refer to the effects of bonding, of any origin, which upon removal by plastic straining or remolding will create the parent "destructured" soil. In this paper, terms "structuration" and "destructuration" will be used in the former and the latter sense, respectively.
As compared to that of destructured clays, the engineering behavior of structured clays have been shown to possess: 1) larger yield stresses than those merely induced by the stress history, 2) higher stiffness and shear strength although being accompanied with a bigger void ratio and 3) brittle behavior with a sudden post-peak decrease of the shear strength, e.g. see [2–4]. Although many experimental studies show that these characteristics gradually unite with those of the parent soil upon destructuration, e.g. [2,3], some other experimental results show that this unification may not take place in one-dimensional/isotropic compression tests, depending on the predominance of the interparticle bonding and the range of applied stresses/strains, e.g. see [5,6]. Interestingly, this non-unification has been reported even for the friction angle at the critical state condition under which the soil is exposed to a large shearing displacement and interparticle bonds are expected to be greatly damaged; e.g. see [6–8]. Performing triaxial compression tests on intact, destructured and remolded samples of Saint-Jean- Vianney clay, Saihi et al  attributed increased critical friction angle to the degree of destructuration at the beginning of the test, i.e. the more the volumetric strain during consolidation is, the less the difference between the critical structured and destructured friction angles will be. The micro-mechanical interpretation of the latter can be linked, to some extent, to the anisotropy of the bond breakage, that is, not all bonds break when the bond yield is initiated, . In other words, a number of strongly cemented clusters of particles may not be destroyed even at the critical state, and hence create more irregularly shaped pseudo-particles which increase the critical friction angle.