The mechanical behavior of soft rocks can be quite complex as it may involve a variety of deformation mechanisms. When loaded under compressive stresses, these rocks often show a semi-brittle response, controlled by dislocations motion causing hardening, and by microcracking producing a degradation of the mechanical properties. In this paper, the authors present a general internal state variable (ISV) model that can be used to describe, in a unified manner, the behavior of low porosity soft rocks. To do so, a damage variable is introduced into an existing model developed for the ductile regime, to accommodate brittle and semi-brittle responses. It is shown that the model depicts the behavior of soft rock samples submitted to conventional triaxial compression tests.
Le comportement mecanique des roches tendres se revèle souvent relativement complexe car il peut impliquer une variete de mecanismes de deformation, Lorsque sollicitees sous des contraintes de compression, ces roches montrent frequemment une reponse semi-fragile, contrôlee par le mouvement des dislocations qui induit un ecrouissage, et par la microfissuration qui provoque une degradation des proprietes mecaniques. Dans cet article, les auteurs presentent un modèle general avec variables d'etat interne (VEI) qui peut être utilise pour decrire, dans un contexte unifie, le comportement des roches tendres peu poreuses. À cette fin, une variable d'endommagement est introduite dans un modèle existant, developpe pour le regime ductile, afin de tenir compte des reponses fragiles et semi-fragiles. On montre que le modèle peut reproduire Ie comportement d'echantillons de roches tendres soumis à des essais de compression triaxtale conventionnelle.
Mechanisches Verhalten weicher Gesteine kann relativ kompliziert sein, weil es eine Menge verschiedener Verformungsmechanismen enthielt. Unter einer Druckbelastung zeigen diese Gesteine oftmals eine halb-spröde Reaktion. Diese Reaktion ist eine Folge der Dislokationsbewegung bedingten Verfestigung und der Mikrosprungbildung verursacheten Degradation mechanischer Eigenschaften des Gesteins. In dieser Veröffentlichung stellt man ein algemeines Modell mit inneren Zustandveranderlichen (lZV) vor, welches das Verhalten wenig poröser weicher Gesteine einheitsweise beschrieben kann. Fuer diesen Zweck wird in das bestehende Modell eine Beschidigungsveranderliche eigefuehrt, um spröde und halb-spröde Antwort behandeln zu können. Es wird gezeigt dass dieses Modell eine gute Darstellung der wichtigsten Eigenschaften des Verhaltens von Proben weicher Gesteine unter konventionellen triaxialen Druckversuchen ermoglicht.
As is the case for most materials of interest to engineers, the deformation of rocks is accompanied by modifications in their internal structure. In soft rocks having a low porosity, substructures are created during inelastic straining due to the creation, propagation and accumulation of defects, such as dislocations and microcracks. Some of these substructures may produce hardening of the material, by increasing its flow strength, while others will induce weakening by reducing some of its mechanical characteristics. The evolution of rocks internal structure and of the ensuing macroscopic behavior depends on the loading history, including the stress path and cumulated strain. A proper representation of the complex relationship that exists between history of the material and its mechanical response is a key element for the analysis and design of openings in soft rocks. This paper deals with such a representation based on the use of internal state variables (lSV). Because deformation creates some form of anisotropy (even for initially isotropic materials), it implies that the adopted ISV must reflect the relevant aspects of not only the magnitude of the internal structure effects, but also its orientation. For that purpose, tensorial variables are used, as they can provide a convenient measure of anisotropy for many complex situations. For the sake of simplicity, this presentation is confined to the case of small isothermal perturbations, considering that the deformation of the rock in the representative volume element (RYE) is homogeneous. In the following sections, the authors first present some of the salient features of low porosity soft rocks showing a semi brittle behavior. Then, a constitutive model with ISV is introduced. It is finally shown, by using some laboratory test results, that the proposed rate-dependent equations can capture many of these features.
