ABSTRACT:
In the semi-brittle regime, the time-dependent behavior of rocksalt results from two types of inelastic deformation mechanisms, which involve dislocations motion producing hardening and microcracks propagation leading to fracturing. In constitutive models, the former type of mechanisms can be expressed from empirical functions using external variables or introduced through the use of the internal state variables (ISV). The latter type of mechanisms, on the other hand, is often neglected or expressed from usual failure criteria. However, because these two types of mechanisms act simultaneously, a coupled approach should be considered. In this paper, the authors present a general ISV model that is used to describe in a unified manner the semi-brittle behavior of rocksalt for a variety of loading conditions. As specific examples, constant strain rate and creep test results under triaxial compression are presented and compared to model predictions.
INTRODUCTION
The mechanical behavior of rocksalt has been studied extensively over the last thirty years or so, mainly because it is considered as host rock for permanent disposal of nuclear wastes. Underground openings in rocksalt are also used for storage of various fluids and solids. For such projects, thorough laboratory testing has been conducted on rocksalt for a diversity of loading conditions, including triaxial compression tests under constant strain rate (CSR) and constant stress (creep) (e.g., Fuenkajorn and Daemen, 1988; Spiers et al., 1988; Senseny et al., 1992). These numerous investigations have greatly improved our understanding of the behavior of rocksalt. Through these, it has been shown that rocksalt behavior is rate dependent and highly non-linear, and that it is strongly affected by its mechanical history. However, despite the progress made over the years, the number of constitutive models that can be used to describe the complex behavior of this rock is still quite limited. This aspect is often recognized as being the weakest link in the design process of underground excavations for long term projects. Thus, the development of improved constitutive equations to predict the deformation and stress fields around openings is still a major area of research (e.g., Munson and Wawersik, 1993).
The present study is concerned with constitutive modeling of the mechanical behavior of rocksalt over the brittle-ductile transition, in the so-called semi-brittle regime. The new model presented here has been derived by combining the elastoviscoplasfie equations developed by Auberfin et al. (1991) with continuum damage mechanics (CDM). It provides a fairly general foundation for simulating hardening and progressive microcracking effects, up to localization of deformation. After a brief presentation on the main features of the mechanical behavior of rocksalt, the authors present the equations of the proposed SUVIC-D model for the case of conventional triaxial compression (CTC) tests. As particular applications of this original model, the new ldnefie law and growth law of the damage variable are used to describe the behavior of rocksalt in the semi-brittle regime under CSR and creep testing conditions.