ABSTRACT:

Viscoplasticity is now regarded as a well established framework for the numerical solutions of plasticity problems. In order to numerically implement viscoplasticity in an efficient way, time step control is essential. The determination of the optimum magnitudes of time steps involves studies of stability and convergence which in turn depend on the failure/yield criterion being used. This work presents such studies in relation to the recent version of Hoek and Brown (Hoek et al.. 1992) failure criterion for intact/fractured rock masses. The details of the criteria used in the definition of the optimum time step are presented.

RÉSUMÉ:

La viscoplasticitc est vue actuellement cornmc un outil pour la solution numeriquc des problèmes de la Plasticite. En cc que concerne l'implementation numerique de la viscoplasticite d'une manière efficiente. Ie contrôle des intervals de temps est fort important. La determination des magnitudes optimales des intervals de temps prends en consideration les etudes de la stabilite et convergence, lesquelles, a son tour, dependent du critère de rupture/ecoulement employe. Cet article presents les etudes cites ci-dessus par rapport au critere de Hoek et Brown mis à jour (Hoek et al.. 1992) pour les massifs rocheux.

ZUSAMMENFASSUNG:

Viskoplasrizitat kann als ein gutes etabliertes Rahmenwerk fur die numerische Lösungen von Plastizitatsprobleme geschen worden. Um die numerische Implementierung der Viskoplastizitat leitstungfahig durchzufuehren. Zeitschrittenkontrolle ist wesentlich. Fuer die Bestimmung der besten Amplituden der Zeitschritten. Stabilitat-und Konvergenzanalysen worden durchgefuehrt. Diese Analysen haugen von dern angewandten Fliess und Versagenskriterium ab. Dieser Artikel stellt diese Analysen fuer die neue Version der Hock-Brown Versagenskriterium (Hoek et al.. 1992) fuer anverschrte/geklueftete Felsen von Die Einzelheiten des in der Definition der besten Zeitschritte angenwandien Kriterium werden vorgesfellten.

I. INTRODUCTION

Most intact rocks behave elastically in the range of stresses experienced in engineering and mining structures. However, in the case of deep mines, the intact rock may yield or fail due to a possible combination of high stresses and fewer joint families. Therefore, in this case, the consideration of rock as an elastic material does not represent the behaviour of this material (Pande et al., 1990) Viscoplasticity is now regarded as a well established framework for the numerical solutions of plasticity problems (Owen et al., 1986) presenting important advantages in the analysis of rock masses: the viscoplastic theory is capable of taking into account the time dependence behaviour of rocks and rock masses and non-associated flow rules and strain-softening features can be trated quite simply In order to numerically implement viscoplasticity in an efficient way, controlling of the time increment is essential In this paper, the study of the computational technique in terms of convergence and stability of the numerical implementation, which, in turn, depend on the failure/yield criterion being used, is emphasized. Such study is presented in relation to the recent version of Hoek-Brown failure criterion for intact/fractured rock masses (Hoek et al., 1992), for its widespread use nowadays. A similar procedure was successfully used in the past for modelling the hydromechanical behaviour of fractured rock masses (Duane, 1994). In a finite element implementation this was done in a self adaptive way, transparent to the user.

2. THE HOEK-BROWN FAILURE CRITERION

Among the available failure criteria, Hoek-Brown criterion is the only one to take into consideration the strength of intact and fractured rock masses. Based on published information on intact rock strength, Hoek and Brown (1980a, 1980b) and Hoek (1983) proposed an empirical failure criterion for rock. The following conditions were taken into consideration in the development of their criterion (Hoek et al., 1994).

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