INTRODUCTION

ABSTRACT:

This paper presents a numerical modeling of damage extension around a keyed underground excavation using a continuous damage model. In the 3-dimensionnal code, FLAC ?ΓΈ, we have implemented an anisotropic damage model. Tfiaxial compression tests provide a verification of the implementation with a good agreement between experiments and predictions. The applicability of the implemented model to predict damaged zone around a keyed tunnel of AECL's Mine by experience is examined. Results indicate that a square key is best to interrupt the continuity of damage zones around the keyed tunnel than the triangular or trapezoidal keys.

An excavation damaged zone (EDZ) is generally formed around an underground opening excavated into brittle rocks in relation to high in situ stresses and high anisotropic stress ratios. The mechanical and hydraulic properties are then changed within such EDZ. The failure mechanism in the damaged zone is the initiation and growth of cracks and fractures and is directly related to the constitutive behavior of rock mass. Indeed, irreversible deformations and failure in brittle rocks subjected to compressive stresses occur by progressive damage as microcracks initiate and grow on the small scale and coalesce to form large-scale fractures and faults. Experimental studies on brittle rocks have shown that there are many different mechanisms by which cracks can be initiated and grown under compressire stresses (Wong 1982, Steif 1984, Martin & Chandler 1994, Homand et al. 1998, etc.). The involved mechanisms include sliding along pre-existing cracks and grain boundaries, pore crushing, elastic mismatch between mineral grains and dislocation movement. To describe these mechanisms, several micromechanical fracture models have been proposed based on experimental studies and Kemeny & Cook (1991) give a quite review. In the Kemeny & Cook model, crack growth conditions are obtained from the principles of linear elastic fracture mechanics and the constitutive equations are evaluated using Castigliano's theorem. The evaluation of the effective elastic tensor is not systematic and the application to 3-D cases is difficult. Further more, micromechanical damage models in which the kinetic equations of microcrack are characterized by the use of fracture mechanics stability criteria have been developed (Ju & Lee 1991). The main advantage of the micromechanical models is the ability to describe the microstructural microcrack kinetics. On the other hand, anisotropic continuous damage models have also been developed, for instance, Costin (1985), Dragon et al. (1994), Homand et al. (1998) etc... These models are based on the thermodynamic principles and the use of an internal variable to describe the damage state of the rock. Moreover, although these models are identified from the macroscopic behavior of the rock, it is possible to relate the internal damage variable to the microstructural mechanism of crack growth in order to make the models physically well funded.

The purpose of this paper is to present an application of an anisotropicontinuous damage model well-calibrated on laboratory tests:

1. to predict the damage development around an elliptical tunnel constructed in the well- documented Underground Research Laboratory (URL) in the Lac du Bonnet granite;

2. to investigate the effectiveness of cut-off keys in bulkhead design to interrupt or no the damage zones around a keyed tunnel.

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