In this paper, laboratory studies and numerical modelling of induced anisotropic damage in a brittle rock are presented. Triaxial compression tests on core samples are first used to determine basic features of deformation and failure of the material. True triaxial block tests are then performed to investigate progressive crack growth and failure mode around an underground opening. A continuous damage theory is proposed and calibrated. The performence of the model is tested through core sample tests and block tests.
The failure mode of underground openings is related to the constitutive behaviour of rocks. In brittle rocks, irreversible deformation and failure occur by progressive growth of microcracks on the microscall and coalescence to form macroscopic fractures. Many experimental studies have shown different mechanisms by which cracks can initiate and grow under compressive stresses (Wawersik and Brace 1971, Wong 1982, Fredrich and Wong 1986, Fredrich et al. 1989, Martin and Chandler 1994). These mechanisms include sliding along preexisting microcracks and grain boundaries, pore crushing, elastic mismatch between mineral grains and dislocation movement. Another important feature of damage in brittle rocks is the subcritical growth of cracks resulting a time dependent behaviour in rocks. Different physical mechanisms of the subcritical growth have been discussed by Atkinson (1984). In order to describe nonlinear rock deformation, micromechanical fracture models have been proposed. A quite complete review was given by Kemeny & Cook (1991). These models are based on the principles of linear elastic fracture mechanics and can directly describe different microcrack growth mechanisms. However the evaluation of the effective elastic tensor is not systematic and the application to 3-D cases is difficult. In the same topic, micromechanical damage models have been proposed by Ju and others (Ju and Lee 1991, Lee and Ju 1990). The self-consistent method is employed to evaluate systematically the inelastic compliances of the cracked material. The main advantage of the micromechanical models is the ability to describe the microstrutural microcrack kinetics. However it is often difficult to implement such models in a numerical code, especially for 3D problems. On the other hand, anisotropic continuous damage models have also been developed, for instance, Krajcinovic and Fonseka (1981), Costin (1985), Singh and Digby (1989), Ju (1989), Dragon et al (1993). These models are based on the thermodynamic principles and use an internal variable to describe the damage state of the rock. The numerical implementation of such models is usually easy. In this paper, the nonlinear deformation of a sandstone will be investigated. Laboratory core sample tests and block tests have been presented. A macroscopic damage theory will be proposed to describe the time independent behaviour of the rock.
The studied rock is a grey Vosges sandstone, it is mainly composed of grains of quartz with a strong cimented cohesion. Its porosity is about 20%. Triaxial compression tests have been performed with different confining pressures. Cylinder samples of 75mm height and 37mm diameter were used and the strain rate was 2.5*10-6 /sec.
