The frictional characteristics of a discontinuity are generally non-uniform and thus slip may start from a region of low shear strength (weak zone) and propagate into a region of high shear strength (strong zone). Hence failure may occur progressively before the peak strength is mobilized on the entire surface of the discontinuity. A series of biaxial tests are conducted on specimens with anisotropic frictional surfaces to explore the mechanisms of slip and rupture. The specimens used in the tests are composed of three individual prismatic blocks: two outer and one inner block, machined and bonded together. Bonding of each contact surface creates in half of the surface a low friction strength (weak zone) and in the other half a high friction strength (strong zone). The specimens are then loaded in biaxial compression in such a way that slip propagates from the weak to the strong zone. The slip process can be treated as a mode II crack propagation and the Stress Intensity Factor (KII) is computed by inversion of the stress field observed at the tip of the weak zone; this is done using near field and multi-parameter solutions. A good correlation between the methods has been found. The results show that KIIC depends on the normal stress, and slightly increases with increasing confining pressures.
1. INTRODUCTION
The process of slip initiation and propagation along frictional discontinuities is an important problem in engineering where design often involves the application of compression to natural or artificial materials that have pre-existing closed discontinuities. The safety and deformation behavior of these materials often depend on the frictional characteristics of the discontinuities. Examples include rock slope stability, tunneling, or even progressive failure in stiff soil deposits.
The frictional characteristics of a discontinuity however are not homogeneous, and as a consequence slip may start in regions of low shear resistance (weak) and propagate into regions of high shear resistance (strong). Hence failure may occur well before the full frictional resistance is mobilized along the entire surface of the discontinuity. If this is the case the choice of a 'uniform' or 'averaged' friction resistance along a particular discontinuity may not be appropriate and may result in an unsafe design.
Extensive research has been performed to study and characterize the behavior of frictional surfaces. The well-known Coulomb formulation is still used extensively in practice. The Coulomb friction law [1], states that, along the slip plane, the maximum shear strength t is given by:
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where c is the cohesion, s'n is the effective stress normal to the slip plane, and µ is the coefficient of friction, also expressed as µ = tan F, where F is the friction angle. The rate and state dependency of the coefficient of friction µ has been investigated by many researchers [2-6] who have proposed a number of well established theories. These theories however assume that along the frictional surface properties are homogeneous and the coefficient of friction can be taken as constant or as an average over the slip surface.
