Rocks are generally more or less anisotropic, depending on their structure at the scale of interest. The anisotropic rocks in civil and mining engineering projects cause non-symmetric deformation and their behaviour is unpredictable. The authors present the results of Brazilian Indirect Tensile (BIT) strength and Cracked Chevron Notched Brazilian Disc (CCNBD) tests for the determination of the fundamental tensile fracturing parameters of Brisbane phyllite specimens. In general, the influence of orientation angle (ψ) and foliation-loading angle (β) were found to influence both fracturing and the failure load.
Anisotropy is a characteristic of foliated metamorphic rock masses (gneisses, phyllites and schists), and intact stratified or bedded sedimentary rocks (coal, shales, sandstones, siltstones, limestones, etc.). At a larger scale, rock mass anisotropy is found in volcanic formations and in sedimentary formations consisting of alternating layers or beds of different rock types and in rock formations cut by one or several regularly-spaced joint sets. Since the deformation and fracture of these rocks is of importance to engineers concerned with the design of mining excavations or of foundations for civil engineering structures, it is obvious that research into the effects of anisotropy on rock behavior is necessary (Hoek, 1964; Chen & Hsu, 2001; Cho et al., 2012).
As in the case of the original Griffith criterion (Griffith, 1924), it is generally assumed that the specimen contains a sufficient number of randomly-oriented cracks for fracture to initiate from those cracks, which are inclined at an angle. If, however, the cracks are oriented preferentially, as in the case of a highly antistrophic material, it is necessary to consider the inclination of the cracks with respect to the applied stress system (Hoek, 1964). Brace (1961) presented evidence indicating that the cracks, from which fracture of the rock propagates, probably lie within the grain boundaries of the rock. Even in rocks of sedimentary origin, which exhibit marked foliation and planar anisotropy, the constituent bedding planes are made up of grains that are cemented together and hence randomly-oriented grain boundary cracks are likely to be present (Brace, 1960).