Elastic parameters of rocks are typically used for design purposes in open pit and underground mining, underground spaces and rock-cutting projects. However, the ultimate strength of rocks is strongly influenced by their micro-fractures, preexisting cracks, and anisotropy due to inhomogeneity, discontinuities, and differing particle sizes or shapes and orientations. Since the fracture behaviour of rocks is important to geotechnical engineers concerned with the design of excavations and underground spaces, it is obvious that laboratory investigations of their anisotropic parameters are necessary for safe designs. The main objective of this paper is to investigate the effect of different orientations of the anisotropy of Brisbane sandstone specimens subjected to diametral compressive (indirect tensile) loading that influences their fracture toughness. To obtain the fracture toughness values of anisotropic Brisbane sandstone, Cracked Chevron Notch Brazilian Disc (CCNBD) specimens were prepared and tested according to International Society for Rock Mechanics (ISRM) standards. The fracture toughness of Brisbane sandstone was found to increase with increasing angle of anisotropy. Based on the experimental results, a statistical regression analysis was conducted to obtain the optimum orientation angle to obtain the highest strength under indirect tensile loading. Statistical analysis showed anisotropy orientations of 45° gave the highest fracture toughness value.
Rocks exhibit different types of anisotropy and discontinuities that affect their fracturing and strength behaviour. The anisotropy of sandstone has been recognised for about 70 years , and fracture propagation in sandstone and other anisotropic rocks is preferentially oriented . Anisotropy also plays a dominant role in determining the velocity of elastic waves in sedimentary rocks. The most critical measure of elastic wave velocity in rocks that exhibit anisotropy is the horizontal velocity . The elastic wave velocity is reduced by the opening of micro-fractures, ultimately resulting in failure [4, 5].
The uniaxial compressive strengths of anisotropic rocks parallel and normal to the orientation of the anisotropy are different, and the most reliable estimates of their resistance to fracturing and their strength is obtained when rock cores are drilled normal or near-normal to the weakest plane . To determine design parameters for anisotropic rocks from laboratory test results, both maximum and minimum strengths must be measured. In addition, it has been observed that the strength of rock joints, another example of rock anisotropy, are a function of the orientation of the applied loading . Generally, the effect of anisotropy decreases with increasing applied vertical stress on the rock joints. The roughness of the joint is another significant parameter that effects their strength.