Effect of Anisotropy on Fracture Toughness and Fracturing of Rocks

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.