Fabric-guided micro-fracturing phenomenon in brittle rock and its effect on crack damage thresholds remains a subject that challenges researchers. Anisotropy of brittle rocks in terms of crack initiation (CI) and crack propagation (CD) thresholds plays a significant role on the extension and density of micro-fracturing in different directions in the walls of underground openings. To better understand the fabric-guided micro-fracturing phenomenon and also to study the effect of type of fabric in layered brittle rocks on their anisotropic behaviour, three types of brittle rock are investigated in terms of crack damage anisotropy in this paper. The rocks that are chosen for this study are limestone from the Cobourg Formation, Queenston shale and Olkiluoto mica gneiss. For each rock type CI and CD thresholds are identified from the unconfined compressive strength (UCS) test data, performed on specimens with different layering orientation in respect to the loading axis. The anisotropic behaviour of the three rock types are investigated at different stress levels and the contributing factors to this behaviour are discussed.

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