In-situ stress and microcracking in granite cores with depth

Microcrack volumes in 87 granite-cored samples, obtained in the depth range from ground surface to 1000 m at Pinawa, Canada and Forsmark, Sweden, were quantified from stress–strain curves to assess the increase in microcrack volume with depth. Digital image analysis of 9 of these samples was used to examine the characteristics of these microcracks. The results indicate that at depths of less than 200 m, the dominant mode of microcracks can be classed as naturally occurring. The volume of stress-induced microcracks was found to increase linearly with sampling depth with the proportion of grain-boundary, intragranular and transgranular microcracks remaining relatively constant regardless of the depth. However, when the mean in-situ stress magnitudes normalized by the laboratory tensile strength was higher than 4, the proportion of intragranular and transgranular microcracks increased significantly for Lac du Bonnet granite. Moreover it was observed that most of the stress-induced transgranular microcracks formed in a plane perpendicular to the core axis. Thus the ratio horizontal to axial transgranular microcrack could be an indicator of the degree of stress-induced core damage. These stress-induced microcracks were responsible for the non-linear stress–strain behavior observed in the initial loading stage in laboratory tests.