Methods for Characterizing Cracks Induced in Rock

A detailed understanding of how induced cracks form and propagate in rock is important in many fields, particularly in mining where size reduction of material is a major cost. This contribution provides a comprehensive review of previously published methods for characterizing rock cracks. The existing methods generate one of the two types of data: crack-related changes in bulk rock properties or the nature of the actual cracks, such as aperture, location, morphology and mineralogical boundaries. However, no single technique provides both types of information. Using a suite of granodiorite specimens which were cracked under microwave irradiation, this contribution illustrates how multiple nondestructive and destructive techniques can be integrated to provide both types of data on cracks induced in rock. Rock property changes were quantified using P-wave velocity measurements and uniaxial compressive strength tests. The cracks were characterized qualitatively and quantitatively using a combined approach of micro-computed tomography, optical microscopy and automated mineralogical analysis of scanning electron microscopy images. The resulting dataset provides information on the mechanical and mineralogical effects of the cracking. The microwave irradiation led to a significant reduction in rock strength, caused by networks of intergranular and intragranular cracks in granodiorite, disproportionately associated with biotite. Based on the strengths and limitations of each method as illustrated by the granodiorite results, recommended protocols are provided for evaluating cracks in four different types of studies: (1) quick assessments of damage; (2) geotechnical investigations; (3) mineralogical investigations and (4) comprehensive evaluations of the effects of cracking, requiring data on mechanical properties and the characteristics of the cracks.