Photogrammetric prediction of rock fracture properties and validation with metric shear tests

An accurate understanding of jointed rock mass behavior is important in many applications ranging from deep geological disposal of nuclear waste, to deep mining, and to urban geoengineering projects. The roughness of rock fractures and the matching of the fracture surfaces are the key contributors to the shear strength of rock fractures. In this research, push shear tests with three normal stress levels of 3.6, 6.0, and 8.5 kPa were conducted on two granite samples with artificially induced well-matching tensile fractures with sizes of 500 mm x 250 mm and 1000 mm x 500 mm. The large sample reached on average a -60% weaker peak shear stress than the medium-sized sample, and a strong negative scale effect was observed in the peak shear strength. The roughness of the surfaces was measured using a profilometer and photogrammetry. The scale-corrected profilometer-based method (joint roughness coefficient, JRC) underestimates the peak friction angle for the medium-sized slabs by -27% for the medium sample and -9% for the large sample. The photogrammetry-based (Z'(2)) method produces an estimate with -7% (medium) and + 12% (large) errors. The photogrammetry-based Z'(2) is an objective method that consistently produces usable estimates for the JRC and peak friction angle.