Statistical characterization of the fracture system in the Stripa granite, Sweden

Abstract An extensive fracture data base was collected from a granitic rock mass at Stripa, Sweden. This was part of a research effort, in the field of high-level nuclear waste disposal, that was aimed at providing a statistical characterization of fracture orientation, size, spacing and density. Data available for this characterization come from about 1500 m of oriented drill cores and from fracture maps of the walls and the floor of a 50 m length of drift. Based on fracture orientation data, from these drill cores and fracture maps, four fracture sets were defined for the rock mass surrounding the ventilation drift at Stripa. For each fracture set, the distributions of trace lengths and spacings were analysed and an average value of fracture density computed. Corrections were made, in the statistical analysis, for orientation bias, trace length censoring and trace length truncation. Appreciable differences were found between fracture sets for fracture densities as well as for trace length and spacing distributions. The differences in trace length distributions were much more apparent after making corrections for biases. A first order approximation of the inter-connectivity of the fracture system was calculated using the parameters of the fracture geometry distributions.