Investigations into borehole breakouts in brittle rock usually treat the phenomenon as a two dimensional problem. In these cases, failure is assumed to initiate at the point of maximum tangential stress concentration at the borehole wall. According to linear elastic theory for a circular hole in an elastic medium, the maximum stress concentrations around the hole are diametrically opposed for anisotropic far-field stress conditions. At the 420 Level of the URL, boreholes drilled at angles not coincident with a principal direction typically developed breakouts that were asymmetric, or not diametrically opposed. Numerical modeling of two large-diameter boreholes demonstrates that the asymmetry in azimuth and depth of borehole breakouts in highly stressed granite can be explained by the maximum deviatoric stress pattern ahead of the advancing borehole.
In Canada, as in many other countries that employ nuclear energy, the concept of deep underground disposal of used nuclear fuel waste is being investigated. As part of its research program, Atomic Energy of Canada Limited (AECL) has constructed an Underground Research Laboratory (URL) approximately 120 km northeast of Winnipeg, Manitoba in the Lac du Bonnet granite batholith. The URL provides a well-characterized in situ environment in a previously undisturbed volume of rock for experiments that address issues of importance in the Canadian disposal concept. In terms of designing engineered barriers and seals, excavation-induced damage around underground openings is of particular interest.
In conjunction with the Mine-by Experiment (Figure 1), an investigation into the excavation-induced respome of granite, borehole breakouts have been studied extensively in situ at the URL (Martin et al. 1994; Read 1994). In a series of boreholes with diameters ranging from 150 to 1240 ram, observed borehole breakouts did not form diametrically opposite one another in the plane orthogonal to the borehole axis. None of the boreholes were drilled parallel to a principal stress direction. The same observation was true of breakouts in the 3.5-m-diameter test tunnel for the Mine-by Experiment, excavated using a non-explosive technique. This phenomenon is contrary to the idea that borehole breakouts initiate at the points of maximum tangential stress concentration around the borehole because, in an elastic medium subjected to an anisotropic stress field, these points are diametrically opposite one another on the borehole wall. Laboratory studies of borehole breakouts in granite, e.g., Lee and Haimson (1993), typically simulate plane strain conditions, and do not consider three-dimensional effects near the borehole face. This paper explores a more complex mechanism, related to stresses ahead of the advancing borehole face, for the in situ development of asymmetric breakouts in highly stressed granite.