Summary
An overcoring program in three nearly orthogonal boreholes, intersecting the same rock volume, was carried out in highly stressed Lac du Bonnet granite at the 240-m level of the Underground Research Laboratory (URL). The program was designed to determine if the more commonly used United States Bureau of Mines borehole deformation gauge (USBM gauge) gave similar in situ stress results as a modified Council of Scientific and Industrial Research (CSIR) triaxial strain cell. A total of 15 successful USBM tests and 17 successful CSIR tests were carried out. The measured mean stress tensor for the USBM results fell within the 90% confidence limits for the CSIR results.
The measured CSIR stress tensors were slightly influenced by the orientation of the boreholes, although this phenomenon may be site specific as it is probably related to the amount of stress-induced microcracking that occurs during the overcoring process. The results of the program provided sufficient confidence in the modified CSIR cell to warrant using it as the main overcoring device for future URL overcoring programs.
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Martin, C.D., Christiansson, R.C. Overcoring in highly stressed granite: Comparison of USBM and modified CSIR devices. Rock Mech Rock Engng 24, 207–235 (1991). https://doi.org/10.1007/BF01045032
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DOI: https://doi.org/10.1007/BF01045032