OVERCORING IN HIGHLY STRESSED GRANITE - THE INFLUENCE OF MICROCRACKING

The influence of microcracking on overcoring results was investigated using data from 81 overcore tests using the continually monitored Council for Scientific and Industrial Research triaxial strain cell. The tests were performed in 13 boreholes drilled in a relatively uniform, massive granitic rock mass covering about a 60 x 60m area at the 240 Level of the Underground Research Laboratory. Overcored samples from boreholes drilled at angles nearly perpendicular to the major principal stress direction showed the greatest amount of stress-induced microcracking. The in situ stresses calculated from tests carried out in boreholes drilled at these angles did not agree with the general in situ stress state for the 240 Level. The investigation concluded that the stress-induced microcracks created anisotropy in the overcore samples, and that the anisotropy could be approximated by an apparent plane of transverse isotropy. The direction of the transverse plane and the anisotropic elastic constants were determined and the in situ stresses recalculated using an anisotropic solution. The anisotropic results indicated a reasonably uniform stress state on the 240 Level except in the immediate area of a single water-bearing subvertical fracture zone. However, the nonlinear behaviour of the overcored samples caused by the stress-induced microcracks limits the confidence for both the linear isotropic and anisotropic closed-form solution for overcoring.