Abstract
Due to the increasing depth of mining and the associated hazard of rock bursting in hard rock mines, the need for detailed stress measurements becomes more important. Mine design processes necessitate detailed knowledge of the orientation and magnitudes of the in-situ principal stresses. This research presents the results of a series of mini-frac tests at KGHM’s Victoria project in the Sudbury Basin, ON. 24 sets of mini-frac tests were conducted at a depth of between 350 m to 750 m. Each test included multiple cycles. The first cycle measured the rock mass's breakdown pressure followed by two or three successive re-opening cycles. One to three step-rate-test (hydro-jacking) cycles were also included in each set to refine the measurements further. In addition to the mini-frac testing, borehole breakouts observed in the Acoustic Televiewer Survey are used to assess the magnitude and orientation of the principal stresses at depth.
It will be shown that the minimum horizontal stress is vertical and is gravity-induced. Assuming that the ratio between the maximum horizontal stress (sH) and the minimum horizontal stress (sh) remains constant, it will be shown that the kh ratio approaches unity at depths below 650 meters. The kH ratio will be equal to 2.4 for depths above 750 m where the mini-frac tests were conducted, however, this ratio falls within the range of 1.50 to 1.65 to describe the magnitude of the maximum horizontal stress that causes the observed borehole breakouts.