Mining Publication: In Situ Stress at the Lucky Friday Mine (In Four Parts): 2. Analysis of Overcore Measurement From 5300 Level

The U.S. Bureau of Mines conducted an overcore stress measurement on the 5300 level of the Lucky Friday Mine, Mullan, Idaho, to investigate the stress regime around an experimental stope in a rock-burst-prone mine. The result indicated a stress field with unexpected magnitude and orientation [maximum principal stress equals approx. 135 MPa (19,600 psi) oriented S 80 deg W]. An unusual amount of rock-burst activity was encountered during subsequent excavation. A careful review of the overcore data suggested the overcore strain measurements were valid. Moreover, concentrated rock-burst activity suggested that the measured stress field did exist, but was localized and therefore did not represent the far-field in situ stress field.