Pre-mining stresses at some hard rock mines in the Canadian Shield

CIM Bulletin, Vol. 84, No. 945, 1991

B. Arjang, Research Scientist, Elliot Lake Laboratory, CANMET — EMR, Elliot Lake, Ontario

Ground stresses are one of the significant factors in the context ofrockbursts and underground instability at some hard rock mines in Canada. Overcoring strain relief measurements using tri-axial strain cells were performed at several mine sites to provide stress data for stability evaluations and mine design. Pre-mining stress determinations carried out at depths between 60 m to 1890 m resulted in the following average stress gradients:Maximum horizontal stress, aHmax = 8.18 + 0.0422 MPa/m depth; Minimum horizontal stress, aHmin = 3.64 + 0.0276 MPa/m depth ;Average horizontal stress, aHa = 5.91 + 0.0349 MPa/m depth ;Vertical stress, av = 0.0266 ±0.008 MPa/m depth The maximum and minimum horizontal compressive stresses, with an average ratio of 1.75 ± 0.45, prevail in east-west and northerly directions, respectively. Horizontal compressive stresses in excess of vertical overburden load were determined, indicating large variations in ratios to a depth of about 1000 m with decreasing trend toward depth. From present data, a particular regional zoning for the magnitude and direction of horizontal stress fields cannot be outlined.###A common feature at mines with near vertical orebodies is that the maximum horizontal stress acts perpendicular to strike while the minimum horizontal stress is aligned on-strike. The vertical stress components approach the gravitational overburden load.

Keywords: Rock mechanics, Ground stress, Underground stress measurements.