ABSTRACT ABSTRACT: Ground stresses are one of the significant factors in the context of rockbursts and underground instability at some hard rock mines in Canada. Overcoring strain relief measurements using triaxial strain cells were performed at depths between 60 to 1890 m at several mine sites to provide stress data for' stability analysis and optimum mine design. Pre-mining ground stress determinations from 64 stress tensor analyses resulted in the following average stress gradients:
(available in full paper)
The maximum and minimum horizontal compressive stresses with an average ratio of 1.7.'5 :0.45. prevail Jn 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 towards 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 compressive stress acts perpendicular to strike while the minimum horizontal compressive stress is aligned on-strike. The vertical stress components approach the gravitational overburden Load.
1 . INTRODUCTION
The structural stability of any underground excavation is primarily dependent on the state of pre-mining stress, the stress re- distribution created by the,' development of the mine as well as the in situ strength of the rock material and geological factors. Underground stress measurements in Canadian hard rock mines have been carried out by the Canada Centre for mineral and Energy Technology (CANMET) since the early 1960's. As a continuation of the stress determination program and as part of a Canada/Ontario/Industry rockburst research project underground stress measurements were conducted in cooperation with operating mining companies. Measurements were taken in the following mine sites located in Northern Ontario and Quebec: Detour Lake, Selbaie, Montauban, Bousquet, Eldrich, Niobec,Kidd Creek, Dome, Macassa, Startleigh and Campbell Red Lake. Overcoring strain relief measurements were made using triaxial strain cells (CSIR and CSIRO). At each site at least two consistent measurements were taken. The following provides a compilation of recent ground stress determinations (Arjang 1984 to 19881. and an evaluation of selected stress data from 84 overcoring strain relief measurements.
Figure 1. Stress determination sites and the direction of maximum horizontal compressive stress in the Canadian Shield.(available in full paper)
Figure 2. Direction frequency plots of maximum and minimum horizontal compressive stresses.(available in full paper)
2. GENERAI. GEOLOGICAL-STRUCTURAL SETTING
Most of the ground stress determination sites are located in the Superior tectonic Province of the Canadian Shield, The characteristic features of the Superior Tectonic Province are the easterly trending. alternating volcanic-plutonic and metasedimentary-gneiss belts upon which major and minor structures are superimposed. Intense structural deformations of the Arcbean rock strata occurred during the Kenoran and the subsequent Grenville orogeny. Available structural data and kinetic analysis derived from the Southern Part of the Superior Tectonic Province suggest a long- standing stress pattern involving north-south horizontal compression and east-west extension (Goodwin and West 1974). On a local scale, kinetic evidence identified a similar stress field a major deformation with compression in a NNW-SSW/subhorizontal direction and a subsequent phase of compression in a NE-SW/subhorizontal direction