Stress measurements at the site of the SM3 hydroelectric scheme near Sept Iles, Quebec

Abstract

A set of 10 complete hydraulic fracturing stress measurements in an anorthosite was conducted as part of the site investigation of the SM3 hydroelectric project, located some 800 km northeast of Montreal, Quebec. These were the first such measurements in that region of Canada. The vertical test hole was N-size, 260 m deep and adjacent to the future powerhouse. Within the relatively short depth range tested two apparent stress regimes were detected. In the 160–220 m range all three principal in situ stresses approach being lithostatic, suggesting stress relief probably induced by natural fracturing in the rock as evidence by considerable jointing the extracted core. At depths comparable to those of the planned powerhouse (220–255 m) the least horizontal stress is quite consistent and averages 11.0 (± 1.4) MPa, the maximum horizontal stress averages 17.7 (± 3.1) MPa and acts in a NE-SW direction, and the vertical stress is 7.5 (± 0.4) MPa. Thus, ratios of 1.6:1 between the two horizontal principal stresses and of 2.5:1 between the maximum horizontal and vertical stresses, can be expected, and should be carefully considered in design, as should the fact that both horizontal stresses are substantially larger than the vertical stress. The northeastern direction of the maximum horizontal stress, as well as the compressive nature of the measured stresses are in accord with most measurements in eastern North America.