STRESS MEASUREMENTS AT THE SITE OF THE SM3 HYDROELECTRIC SCHEME NEAR SEPT-ILES, QUEBEC

A set of 10 complete hydraulic fracturing stress measurements in an an orthosite was conducted as part of the site investigation of the SM3 h ydroelectric project, located some 800 km northeast of Montreal, Quebe c. These were the first such measurements in that region of Canada. Th e vertical test hole was N-size, 260 m deep and adjacent to the future powerhouse. Within the relatively short depth range tested two appare nt stress regimes were detected. In the 160-220 m range all three prin cipal in situ stresses approach being lithostatic, suggesting stress r elief probably induced by natural fracturing in the rock as evidenced by considerable jointing in the extracted core. At depths comparable t o those of the planned powerhouse (220-255 m) the least horizontal str ess is quite consistent and averages 11.0 (+/-1.4) MPa, the maximum ho rizontal stress averages 17.7 (+/-3.1) MPa and acts in a NE-SW directi on, and the vertical stress is 7.5 (+/-0.4) MPa. Thus, Patios of 1.6:1 between the two horizontal principal stresses and of 2.5:1 between th e maximum horizontal and vertical stresses, can be expected, and shoul d be carefully considered in design, as should the fact that both hori zontal stresses are substantially larger than the vertical stress. The northeastern direction of the maximum horizontal stress, as well as t he compressive nature of the measured stresses are in accord with most measurements in eastern North America. Copyright (C) 1996 Elsevier Sc ience Ltd