HYDROFRACTURING STRESS DETERMINATIONS AT DARLINGTON, ONTARIO.

Abstract

A 303-m-deep test hole at Ontario Hydro's Darlington Generating Station site, 65 km east of Toronto, was used to determine the state of stress in the Ordovician limestone bedrock and the granitic gneiss in the Precambrian basement. The results of the hydrofracturing measurements indicated a state of high horizontal stress along the depth of boring and a consistent orientation of principal stresses in each geologic unit. An unexpected result is the apparent decoupling between the Precambrian stress field and the Paleozoic one. These results were confirmed by a borehole TV camera survey and by overcoring tests. The magnitudes and orientations of the measured in-situ stresses were used in the conceptual design and evaluation of an underground nuclear power plant, and in particular in the design of reactor caverns in Precambrian rocks.