1. INTRODUCTON
The Province of Ontario, Carina, has a very large outcrop of Precambrian, cystalline rocks in the north, which forms part of the Canadian Shield. The Precambrian surface dips towards the south, and becomes overlain by Paleozoic rocks of increasing thickness. These Paleozoic rocks consist of flatlying limestones, dolomites, shales and sandstones of the Ordovician, Silurian and Devonian periods. The entire area was heavily glaciated during the Ice Ages of the Pleistocene. Thus, it is known that a major depression of the area took place during the Wisconsin Glaciation followed by isostatic rebound. It has also been suggested that some heavy erosion might have occurred since Paleozoic t/me (Chamberlin, 1910; Gilluly, 1964; Asmis and Lee 1980).
The state of stress in a rock mass is a product of its geologic/tectonic history. In much of Ontario, this history apparently led to the evolution of a high horizontal stress field, which manifests itself as compression features in bedrock (e.g. folds, pop-ups and high-angle reverse faulting, see Oliver et al 1970), and as a force to be reckoned with in rock excavation projects. This paper summarizes the rock stress measurements made in southern Ontario to date, along with their implications on engineering design. These measurements will be presented chronologically in an order as shown in Table 1, from Silurian sediments to Precambrian basement rocks. Reference will be made periodically to Figure 1, for the location of the test sites involved.
It is apparent from Table 1 that almost all of the measurements listed have been obtained from sedimentary rocks of the Silurian and Ordovician periods. An attempt was rode in 1979 to measure the in-situ stresses in Devonian dolomite beds belonging to the Detroit River Group, at a site on the east shore of Lake Huron. The attempt was not successful due to the relatively high fracture density of the bedrock. Stresses in Precambrian basement rocks have been measured only at one site in southern Ontario, namely, Darlington. For measurements obtained in the Precambrian Shield portion of northern Ontario, the reader is referred to Herget (1980).
2. SILURIAN ROCKS OF THE NIAGARA SERIES
2.1 Thorold
The first series of in-situ stress measurements was made in 1974, in Silurian dolomites, limestones and shaly limestones of the Niagara Series near Thorold, Ontario (Figure 1). The measurements were made as part of an investigation into the cause of structural distress in the walls of a service building at the Thorold Tunnel, on Highway 406 and beneath the new Welland Canal (Lo et al, 1975; Bowen et al, 1976). The measurements were made in a vertical hole to a depth of 25 m below the surface (or 16 m into bedrock), using the U.S. Bureau of Mines borehole deformation gauge (Hooker and Bickel, 1974) and the strain-relief, overcoring technique. The test hole was located at a distance of 336 m away from the tunnel to alleviate the latter's influence on the stress field.
A simplified stratigraphy of the test site, along with the results of measurement, is given in Figure 2.
