AECL Research carried out the first hydraulic fracturing stress measurement tests at the proposed site of the Underground Research Laboratory (URL) in 1982. Since then, both hydraulic fracturing stress measurements and overcoring stress measurements have been successful in determining the in situ stress at the URL to a depth of 273 m, the depth at which a major fracture zone (Fracture Zone 2) intersects the URL shaft. Below Fracture Zone 2, high horizontal stresses, up to 60 MPa, have been determined from overcoring and from convergence measurements in the URL shaft. However, hydraulic fracturing attempts below the fracture zone have produced only subhorizontal fractures, making it difficult, if not impossible, to determine the horizontal in situ stresses with this method. Laboratory tests on large-diameter core and an analysis of in situ tests suggest that the poroelastic effect, enhanced by high horizontal stresses, is the governing mechanism for the occurrence of horizontal hydrofracs at the URL.
The Underground Research Laboratory (URL) is located within the Lac du Bonnet batholith on the western edge of the Precambrian Canadian Shield. The batholith is an elongated body, about 75 by 25 km in surface area and 10 km in depth. It is considered to be typical of many granitic intrusions in the Canadian Shield. Results from research at the URL are being used by AECL Research in its assessment of the feasibility and safety of deep geological disposal of nuclear fuel waste in plutonic rock. The Canadian concept for nuclear fuel waste disposal calls for the location of the disposal vault to be at a depth of 500 to 1000 m. Design of a nuclear waste disposal vault will require knowledge of the in situ stress state, and hence, the in situ stress program is an integral element of the research being conducted at the URL (Martin et al. 1990a).
The two most common methods of in situ stress measurement are overcoring and hydraulic fracturing. Either method has its advantages and limitations, and the choice of one method over another is governed by the particular application. At the URL the in situ stress state has been determined primarily by overcoring continuously monitored CSIR (Council for Scientific and Industrial Research) triaxial strain cells (Thompson et al. 1986) and USBM (United States Bureau of Mines) borehole deformation gauges (Hooker and Bickel 1974). Although overcoring is generally conducted in shallow boreholes (less than 50 m), recent equipment developments, such as the AECL Deep Borehole Deformation Gauge (Thompson 1990) and the Swedish State Power Board (SSPB) triaxial strain cell (Christiansson 1989), have been successfully used to measure the in situ stresses at locations up to several hundred metres in depth, with the potential to go to 1000 m. The interpretation of deformations associated with overcoring is based on the theory of elasticity. However, as boreholes are deepened and stress magnitudes increase, the overcored rock is affected by microcracking.
