An extensive site investigation program was carried out between 2002 and 2007 to characterise the rock mass at Forsmark Sweden, to assess its potential for hosting a used nuclear fuel repository at a depth between 400 and 700 m. Stress campaigns using both overcoring and hydraulic fracturing, including the hydraulic testing of pre-existing fractures were carried out to establish the in-situ stress state. Using only those measurements that had approximately the same Azimuth, the minimum horizontal stress from the overcoring measurements were compared to the minimum horizontal stress from hydraulic fracturing. The hydraulic testing was carried out using special testing procedures designed to minimise the risk for inducing horizontal fractures or fracture rotation. Despite these efforts the hydraulic fracturing results indicated that the minimum horizontal stress was approximately 50% of the magnitude given by the overcoring measurements and approximately equivalent to the vertical stress. It appears that the prominence of the open subhorizontal fractures at Forsmark combined with high horizontal stresses prevented both the hydraulic fracturing and HPTF from providing reasonable estimates of the minimum horizontal stress. The horizontal stress magnitudes from the overcoring were found to be in agreement with indirect measurements and observations made during the site characterisation program.
Between 2002 and 2007, the Swedish Nuclear Fuel & Waste Management Co. (SKB) conducted an extensive site characterization program at Forsmark, Sweden for a potential repository for used nuclear fuel. The surface investigations, which covered several km2, were primarily carried out using 14 76-mm-diamter cored boreholes drilled to a maximum depth of 1000 m and 11 drilled to a maximum depth of 500 m. The target depth of the repository was between 400 and 700 m. Several stress campaigns involving both overcoring and hydraulic methods (e.i. hydraulic fracturing and the hydraulic testing of pre-existing fractures - HTPF) were carried out in surface drilled boreholes during the site investigation period to establish the in-situ stress state at the proposed repository depth.
As noted by Doe et al. (2006) not all stress measurement methods carried out in deep boreholes will work in all geological environments. They note that in over-stressed rock, e.g., rock masses where the horizontal stresses significantly exceed the weight of the overburden (a thrust regime) none of the stress measurement techniques work particularly well. Because hydraulic fracturing produces a fracture normal to the minimum stress, hydraulic fracturing in such a stress regime tends to produce horizontal fractures and hence measures the weight of the overburden /Evans and Engelder 1989/. The overcoring method relies on elastic theory and in such a stress regime micro-cracking may occur inducing a nonlinear stress-strain response which makes it difficult to interpret the results (Martin and Christiansson 1991).
While it may be difficult to measure the in-situ stress state directly using traditional overcoring and hydraulic methods in some in-situ stress regimes (geological environments), the results from such measurement campaigns, nonetheless, contribute significantly to understanding the in-situ stress state at a site.
