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1 IntroductionSite investigations are currently being performed by SKB for location and safety assessment ofa deep repository for high level radioactive waste /1/. The investigations are carried out in twoSwedish municipalities, Östhammar and Oskarshamn. The site investigation area in Östhammaris situated close to the Forsmark nuclear power facilities /2/, see Figure 1-1.Drilling is one important activity within the scope of the site investigations. Three main typesof boreholes are produced, 1) core drilled respectively 2) percussion drilled boreholes in solidrock and 3) boreholes drilled through regolith. The last type may be accomplished by differentdrilling techniques, e.g. percussion drilling and auger drilling.The deepest boreholes drilled at the site investigation are core drilled boreholes in hard rock.So far, three sub-vertical and seven inclined, 950–1,000 m long, cored boreholes have beendrilled within the investigation area. Five semi-deep (500–850 m) cored boreholes and sevenshorter (100–500 m) cored boreholes have also been drilled. The locations of the twelve drillsites in question, DS1 to DS12, are illustrated in Figure 1-1.Figure 1-1. The site investigation area at Forsmark including the candidate area selected for moredetailed investigations. Drill sites DS1–12 are marked with blue dots.