Heikkinen, E.; Ahokas, T.; Heikkinen, P.; Kristiansson, S.; Tiensuu, K.; Pere, T.
Posiva Oy is preparing for disposal of spent nuclear fuel in the bedrock at Olkiluoto, Eurajoki. This act is in accordance with the application filed in 1999, the Decision-in- Principle of the State Council in 2000, and the ratification by Parliament in 2001. The ONKALO underground characterisation facility has been under construction since 2004. Construction work has included excavation of an access tunnel (currently at a depth of c. 350 m) and driving of three vertical shafts using raise-boring. In 2008, geological, hydrological and geophysical measurements were carried out in three vertical drillholes intended for pre-grouting of personnel shaft ONK-KU1 at depth level -177 to -280 m. The drillholes are 76 mm in diameter, 104.5 m long and located 3.2 m apart within the perimeter of vertical shaft to be constructed. The shaft also contains geological and geophysical information from deep drillhole OL-KR24. Goal of survey and integrating interpretation was to investigate variation in bedrock properties and their predictability in small scale. Goal was also to demonstrate appropriate survey techniques. These techniques can be applied for characterization of bedrock in disposal tunnel and specifically in the near field of disposal holes. Information is gained from fracturing with orientations and continuity as well as hydraulic conductivity, and position and continuity of electrically conductive layers and fractures. Surveys included flow logging, crosshole mise-a-la-masse measurements, optical imaging and drillhole radar surveys. Data sets were adjusted to common depth level with core logging according to geological reference points. Results were interpreted for hydraulically conductive fractures and their orientations, fracture orientations, and electrical conductivity features from mise-a-la-masse and radar. Geophysical results were compared with geological information. The drillholes are located in a sparsely fractured rock mass. Rock quality is very good and lithology heterogeneous, consisting of veined, diatexitic and mica gneisses and granite pegmatite. Prediction of continuous features from several drillholes on hydraulically conductive features, orientated with drillhole imaging, with crosshole mise-a-la-masse electrical connections, and with radar reflections. Hydraulic conductivity is low and concentrated to a few fractures. Resistivity varies strongly and is associated with mineral conductors in 3 to 5 zones. The continuity of conductive bodies between the drillholes was defined with mise-a-la-masse. Zones themselves may not appear as reflectors but their boundaries are radar reflecting surfaces. Many reflectors originate from lithological variation or foliation, and some from hydraulically conductive fractures. Recognition of these reflectors was based on geological relationships. Only a few core fractures were found to be continuous. Their length cannot be directly measured from drillholes. 3D presentations were compiled to illustrate the results. Proposed techniques for characterisation in ONKALO were demonstrated. However, methods and interpretation can be developed further, possibly to include a seismic reflection survey within the method palette, to describe fracturing. (orig.)