Vertical Shaft Pilot Hole Investigations in Pre-assessment of Orientation, Transmissivity and Continuity of Fractures

Spent nuclear fuel disposal in crystalline bedrock requires knowledge on long and hydraulically conductive fractures near disposal holes and tunnels. It is possible to investigate fractures with different techniques before drilling of pilot holes, before excavation from pilot holes, between tunnels as crosshole survey, or from the surfaces of excavated tunnel. This work included review of materials obtained in a 105 m long section of vertical shaft, investigated before raise-boring, and using single-hole and detailed crosshole investigations performed in and between three drillholes located at 3.2 m distances from each other, within shaft perimeter. The geological mapping of shaft took place after the raise-boring. From the results it was possible to draw conclusions on applicability of these methods in detailed model compilation. Combined use of geological, imaging and geophysical logging will assist detecting the fracture responses and assigning the orientation. The hydraulic difference flow logging will assign the transmissivity. Crosshole hydraulic measurement will detect the hydraulic connections between drillholes. Crosshole mise-à-la-masse survey grounded on fractures and other conductive zones will effectively reveal electrical connections. Single-hole radar survey indicates continuity of conductive features. Explanation of the features will require geological correlation.