Geophysical Mapping Techniques of Excavation Damage Zone for Quality Control of Drill and Blasting in Crystalline Rock

The Excavation Damage Zone in crystalline bedrock was mapped under controlled conditions. The work was carried out in drill and blasting excavated Investigation Niché at depth level -350 m along access tunnel to ONKALO underground characterization facility, Olkiluoto. The work belongs to geological disposal project of spent nuclear fuel in Finland, carried out by Posiva. The work was co-operated with SKB, Sweden. EDZ may form hydraulic flow path on the tunnel surface and thus impact to retardation and transportation of radionuclides and further to long term safety of disposal. Quality control of drill and blasting excavation is advised for ensuring that the constructed disposal tunnels will meet the set requirements. Disposal tunnels need to express low hydraulic conductivity. The excavation method to be applied has to enable discontinuous EDZ along the tunnel surface. It was found necessary to develop a reliable mapping and assessment technique of EDZ. Geophysical investigations were applied before and after excavation on same locations. The results were compared between baseline and repeat surveys. Results were compared also between several geophysical techniques including seismic tomography and GPR mapping to understand EDZ property. Mapping and processing technique of GPR dispersivity was developed for efficient EDZ characterization.