In ONKALO, 110 km north of Turku, Posiva are constructing an underground
facility for disposal of nuclear waste. The facility today consists of an access
tunnel down to 455 m at its lowest point, three shafts and some test tunnels.
All tunnels have been excavated with drill and blast (D&B) method, which is the
current reference method for tunnelling in the final disposal facility.
One of the many requirements for tunnel excavation is to avoid producing
an excavation damaged rock zone (EDZ) behind the tunnel surfaces. As part
of this, intensive studies of the EDZ have been conducted. In one of the latest
study 4 blocks were excavated from the floor in a test tunnel and then cut into
40 slices. Each slice was then analyzed for fractures by using penetrants and
high definition photography. A 3D model was made connecting the fracture
traces to fracture plans. The paper describes the excavation, the analysis of
the fractures and the modelling.