Probe and Pilot Holes

Probe holes are bored and pilot holes drilled from ONKALO in order to gather information about the rock to be excavated.

In the rear end of the tunnel, four probe holes are drilled at a distance of approximately 20 metres from each other. Measurements related to leak water, water loss and flowing are performed through the holes. Information gathered on the basis of the probe holes is then employed in the tunnel excavation planning and execution.

From the rear end of the ONKALO tunnel, a number of pilot holes will be drilled at pre-determined locations. These pilot holes will remain within the tunnel profile. The purpose of these pilot holes is to verify the rock quality at the location the tunnel is going to be before any excavation takes place, and in particular to locate any water-conducting fracture zones and other rock characteristics that may be significant for the construction.

Pilot holes are used for geological, geophysical, hydrogeological and hydrochemical studies. Samples taken include the drill core sample that can be used for determining the rock type and the fracturing of the rock to be excavated.

The length of the pilot holes varies between 50 and 300 metres. By the end of 2008, nine pilot holes had been drilled. According to plans, five more pilot holes will be drilled from ONKALO.