The Construction of ONKALO

The method used for excavating ONKALO is drilling and blasting. The shafts are constructed by raise boring.

ONKALO is being constructed five days a week, in three shifts. Some 25 metres of tunnel is excavated each week. The work proceeds in five-metre long sections, also known as rounds.

The Tunnel Grows by Drilling and Blasting

The access tunnel is excavated by drilling and blasting: at the beginning of excavation, a number of holes are drilled and then filled with explosives. Once the rock material has been blasted, the rock waste is moved away. The rock walls are then washed and any broken stones are dropped down.

The rock is sealed, where necessary, by grouting before and after excavation, as well as by using structural water insulation. Cemented anchor bolts made of ribbed steel bars as well as shotcrete are used to strengthen the rock.

Shafts Constructed by Raise Boring

There will be three shafts in ONKALO: a personnel shaft, a supply air shaft and an exhaust air shaft. The diameter of the passenger shaft is 4.5 m and the diameters of the supply air shaft and exhaust air shaft are 3.5 m.

While ONKALO is being constructed, the personnel shaft is used as a combined exhaust air and smoke outlet shaft. This shaft will be used for its nominal purpose once the ONKALO construction project is finished and the permanent personnel lift to the research tunnel is installed in the shaft.

The exhaust air shaft is used as supply air shaft while ONKALO is being constructed. The exhaust air shaft is also equipped with a maintenance and hoisting lift that can be used when performing installations and controls and, in emergency situations, for lifting people up from lower levels and as a connection path for the fire brigade.

The shafts are constructed with the raise boring method. In the raise boring method, a pilothole is first drilled down. Once the drill is down, a reamer bit is installed to the drill, which is then pulled up while the bit is rotating. The reamer bit moves upward at about half a meter per hour. Approximately 100 m of shaft is drilled at one time.