Strong restrictions on environmental aspects in respect to ground water table has included an extensive and systematic grouting process through out the whole tunnel length. After 90 % of the main tunnel excavated (February 2004), the project has reached the strict demands on water leak (less than 8 liters/minute and 100m tunnel) after a well prepared and implemented grouting program.
One of the major challenges has been the low rock cover from the Asker access. Long parts of the tunnel (apprx 100 meters) with rock cover from 2,7 to 5 meters in a tunnel 13 meter wide (cross section 104 m2), has caused extensive efforts both in excavation methods, extensive rock support, and vibration control on sensitive buildings above.
Figure: Digitalized drawing for geological follow up.
Continuous geological mapping and decision of rock quality and support after each blast is implemented into digitalized drawings.
Each drawing covering approximately a sequence of 140 meter tunnel including geology, Q-values
and permanent rock support.
The geology consists mainly of Cambro-Silurian schist and calcitic schist. The last part of the Tanum tunnel has reached badly consolidated rock in the so-called "Asker-group", which has slowed down the production. Q-values are here between 0,01 and 0,05 and extensive use of steel ribs cealed with 20cm layers of alkali-free shotcrete for every 3 meter tunnel length has been implemented. Use of longer rock bolts in this area (4-5 meters) has also caused problems with penetration of the grouting curtains with increasing water leak as a result.
Tunnel & GeoConsult is engaged by the client JBV as geologist and control engineer throughout the construction period.
Breakthrough on the Tanum tunnel was February 05, 2004, and for the Skaugum tunnel expected breakthrough is May 2004.