ABSTRACT
The project «New Water Supply Oslo» is currently using three Tunnel Boring Machines (TBMs) for boring in total 30 km of water tunnels for the Municipality of Oslo. From the western access point the first TBM is boring through different layers of rhomb porphyric rock, which is known for its exceptionally high hydraulic conductivity. With overburden up to 350 meters, combined with the extremely high hydraulic conductivity of the rock mass, the TBM was expected to face very challenging water conditions. As the tunnel is bored with a declination towards the tunnel face, the rock mass in front of the TBM must be treated to prevent the TBM from potential flooding. It was therefore foreseen a lot of pre-grouting from the TBMs to keep the water under control. All three TBMs in the project have been equipped with a new and innovative drilling shield to be able to handle the challenging conditions that were expected. This drilling shield makes extra room for the high-tech drilling jumbos that are used for fast and efficient probe drilling. The first two TBMs started up in January and March 2023, and the encountered conditions were as expected. Open fractures in the rhomb phorphyric rock connected to layers of agglomerate in the transition zones have been the most challenging to handle. Leakages of several thousand liters a minute with high water pressure have been encountered. The TBM from the east met lower water pressures, but it was still performed considerable amounts of pre-grouting to prevent settlements to buildings on top of the tunnel. The drilling and grouting equipment installed on the TBMs worked as expected and helped reducing the time spent in challenging conditions to a minimum. The experience from the New water Supply for Oslo project is that pre-grouting can be an effective and preferred method for groundwater control, also in TBM projects.
