Pressurized TBM-shield tunneling under the subsidence sensitive grounds of Oslo: Possibilities and limitations

The marine clay deposits covering the bedrock under the Norwegian capital Oslo are very sensitive to pore pressure reduction. Existing tunnels under the city are draining the ground to varying extents, which have led to settlement damages to buildings and infrastructure. Oslo is the fastest growing capital in Europe, and there is great demand for underground excavations for trains, roads, subways, power cables, water, and wastewater. It will be very important to limit drainage into new tunnels during the construction phase and in the permanent state. This paper discusses the possibilities and limitations of pressurized (closed-mode) TBM-shield tunneling as an alternative to traditional drill and blast (or open-mode TBM excavation) with pre-excavation grouting. Earth pressure balance and slurry shield machines with gasketed precast concrete segmental linings, which are normally used in soil and soft rock tunneling, may give superior drainage control compared to non-pressurized (open) methods; but these machines have challenges with high wear when used in hard rock. The use of traditional hard rock single-shield machines built with the option to convert into closed-static-mode is another, and probably a more realistic alternative, which should be considered in order to better control drainage during the construction phase.