Feasibility of tunnel boring through weakness zones in deep Norwegian subsea tunnels

Norwegian subsea tunnels have all been excavated with the drill and blast method. The prevailing rock mass quality is generally favorable for tunneling, but the encounter of weak and/or water bearing zones is normal, and sometimes leads to extreme challenges. Future Norwegian subsea tunnels might benefit from the use of tunnel boring machines (TBMs), but the less flexible nature of a TBM will require more effort with regards to investigations and evaluations in the pre-construction phase. This paper summarizes some of the extreme challenges encountered in Norwegian subsea road tunnels, and reviews experience from international TBM projects considered relevant for Norwegian tunnels. The focus is on geological hazards, their implications, and mitigation measures. The aim is to assess the feasibility of tunnel boring through subsea weakness zones. Due to uncertainties and limitations with pre-construction investigations/interpretations for subsea tunnels, there will always be a remaining risk of encountering difficult ground. It is shown that it can be hard to predict adverse rock mass behavior ahead of the face during tunneling. Based on recent state of the art large diameter ( > 12 m) TBM technology, it is concluded that closed-mode excavation may be considered feasible for water pressures up to ca. 100 m. Pressurized TBMs can reduce risk and may enable excavation through unfavorable rock mass conditions, but this will require continuous installation of a gasketed segmental concrete lining (undrained solution), which can mean a conservative lining design for the rest of the tunnel. Adverse rock mass behavior and/or sudden large water inflow at high pressure can be challenging to handle with open-face TBMs. Based on the above large diameter tunnel boring is considered to involve a high risk for water pressures above ca. 100 m, and is therefore not recommended. The use of a pilot tunnel to investigate and treat the ground ahead of the main tunnel(s) can be a way to reduce risk. In order to reduce contractual risk, the inclusion of a drill and blast section to be used in the case of extreme challenges, can be wise. The potential for squeezing should be evaluated for weakness zones of substantial width, and 3D numerical analysis are encouraged for zones where squeezing challenges are expected.