Challenges related to water inflow during construction of deep Norwegian subsea tunnels

Large water ingress represents a main challenge in subsea tunnelling, particularly when occurring at deep tunnel sections of poor rock mass quality. Water inflow with pressure of up to 2.4 MPa have in some cases been encountered when crossing major faults/weakness zones during excavation, and special procedures for stability support and grouting have been required to cope with such conditions. This paper is discussing engineering geological characteristics of water ingress and how large, high pressure water inflow has been coped with in selected cases of particularly challenging Norwegian subsea tunnels. Based on the experience from Norwegian subsea tunnel projects it is concluded that continuous follow up by experienced engineering geologists, systematic probe drilling and pre-grouting adapted to the prevailing rock conditions and performed at sufficient distance from the leakage zone are the most important factors for coping with water ingress and ensuring stability.