Tunnel Drammen (Norway) – Solution for excavating a tunnel in loose soil with the presence of groundwater

Abstract

AbstractThe soil tunnel in Drammen (Norway), part of Bane NOR's ‘New Double Track Drammen–Kobbervikdalen' project, has a length of 290 m; however, the different unfavourable boundary conditions combined with a large excavation cross section (156 m2) make this section the most technically challenging part of the project. The soil tunnel is driven mostly in saturated glaciofluvial sediments, which consist primarily of sand and gravel with a low content of fines, resulting in a low cohesion to cohesionless soil. The groundwater level, always within or above the tunnel section, cannot be lowered during the construction due to project requirements. These geological/hydrogeological conditions together with the large cross section present a high‐risk scenario for any tunnelling project and, in the case of Norway, an unprecedented challenge. Moreover, the tunnel is located in an urban area and has a low overburden of just 8 to 9 m during the first 80 m. For the client it was critical that the design solution was safe, robust, and able to respond to the existing level of uncertainty and potential unexpected occurrences. ILF's design is able to answer these technical challenges while removing many of the typical risks associated to saturated soil tunnelling with a solution that relies mainly on extensive pre‐excavation ground improvement works in the form of jet grouting executed from the surface.