Slope Stability Assessment for Deep Fjord Crossings Using VHR Geophysical Data – An Example from Bjørnafjorden, Norway

The Norwegian government will replace many ferry crossings along the coastal highway E39 with bridges or tunnels. Geophysical data from most fjords show ample evidence of repeated slope instabilities, which are a hazard and risk for the foundations of the infrastructure. The fjords are often several km wide, and hundreds of metres deep. In this study, we combined very-high-resolution swath bathymetry and sub-bottom profiling data as well as in situ geotechnical data to investigate the hazard potential of such landslides in Bjørnafjorden. We used a 1D slope stability screening study to find suitable anchoring locations. Back-analysis of a well-constrained landslide using a geometry reconstructed from the interpretation of the geophysical data and validated by 2D finite-element slope stability modelling provides key input to constrain the rheological parameters. The results were then used to assess slope stability across the fjord basin, as well as landslide dynamic simulations in quasi-2D and quasi-3D, resulting in flow velocities above 10 m/s and run-out distances of several hundreds of meters. The work concludes with some recommendations for similar investigation and site characterization.