Geohazard assessment related to submarine instabilities in Bjørnafjorden, Norway

Abstract

This paper presents the geohazard assessment for a proposed bridge across Bjørnafjorden in western Norway. The fjord is c. 5 km wide with a maximum depth of 550 m at the proposed bridge crossing. The main geohazards of concern are submarine slope instabilities. To identify locations of instability, their susceptibility to failure, and their potential runout distances, we performed the following analyses: (1) static and pseudo-static limit equilibrium analyses for the entire fjord crossing area; (2) 1D seismic slope stability sensitivity analyses for different slope angles and soil depths; (3) 2D static and pseudo-static finite element analyses for selected profiles; (4) back-analysis of a palaeolandslide; and (5) quasi-2D and quasi-3D landslide dynamic simulations calibrated using results from the back-analysis. The workflow progresses from simplified to more advanced analyses focusing on the most critical locations. The results show that the soils in many locations of the fjord are potentially unstable and could be the loci of landslides and debris flows. The evidence of numerous palaeosubmarine landslides identified on geophysical records reinforces this conclusion. However, the landslide triggers and timing are currently unknown. This paper demonstrates the need for comprehensive and multidisciplinary geohazard analyses for any infrastructure projects conducted in fjords.

Advancing from subaqueous mass movement case studies to providing advice and mitigation

“…The Geological Survey of Canada has undertaken a multiyear and multidisciplinary study (Lintern et al 2019) to understand the magnitudes, frequencies and causes of tusnamigenic landslides in Kitimat Arm, British Columbia, an area with high port development potential which happens to be a very well-known example of submarine mass movement. The Norwegian Geotechnical Institute has moved on from a similar multidisciplinary and multiyear study in coastal and deepwater areas (Vanneste et al 2012) to now providing geotechnical advice to government on major infrastructure projects, such as a bridge project in the Norway's Bjornafjorden (Carlton et al 2018). Both of the above examples demonstrate the need for comprehensive and multidisciplinary geohazard analyses for any infrastructure projects conducted in fjords.…”

Geotechnical characterization and stability analysis of subaqueous slopes in Lake Lucerne (Switzerland)

“…Among the most widely used approaches to evaluate the static slope stability are the limit equilibrium method (LEM; Morgenstern and Price 1965;Kramer 1996), the infinite slope method (1D LEM; Morgenstern 1967), the finite element (FEM) and finite difference (FDM) methods (Mansour and Kalantari 2011;Baba et al 2012;Chatzi and Escallon 2013). FEM or FDM allow a comprehensive analysis of the target structure (De Martin 2010;Smith et al 2016;Stoecklin et al 2017;Carlton et al 2019) but require also detailed site information, uncertainty estimates and understanding of the nonlinear behavior of sediments under seismic loading. Consequently, they have high computational demands and are rarely used to analyze the stability of slopes on large areas.…”

“…Consequently, they have high computational demands and are rarely used to analyze the stability of slopes on large areas. In contrast, the infinite slope and LEM methods are quite conservative and simplistic but allow a rapid assessment of the slope stability at multiple locations and large areas, thus they are widely used in different case studies (Strasser et al 2011;Duncan et al 2014;Strupler et al 2018a;Carlton et al 2019). The most crucial parameters for the stability analysis are the undrained shear strength and the unit weight of the sediments together with the slope angle (e.g., Sultan et al 2010;Ai et al 2014;Wiemer et al 2015;Strupler et al 2020).…”