A risk-based approach to the quick clay landslide at Flatanger, Norway – comparison of national regulatory guidelines

¹ Norwegian Geotechnical Institute, Trondheim, Norway
² Multiconsult Norge AS, Trondheim, Norway

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

The quick clay landslide that struck Flatanger in Mid Norway on March 10, 2023, posed significant challenges for the safe reopening of County Road 6928. Key concerns included the risk of additional slide debris reaching the road, potential for new retrogressive failures from the shoreline, and the stability of the road itself. According to Norwegian Public Roads Administration regulations N200, the safety levels for repairing roads are in general the same as required for building new roads. For Flatanger, this would have implied extensive stabilization measures with substantial environmental, climatic, and economic impacts. Through an analysis of relevant legislation, regulations, and guidelines, this study explores how safety requirements can be interpreted and adapted while maintaining public safety. At Flatanger, a design strategy was chosen that prioritized addressing triggering factors rather than imposing large-scale interventions to achieve absolute safety factor requirements. The post-slide work highlights the importance of a flexible approach to regulatory interpretation and risk management in complex quick clay environments. This paper investigates and compares the regulatory guidelines for Norway, Sweden and Finland in light of the Flatanger landslide. To what extend does these different guidelines open for a risk-based approach, and under which prerequisites? The key findings show that Norway and Sweden have similar guidelines which would have led to the same solution at Flatanger. But also, it is evident that the Swedish regulations are more consistently founded on probabilistic principles. As Finland has less issues with sensitive clays and poor stability, their regulations are to a lesser extent adapted to situations like the one at Flatanger, resulting in massive stabilizing measures if applied.