Reconstruction of the Kråknes landslide event, Alta

Abstract

On June 3rd, 2020 a major quick clay landslide occurred at Kråknes in Alta municipality, Northern Norway. The landslide event destroyed eight buildings and was estimated to be 650 meters wide and 40 meters high. Two days later, another landslide occurred, this time causing the old E6 road above the previous landslide area to slip out towards the sea. People lost cabins and other valuable possessions, but there were no casualties. Landslide events in sensitive clay along the Norwegian coastline pose a threat to coastal communities and infrastructure and have frequently caused fatalities and damage throughout recent history. To avoid and prepare for potential landslide disasters, it is crucial to study and understand the development of sensitive clay landslides. In this study, the Kråknes landslide in Alta, Northern Norway has been reconstructed based on field observations, geotechnical data and numerical modeling in DAN3D to gain insight into the sequence of events and better understand and prepare for potential landslide events in the future. The results show that the topography and stratigraphy of Kråknes make the area unstable and sensitive to external forces due to alternating permeable and impermeable layers in a moderately dipping fjord sequence created by the ice sheets during deglaciation and exposed by isostatic uplift. This stratigraphy provides good conditions for groundwater flow and the creation of highly sensitive clay and weak layers when exposed to excess pore pressure. Human activity has further increased the instability through loading of fill material during the construction of a cabin in 2015. After heavy amounts of snowfall during winter season 2019/2020, hydrometric data show that the ground was up to 100% saturated due to snowmelt in the days prior to the landslide. The high amounts of meltwater caused an increase in the pore water pressure in the already unstable study area and initially triggered the landslide event.

Two scenarios on how the landslide event occurred are proposed and simulated in DAN3D, where one is initiated below sea level and the other is initiated above sea level. Based on the results, it is concluded that the landslide was triggered on land as a progressive landslide, which continued to retrogress backward and laterally in several slabs. The rapid deposition of landslide mass on the seafloor initiated an underwater slide, which later caused further retrogression on land due to lack of support from the buttressing mass in front.

The expansion of residential areas and infrastructure below the marine limit can cause drastic changes in the stability of areas with sensitive clay. With the ongoing climate change we see today, the shifts in temperature and rain and snow patterns may increase the amount of groundwater compared to previous years. In fjords, the stratigraphies' ability to feed already unstable slopes with high amounts of groundwater should therefore be examined closely before constructions are made that could create weak layers and shear bands.