An investigation into the causal factors of the Kråknes landslide

Abstract

On the 3rd of June 2020, a landslide measuring 600 meters long and 160 meters wide initiated along the shoreline at Kråknes, in Alta municipality in Troms and Finnmark County. The landslide was comprised of a series of smaller slides, which destroyed eight houses and a caravan in the process. Fortunately, no human lives were lost in the event. Due to the widespread presence of sensitive clays along the Norwegian coast, numerous areas are at the potential risk of landslides. As these landslides pose a risk to human life and infrastructure, a better understanding of what promotes instability and triggers these types of landslides is necessary. The goal of this thesis is to determine the causal factors responsible for the slope failure at Kråknes. To do this, hydrometeorology, ground conditions and human activity were analyzed in combination with slope stability modeling. The array of data used in this thesis comprises meteorological reports and models, field observations, unmanned aerial vehicle surveys, LiDAR- and bathymetry data, and a consultant geotechnical survey. The results indicate that the initial stability at Kråknes was low. Signs of quick- and sensitive clay were observed during fieldwork, and were also detected during the geotechnical survey. The layered stratigraphy of the ground conditions, with permeable- and non-permeable layers are interpreted to have intensified the leaching process of the marine clay and promoted the development of excess pore pressure. With the added load of the cabin building in 2015, it is suspected that the average stability at Kråknes was lowered enough to be triggered by an abrupt snowmelt event in the second half of May. As a result, the formation of a tension crack on the 2nd of June is interpreted to be the initiation of a progressive flake-type failure that spread north overnight to where the slide initiated below Kråknesveien 416 on the 3rd of June. The slope stability modeling supported the expected negative effects higher groundwater levels, excess pore pressure and external loading from the cabin building might have had. However, some model results regarding the dimension of the failure surface and factor of safety did not reflect reality.