Some Aspects of the Norwegian Risk Evaluation System for Quick Clay Based on Recent Landslide Events

Mapping of landslide hazard zones in Norway started after the famous Rissa landslide in 1978. In the following decades, quick clay hazard zones were mapped for large areas. A semi-quantitative risk evaluation system has been established for prioritizing between the zones for more detailed geotechnical investigation and mitigation measures. Mapping of quick clay zones is still ongoing, however, the majority of already known zones have not yet undergone detailed examination. Together with the fact that the quick clay areas in Norway coincide with the most densely populated areas of the country, this means that quick clay landslides pose a considerable threat to the society. Risk is related to extent of hazards zones, to landslide triggering from natural and human factors, and from objects at risk within (known and unknown) hazard zones. The Plan and building code, geotechnical standards and guidelines give clear restrictions for new projects. However, for existing (older) residential areas, no legal requirements exist. In this paper, some aspects related to the methods used for assessing the landslide risk are discussed. Lines are drawn from the current risk assessment system to experiences from the last fatal quick clay landslide in Norway that occurred in Gjerdrum, 30th of December 2020

Keywords: Quick clay, Hazard mapping, Risk evaluation, Gjerdrum quick clay landslide