Forensic Geotechnical Investigation of the Skjeggestad Quick Clay Landslide, Norway

On February 2 2015, a quick clay landslide occurred adjacent to Mofjellbekken's two parallel bridges (referred to as the Skjeggestad landslide) on the E18 motorway from Oslo to the southeastern coast of Norway. The landslide damaged one bridge foundation which led to partial collapse of the southbound bridge. The damaged bridge had to be demolished and rebuilt. NVE, the Norwegian Water Resources and Energy Directorate, established an independent investigation commission to study the cause and the failure mechanism of the landslide. The Commission examined evidence from the soil investigation, available topographic data, observations in situ, eye witness statements and photographs taken by drones. These evidences were used to reconstruct the most probable sequence of events during the landslide. Both natural causes (erosion, rainfall, snow/ice melting) and man-made causes (vibration, fill construction or other disturbance) were investigated using the available data and stability analyses. The Commission concluded that fill placement on top of the slope in the period 1998–2006 reduced the safety margin considerably. The last fill placed shortly before the landslide occurred in 2015 was the trigger of the landslide which had by then a marginal factor of safety. This paper presents the forensic investigative process by the NVE investigation commission.