Anthropogenic fill and geological-hydrological controls on progressive failure in the 2023 Stenungsund quick-clay landslide (SW Sweden)
On 23 September 2023, a rapid quick-clay landslide occurred at the Stenungsund interchange on the E6 highway in southwestern Sweden, severely damaging national and local infrastructure and closing a key transport corridor for 285 days. The landslide affected approximately 15 ha, with a runout of 620 m and an estimated displaced volume of 1.2–1.85 million m 3 . This study reconstructs the geological, hydrological, and anthropogenic conditions that led to failure by integrating field observations, stratigraphic and geotechnical data, LiDAR-based elevation models, aerial and satellite imagery, and hydrological flow modeling. The site is underlain by thick late- and postglacial marine clays interbedded with permeable silt, sand, and shell-rich layers within a fracture-valley landscape. These sediments facilitated groundwater flow and salt leaching, resulting in high clay sensitivity. Results suggest initiation as a deep-seated translational progressive failure beneath anthropogenic fill, as inferred from finite element modeling and field observations/stratigraphy. Construction-related loading was interpreted as the primary trigger, while altered drainage patterns, increased infiltration along fractured bedrock, and elevated pore-water pressures—likely intensified by heavy rainfall shortly before the event—controlled the timing and magnitude of the failure. Loss of resistance downslope enabled the rapid mobilization of quick clay and resulted in large horizontal displacements. The Stenungsund event demonstrates how cumulative anthropogenic modifications can destabilize sensitive clay slopes. The findings highlight the importance of integrated geological–hydrogeological assessments in infrastructure planning and the need for improved risk management in quick-clay terrain.