The Refne landslide, Halden, Norway: case history and use of risk assessment
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In 2014, a landslide in a steep slope beneath an apartment block in Refne in the city of Halden, Norway led to the evacuation of over 60 residents from their homes. The landslide is assumed to have been triggered by unusually high levels of rainfall over the preceding months, leading to accumulative long-term rainfall not previously seen in the lifetime of the block. Raised groundwater level and increased pore water in the saturated and unsaturated (vadose) zones are assumed to have contributed to reduced stability of the slope. An initial risk assessment was performed to aid communication with local authorities and stakeholders and to clarify necessary actions. Following the evaluation of potential scenarios affecting the structural integrity of the pile foundations, the risk to residents was found to be unacceptable, and evacuation was therefore maintained. In particular, the potential development of a quick clay slide would have had catastrophic consequences. In order to improve the basis for further risk analyses and allow the design of stabilising measures, geotechnical site investigations (SI) were undertaken. Following the SI results and the completion of stabilising measures, an updated risk analysis was issued. This concluded a reduced level of risk for all landslide scenarios previously evaluated, and that the risk to residents was adequately reduced. This paper shows how the visualization of risk using diagrams for risk analysis enabled communication and understanding between the geotechnical and structural engineers, police, local authorities and other stakeholders throughout the project, and formed the basis for decisions regarding evacuations and necessary mitigation measures. Engineering judgment is essential when reviewing risks in any natural hazard situation. Graphical risk analysis tools allow experience to be quantified and communicated to both experts and laymen alike so that operational decisions regarding public safety can be taken.