The cause of the landslides of 1st and 2nd April 2016 in Tosbotn = Norway La cause des glissements de 2016 à Tosbotn, Norvège
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On April 1st 2016 a 50 000 m3 landslide, partly in clay, took a coastline road in Nordland, Norway. While geotechnical engineers and construction workers focused on reopening the road, a much larger slide of about 130 000 m3 took place the following day, forcing the road to remain closed for 3 months. No lives were lost and no persons were injured by the slides, but the road and three houses were destroyed. The two landslides were very close but not in physically connected. Sensitive clay was involved. Shortly before the slides a water filling took place in a new hydropower tunnel in the mountainside behind the landslides. An investigation committee consisting of the authors of this paper was appointed by the local government in Nordland to identify the technical cause of the slides. The committee’s final conclusion was that the landslides most likely were initiated by the increased pore-pressures in the soil after hydraulic fracturing and leakage from the unlined pressurized head race tunnel for the hydropower plant. The paper presents geological and geotechnical conditions, measured rock stresses around the hydropower tunnel, soil properties and results from stability analyses.