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Natural Hazards and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/nhess-2017-410
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/nhess-2017-410
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.

  20 Nov 2017

20 Nov 2017

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This discussion paper is a preprint. It has been under review for the journal Natural Hazards and Earth System Sciences (NHESS). The manuscript was not accepted for further review after discussion.

Influence of extreme long-term rainfall and unsaturated soil properties on triggering of a landslide – a case study

Håkon Heyerdahl1,2 Håkon Heyerdahl
  • 1Department of geosciences, Faculty of Mathematics and Natural Sciences, University of Oslo (UiO), Oslo, 0316, Norway
  • 2Norwegian Geotechnical Institute (NGI), Oslo, 0806, Norway

Abstract. A case study of a natural slope in Eastern Norway that failed after extreme long-term rainfall in year 2000 was performed. Effect of soil suction on soil shear strength was investigated for intact specimens of Quaternary silt/sand, using an unsaturated shear box apparatus. Common prediction models under-predicted the unsaturated shear strength, particularly for small suctions. Analyses of rainfall infiltration were performed for silt and sand slopes, based on retention curves measured in laboratory. For normal annual rainfall of 800 mm/year the model slope is theoretically stable. Extreme rainfall (240 mm in 30 days) during the autumn of year 2000 results in a rise of groundwater and loss of soil suction in the vadose zone. To reach theoretical slope failure, lower cohesion had to be assumed than measured in laboratory. High cohesion may be caused by cementation in shallow soil layers, and lower cohesion may be appropriate. Slope stability analyses based on transient seepage analysis of rainfall show gradual decrease of slope stability towards slope failure (for the silt slope). With expected future increase in rainfall, more attention is needed on the role of unsaturated soil properties in rainfall triggering of landslides in different soil types, climatic conditions and geologic settings.

Håkon Heyerdahl

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Håkon Heyerdahl

Håkon Heyerdahl

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Short summary
This paper deals with effects of an autumn with very high rainfall in Eastern Norway. A slope very close to a house failed after a long period of continuous rainfall. In order to improve understanding of the processes involved, a case study was performed. Soil properties relevant for soil strength and infiltration were investigated in laboratory. Stability analyses show that the rainfall gradually destabilized the slope through the autumn of year 2000.
This paper deals with effects of an autumn with very high rainfall in Eastern Norway. A slope...
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