Back Analysis of a Rainfall Induced Landslide, Eidsvoll, Norway

Landslides in Quaternary soils are common in Norway and constitute a substantial risk to society. Landslides can be of several types: Debris flowsoccur as the result of erosion and entrainment. Smaller local landslides described as rotational or translational slides are frequently released by infiltration from rainfall and snowmelt,including effects of ground water flow. Many landslides occur onsteep slopes that depend onunsaturated conditions to remain stable. This paper presents a case study of one landslide occurring in the autumn of year 2000, during which Eastern Norway received unusual amounts of rainfall. Slopes in the area are stable for normal annual rainfall. For rainfall during the autumn of year 2000, with an estimated value of about 240 mm in 30 days based on gridded values from near-by metrological stations, infiltration of surface water resulted in a gradual decrease in slope stability, leading to slope failure. Long-term rainfall triggered numerous landslides in the region, most of them shallow, involving mostlyintermediate soils (some shallow landslides were also observed in clay slopes). The case study involvesground investigations including soil sampling and laboratory testing of unsaturated soil properties, followedby stability analyses taking retention properties and unsaturated strength into account.Long-term infiltration is shown to gradually reduce the stability of the actual slope, until critical stability was reached.

Keywords: Landslide, Unsaturated, Rainfall, Infiltration, Stability, Water retention.