Modelling Seasonal Stability of Shallow Glacial Till on Steep Rock Slopes in Southeast Norway via Finite Element Analysis - A Case Study

Abstract

One of the most common geohazards in the mountainous regions of Norway are debris slides in shallow glacial till horizons overlying steep valley slopes of various bedrock. The predictions pro-duced by the established Landslide Early Warning System (LEWS) in Norway is based on mete-orological data, which are precipitation and temperature for the main part. This is the commonly established method in LEWS given that water conditions are a major control factor of slope sta-bility. The effects of seasonal frost in soils on aggregate stability and overall soil structure in the Nordic regions are poorly documented and disregarded in the established LEWS algorithms. It is however a common feature in Norway, which strongly controls the water regime. Based on projections of temperature and precipitation data only, it could not be proven that climate change will lead to an increase in slush avalanches and debris flows. However, all slope stability models including partially frozen subsoil lead to failure. Hence, the presence of frost in the sub-soil and the frequency of frost-thaw cycles are expected to be more critical in a thawing shallow till layer than water conditions alone. Main triggers for debris slides in shallow glacial tills on steep rock slopes in Norway are the antecedent pore pressure conditions combined with high intensity rainfall events and loss of friction strength along a boundary such as frost. The back-calculation of a slide in Aurdal from 2013 was moderately successful. Desk studies have limitations when it comes to modelling input parameters establishment. The most influential parameters are water contents and hydraulic conducitivity of the materials involved. Loose glacial surface till can be approximated as friction soil. A frost horizon in the subsoil can be modelled as impermeable low-friction layer. The impacts of climate chance scenarios on slope stability do not entirely unfold from the results of this project, but assumptions are made on the influence of thawing on slope stability. Aggregate stability and shear strength of a soil are at a minimum at the end of the winter season.