Engineering geology of the Jettan rockslide, Kåfjorden

Abstract

The unstable rock slope (URS) Jettan is located at Nordnesfjellet in Kåfjorden municipality in Troms and Finnmark County, 800 m above the fjord. The rockslide has an estimated volume of 6 Mm3, and moving at a rate of up to 50 mm a-1. Jettan is considered a “high risk object” within the Norwegian unstable mountain framework. This is due to the potential displacement wave impact to housings, infrastructure and industry. As such there has been a scientific interest in the site since 1999 generating several studies, reports, investigations and theses. Continuous monitoring began in 2007. The goal of this thesis was to gain a larger understanding of the unstable rock slope and its driving mechanisms. To do this, exiting data on lithology and structure, geophysics, borehole investigations, seasonal movement trends, past avalanche activity, published geological models and engineering geology studies were reviewed. This array of data was complimented by the work of this thesis including in-depth lithological study, rock mass descriptions, detailed geomorphological mapping, an updated analysis of movement and external drivers. The existing data, and the data gaps covered by this thesis, allowed the construction of a 3D model. Previous studies have shown that Jettan is highly seasonally controlled. High movement rates are recorded in the spring due to snow melting and a continuous deformation in the autumn is considered to be due to permafrost processes. Analysis in this thesis confirmed seasonal variations at Jettan, with high deformation in the summer, lower deformation in the winter and a lower but continuous deformation in the autumn. Jettan is a complex URS with areas showing different morphology, movement direction and movement rates. As dip of the foliation is rarely above 25°, a possible sliding surface is assumed to be a combination of foliation planes and joint sets building a stepped sliding surface. In field a repeating weakness zone was found parallel to the foliation, and it is suggested that it contributes to the overall reduction of the stability for the slope together with groundwater processes. In the boreholes the main sliding surface was interpreted to be at 45 m bgl. The 3D model supports both a stepped and planar sliding surface, and suggested several possible failure scenarios. New volume estimates gave a volume of 7.87 Mm3, for the most realistic larger failure scenario. This is a larger estimate than previous studies, and the greater depth to the sliding surface in this interpretation is seen as the main reason for a larger volume in the 3D model.