Characteristics and failure mechanisms of the unstable rock slope Dusnjárga

Abstract

Large slowly creeping rock slope deformations, with an annual displacement in the range of millimeters to centimeters, have been prone to rapid catastrophic failures in the past. This master thesis focuses on the unstable rock slope of Dusnjárga in Northern Norway. A sudden rapid failure of Dusnjárga would pose a threat to the local society in Lille Altafjord and Burfjord, due to potential secondary effects such as displacement waves. A detailed investigation of the rock slope is needed to understand its mechanical and kinematic characteristics and to properly assess its hazard. A two-dimensional (2D) satellite-based method, based on Synthetic Aperture Radar Interferometry (InSAR) from two satellite geometries (ascending and descending orbits), was combined with morphological and structural analyses to assess the kinematics and failure mechanisms of the slope. The 2D InSAR results were decomposed into vertical and horizontal velocities along terrain profiles and compared with simple finite element simulations of known failure mechanisms. Additionally, 3D InSAR was utilized for a certain part of the slope, by combining InSAR from the two satellite geometries with a ground-based radar (GB-radar). Displacement data from differential Global Navigation Satellite System (dGNSS) and InSAR, indicates that the slope can be separated into an inactive head domain and an active rock slope underneath. Although the head domain is generally inactive, it contains a minor local block with a noticeable displacement rate between 7–15 mm/year. The active rock slope deformation has a peak velocity of 15–20 mm/year in its central domain, with velocities decreasing downslope until it is halved at the toe domain. From examining morphological structures and carrying out structural analyses, the active parts of the rock slope are found to be joint- and foliation-controlled. The foliation and a single joint set are controlling the morphologic elements oriented perpendicular to the slope movement, such as the backscarp, while two sub-vertical joint sets are controlling the lateral limits and spatial lateral variations in velocity. From comparing the 2D InSAR velocity vectors to simple finite element simulations, the rear rupture surface was interpreted to have a roto-translational failure mechanism. The controlling structure of the basal rupture surface was proposed to be the foliation, as velocity vectors follows the orientation of the foliation and strength tests revealed anisotropic properties. The foliation at Dusnjárga follows both an anticlinal and synclinal fold, causing diverse stress-regimes and an upwards moving compressed toe area. The rock slope deformation has been classified as an irregular complex bi-planar compound slide. A hazard rating of medium was proposed, but the special geometry at the toe domain could suggest that Dusnjárga will have increased stabilisation in the future.