Analyzing complex rock slope deformation at Stampa, western Norway, by integrating geomorphology, kinematics and numerical modeling

• Published in: Engineering geology Vol. 154; pp. 116 - 130
• Main Authors: Böhme, Martina, Hermanns, Reginald L., Oppikofer, Thierry, Fischer, Luzia, Bunkholt, Halvor S.S., Eiken, Trond, Pedrazzini, Andrea, Derron, Marc-Henri, Jaboyedoff, Michel, Blikra, Lars H., Nilsen, Bjørn
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier B.V 28-02-2013; Elsevier
• Subjects: Applied sciences; Buildings. Public works; Cliffs; Computation methods. Tables. Charts; Continuum modeling; Deformation; Discontinuum modeling; Exact sciences and technology; Failure; Geomorphology; Geotechnics; GNSS; GPS; Mathematical models; Rock; Rockfall; Rockslide; Soil investigations. Testing; Soil mechanics. Rocks mechanics; Structural analysis. Stresses; Structural geology; Terrestrial cosmogenic dating; Norway; Europe; Rockslide; GPS; Terrestrial cosmogenic dating; Continuum modeling; Structural geology; Discontinuum modeling; Discontinuous; Geomorphology; Rock mechanics; GPS system; Geology; Site analysis; Rockfalls; Modeling; Continuum; Kinematics; Displacement measurement; Numerical simulation; Structural analysis
• ISSN: 0013-7952; 1872-6917
• DOI: 10.1016/j.enggeo.2012.11.016

The unstable rock slope, Stampa, above the village of Flåm, Norway, shows signs of both active and postglacial gravitational deformation over an area of 11km2. Detailed structural field mapping, annual differential Global Navigation Satellite System (GNSS) surveys, as well as geomorphic analysis of high-resolution digital elevation models based on airborne and terrestrial laser scanning indicate that slope deformation is complex and spatially variable. Numerical modeling was used to investigate the influence of former rockslide activity and to better understand the failure mechanism. Field observations, kinematic analysis and numerical modeling indicate a strong structural control of the unstable area. Based on the integration of the above analyses, we propose that the failure mechanism is dominated by (1) a toppling component, (2) subsiding bilinear wedge failure and (3) planar sliding along the foliation at the toe of the unstable slope. Using differential GNSS, 18 points were measured annually over a period of up to 6years. Two of these points have an average yearly movement of around 10mm/year. They are located at the frontal cliff on almost completely detached blocks with volumes smaller than 300,000m3. Large fractures indicate deep-seated gravitational deformation of volumes reaching several 100million m3, but the movement rates in these areas are below 2mm/year. Two different lobes of prehistoric rock slope failures were dated with terrestrial cosmogenic nuclides. While the northern lobe gave an average age of 4,300years BP, the southern one resulted in two different ages (2,400 and 12,000years BP), which represent most likely multiple rockfall events. This reflects the currently observable deformation style with unstable blocks in the northern part in between Joasete and Furekamben and no distinct blocks but a high rockfall activity around Ramnanosi in the south. With a relative susceptibility analysis it is concluded that small collapses of blocks along the frontal cliff will be more frequent. Larger collapses of free-standing blocks along the cliff with volumes >100,000m3, thus large enough to reach the fjord, cannot be ruled out. A larger collapse involving several million m3 is presently considered of very low likelihood.
► Strong control of the gravitational slope deformation by inherited structures. ► A complex failure mechanism composed of toppling, subsiding bilinear wedge failure and planar sliding is proposed. ► Different deformation styles with larger unstable volumes in the north and high rockfall activity in the south are explained.