Analyzing complex rock slope deformation at Stampa, western Norway, by integrating geomorphology, kinematics and numerical modeling
Highlights
► 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.
Introduction
In order to understand the deformation of a complex unstable rock slope and to develop a coherent geomechanical model the integration of different data sources and results from different analysis methods is necessary (e.g. Zangerl et al., 2010, Gischig et al., 2011). Numerical modeling techniques are especially helpful to understand the failure mechanisms of complex rock slope deformations, where basic kinematic analyses often present an oversimplification (Stead et al., 2006). However, especially for structurally controlled instabilities, the latter are still essential for a first evaluation of the slope situation (Coggan et al., 1998, Stead et al., 2006). Kinematic models yield valuable information as inputs for more advanced numerical models. It has been demonstrated in numerous studies that structural control plays an important role for large rock slope instabilities (e.g. Terzaghi, 1962, Cruden and Varnes, 1996, Hermanns and Strecker, 1999, Agliardi et al., 2001, Sartori et al., 2003, Jaboyedoff et al., 2009). The influence of inherited structures, mainly pre-existing joint sets and the metamorphic foliation, has been pointed out to be especially important in western Norway, where the study area is located (Braathen et al., 2004, Böhme et al., 2011, Henderson and Saintot, 2011, Saintot et al., 2011).
Historical records and geological studies of western Norway show a high concentration of post-glacial gravitational slope failures as well as current rock slope instabilities (Blikra et al., 2006, Böhme et al., 2011, Saintot et al., 2011). Several catastrophic failures causing tsunamis in the inner fjord areas of western Norway resulted in large loss of life in the last century (Furseth, 2006). The unstable rock slope, Stampa, above the village Flåm, Aurland municipality, is one of the largest actively deforming rockslide areas known today in Norway (Braathen et al., 2004). An area of up to 11 km2 that extends 7 km N–S and 2 km E–W exhibits signs of both active and postglacial gravitational deformation (Fig. 1, Fig. 2). Movement rates vary considerably over the entire unstable area, ranging from average 3D movement vectors of 15 mm/year at one delimited block down to below significance level (Hermanns et al., 2011a).
We carried out detailed structural field mapping, yearly differential Global Navigation Satellite System (GNSS) surveys from 2005 to 2011, as well as an analysis of a high-resolution digital elevation model (HR-DEM) based on airborne laser scanning (ALS) data and several terrestrial laser scanning (TLS) surveys. In addition, continuum and discontinuum numerical modeling were used to understand the influence of former rockslide activity as well as the deformation of the unstable slope.
Section snippets
Overview of the study area
The unstable rock slope, Stampa, is located on the eastern slope at the southern end of Aurland Fjord, a branch of Sogne Fjord in western Norway (Fig. 2). The bedrock in this area consists of Lower Palaeozoic and Precambrian metamorphic rocks. During the Caledonian Orogeny, the rocks were intensely reworked by a general NW–SE oriented crustal shortening that resulted in a thrust sheet transport towards SE onto the Precambrian basement (Roberts and Gee, 1985). The instability is completely
Methods
Detailed structural field mapping was undertaken between 2008 and 2010. More than 2500 orientation measurements of joints and foliation were recorded at 122 localities (Fig. 2, Fig. 4). Terrestrial laser scanning (TLS) surveys were carried out in 2008 and 2009 on two different locations, TLS 1 focusing on the cliffs at Ramnanosi and TLS 2 focusing on block A1 (Fig. 2). A detailed structural analysis of the TLS data was conducted using the Coltop3D software (www.terranum.ch; Jaboyedoff et al.,
Method
Simple kinematic feasibility tests for planar sliding, wedge failure and toppling have been made based on the criteria defined by Richards et al. (1978) and Hoek and Bray (1981). However, we used a larger direction tolerance of 45° for toppling failure and no direction tolerance at all for planar failure to be more conservative. This study investigates rockslides with complex structures and a small direction limitation might thus not be suitable (Oppikofer, 2009). In addition, a low friction
Method
Differential GNSS surveys have been undertaken yearly since the establishment of the first survey points in 2005. At that time, 3 fixed points were installed in stable areas and 16 rover points in potentially unstable regions. Two additional rover points (AU15 and AU16) were installed in a potentially unstable area at Furekamben in 2006. The locations of all GNSS-points are illustrated in Fig. 2.
GNSS-points with a significant movement are in this publication defined as points with a registered
Dating of rockslide deposits
Terrestrial cosmogenic nuclides (TCN) dating was used in the Flåm study area to help further constraining the chronology and nature of rockslide events at two different lobes, belonging either to single rockslide events or to creeping lobes of successive rockfall deposits. TCN dating has been previously used for separating complex rock avalanche deposits of several generations into various events (Hermanns et al., 2001, Hermanns et al., 2004). A comprehensive review of TCN dating is provided by
General method
Two different numerical approaches were conducted in this study. First, a continuum model was used to analyze the stress distribution within the slope and to investigate the influence of unloading the slope due to a prehistoric rockslide. In a second step, the potential failure mechanism was investigated in more detail using a discontinuum model with the aim to reproduce the current slope morphology from an assumed pre-deformation topography.
A profile parallel to the main measured movement
Discussion of the geomechanical model
Movement trends from the differential GNSS analysis are consistent with a wedge failure along foliation and J2 as well as toppling along J3 taking into account the variability of the involved structures, but not with a planar sliding along foliation (Fig. 6). However, calculated movement plunges are too steep for both mechanisms. As demonstrated by the numerical modeling results, combined toppling and subsiding bilinear wedge failure result in steeper displacement vectors (Fig. 11b).
The
Relative failure susceptibility and implications for hazard
The village of Flåm is a popular tourist destination during summer, receiving around 450,000 visitors and more than 130 cruise ships each year. Due to the high public interest and the large amount of potentially affected persons, questions about the likelihood of a catastrophic failure are important to answer. Based upon geomorphological studies of prehistoric events, structural variability and differential GNSS movements it is interpreted that slope deformation and collapse of the slope in
Summary and conclusions
This study of the Stampa slope demonstrated the importance of integrating different data sources and analysis methods in order to understand the deformation mechanism of a complex unstable rock slope. Only the joint interpretation of all results led to a coherent model of the deformation and failure mechanism.
All analyses indicated that gravitational slope deformation in the study area is strongly influenced by inherited structures, like pre-existing joint sets and the metamorphic foliation of
Acknowledgments
The authors are grateful to A. Günther for support in the field and the master student Å. Tukkensæter from the Norwegian University of Science and Technology, who was involved in the fieldwork of 2009. Partial funding was received by the Research Council of Norway through the International Centre for Geohazards (ICG). Their support is gratefully acknowledged. This is ICG contribution no. 406. The authors would also like to acknowledge constructive comments on this article from C.R. Froese, an
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