Composite rock slope kinematics at the current Randa instability, Switzerland, based on remote sensing and numerical modeling V. Gischig a, ⁎, F. Amann a , J.R. Moore a , S. Loew a , H. Eisenbeiss b , W. Stempfhuber b,1 a Geological Institute, Swiss Federal Institute of Technology, ETH Zurich, Switzerland b Institute for Geodesy and Photogrammetry, Swiss Federal Institute of Technology, ETH Zurich, Switzerland abstract article info Article history: Received 5 May 2010 Received in revised form 19 November 2010 Accepted 23 November 2010 Available online 29 November 2010 Keywords: Rockslide Randa Kinematics Remote sensing Numerical modeling Kinematic analysis of slope instabilities in brittle rock is crucial for understanding the reaction of the rock mass to external forcing factors. In steep terrain, inaccessibility often limits collection of relevant data and remote sensing techniques must be applied. This is the case at the current Randa rock slope instability in southern Switzerland, where a total volume of about 5–6 million m 3 moves at a rate of up to 30 mm/yr. A large portion of the unstable rock mass is exposed in an 800 m high inaccessible cliff; the main scarp of the May 1991 rock slope failure. Between 2005 and 2007, a comprehensive suite of remote sensing techniques, including photogrammetry, LiDAR, and GB-DInSAR, was combined with 3D geodetic measurements to characterize the rock mass structure and displacement patterns. Photogrammetry and LiDAR data were measured simultaneously from a helicopter using a system allowing for oblique view angles, which provided optimal observations of the steep rock cliff. We used these datasets to map large-scale structures and extract their orientation and minimum persistence, as well as to characterize the 1991 failure scarp. The northern part of the May 1991 failure surface shows a transition from stepped planar sliding at the base, to failed rock bridges in the center, to tensile failure close to the vertical head scarp. Kinematic analysis of the discontinuity sets in the currently moving rock mass shows that both toppling and translational sliding are feasible failure mechanisms. Toppling is more likely for steep faces above 2200 m, whereas translational failure is more likely in the lower portion of the instability. Interpretation of GB-DInSAR displacement maps revealed similar kinematic behavior, and also allowed identification of a basal rupture zone and lateral release plane bounding the instability. Displacement vectors derived from geodetic surveying provided new insights into the 3D kinematic behavior of the instability. All information extracted from different data sets were integrated in a conceptual model, which was then investigated with 2D numerical simulation using the discontinuum code UDEC. The numerical models were able to reproduce the hypothesized kinematic behavior well. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Hazard assessment and analysis of failure mechanisms for an unstable rock slope require detailed knowledge of the kinematic behavior of the rock mass (e.g. Goodman and Kieffer, 2000). Reconstructions of catastrophic failure events have shown that rock mass kinematics not only controls stability, but can also influence run- out distance; e.g. Sitar et al. (2005) showed for the Vajont landslide that a greater number of kinematic degrees of freedom prior to failure ultimately resulted in higher run-out velocity. Kinematic analyses of rock slopes take into account discontinuity properties, such as orientation, persistence, strength, large- and small-scale roughness, in addition to the geometry of the rock slope (e.g. Goodman, 1989; Wyllie and Mah, 2004). Measuring and characterizing such discon- tinuity properties have become standard practice in rock mechanics, and many tools for analysis and statistical description of structural data are available (Goodman, 1989; Priest, 1993; Jing and Stephans- son, 2007; Tran, 2007). However, in the case of steep rock slopes in alpine terrain, limited accessibility for field investigations presents a major problem, since outcrops yielding most information are often too steep or dangerous to investigate in situ. Remote sensing techniques, such as laser scanning and photo- grammetry, have proven to be appropriate tools for characterizing the structure of rock masses (e.g. Lemy and Hadjigeorgiou, 2003; Buckley et al., 2008; Oppikofer et al., 2009; Sturzenegger and Stead 2009). Recent developments have led to helicopter-based systems that integrate both laser scanning and photogrammetry, operated manu- ally (Vallet and Skaloud, 2004) or via remote control (Eisenbeiss, 2008). Due to the advantage of flexibility in selecting the range and view angle according to site requirements, such instruments are ideal for rock slope characterization in steep, inaccessible terrain. An essential prerequisite for complete kinematic analysis is the spatial displacement field of the moving rock mass. Traditional methods relying on in situ monitoring systems, such as geodetic measurements, Engineering Geology 118 (2011) 37–53 ⁎ Corresponding author. E-mail address: valentin.gischig@erdw.ethz.ch (V. Gischig). 1 Now at Beuth Hochschule für Technik, Berlin, Germany. 0013-7952/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2010.11.006 Contents lists available at ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo