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Abstract: The monitoring of slope displacements over time provides the basis for most rockslide early warning systems, yet the prediction of catastrophic failure from these records is highly problematic and tenuous, especially if the underlying kinematics and instability mechanism are poorly understood. An example is the moving slope above the town of Randa in the Swiss Alps. This slope is considered typical of those in crystalline rock that lacks a natural, highly persistent, weakness plane dipping towards the valley that can serve as a through-going detachment surface for kinematic release. In Part I (the companion paper to this), the findings from a comprehensive geological and geophysical investigation to image the internal structures of the unstable rock mass were presented. In this paper we develop a kinematic model that describes the pattern of displacement vectors for the rock mass along these structures, both on surface and at depth. The displacements were estimated from 5 years of data from an extensive monitoring system which included surface geodetic and crackmeter measurements, borehole inclinometer and extensometer measurements (up to 120 m depth), and microseismicity. The results showed that the displacement field is highly heterogeneous. Internal deformation is accommodated by both shear and opening-mode dislocation of faults and fracture zones which dip moderately to steeply into the slope. Microseismicity is most intense near the front of the scarp where surface translation in large part reflects the toppling of blocks accommodated by slip along the steeper dipping fractures. The small displacement rates at the study site of up to 2 cm/year coupled with a modest deviation of up to 15° from vertical of the boreholes posed severe problems for the estimation of horizontal displacements from the inclinometer data. We describe the error analysis of these data in some detail since it is relevant for similar installations elsewhere. Conclusions drawn from this work highlight the importance of integrating various types of data in order to better understand the complex block-kinematic processes whose evolution governs the long-term progressive failure of unstable rock slopes. [Copyright &y& Elsevier]

Abstract: In 1991, the steep slope above the village of Randa in southern Switzerland failed during two successive rockslide events three weeks apart. The failures, which resulted in the release of approximately 30×106 m3 of material, occurred in fractured gneissic rock whose principal foliation, faults and fracture zones dip into the slope. Above the scarp of the second rockslide, the mountain continues to move at rates of up to 2 cm/year. In an attempt to understand failure mechanisms in fractured crystalline rock masses, we have carried out a comprehensive multidisciplinary study that included detailed geological mapping, borehole logging and imaging, and surface and borehole geophysical surveys. An integrated interpretation of the resultant data has revealed the presence of three prominent fault- and fracture zone sets that define the internal structure of the rock mass. Some faults and fracture zones could be traced up to 85 m into the subsurface using borehole radar reflection methods. The dips of the faults and fracture zones estimated from surface mapping matched those from radar reflection and borehole televiewer surveys. No evidence was found for significant faults or persistent weakness planes that paralleled the 1991 rockslide failure surfaces. Instead, the new observations were consistent with dominant failure mechanisms that involve the progressive development of major shear and brittle fracture zones and the eventual destruction of intact rock bridges immediately prior to catastrophic collapse. This integrated study provides the first reliable model of the 3-D fault structure associated with and contributing to a large rock slope instability in a brittle crystalline rock mass. [Copyright &y& Elsevier]