ABSTRACT:

Extensive field studies show that the occurrence of deep-seated slope instabilities (i.e rockslides) is influenced by the discontinuity network and its intersection in relationship to the slope orientation. Furthermore in similar rock types slope failure can develop even at moderately inclined slopes, whereas the steeper slopes nearby remain stable over a time span of thousands of years. A possible explanation of this slope behavior may be attributed to rock mass anisotropies caused by geological discontinuities i.e. tensile joints, shear fractures, foliation planes and brittle fault zones. A deep-seated paragneissic rockslide located in the Tyrol (Austria) was investigated by means of detailed field investigations and numerical 2-D discontinuum modeling in order to study a) the failure initiation and formation processes of a persistent sliding zone, b) the structural influence on rockslide geometry, c) the interrelationship between the dip angle of the sliding zone and the pre-existing fracture network, d) the sliding mechanisms along this sliding zone and e) the internal deformation behavior that is induced from large-scale shear displacements.

1. INTRODUCTION

The growth of settlement areas and new infrastructure projects in mountainous regions (e.g. buildings, traffic routes, tunnels, ski resorts, reservoirs), necessitate that established landslide simulation and prediction methods should be enhanced and new forecasting tools developed. Therefore, a fundamental understanding of the underlying landslide failure and deformation processes is crucial. Furthermore, geological-geotechnical and mechanical models based on limit equilibrium methods or numerical techniques are only reliable if the geometry of a landslide, the failure mechanism and the slope kinematics are sufficiently known. In soils, slope failure is predominately characterized by rotational slides (representing an isotropic material behavior) whereas slope failure in fractured rock masses (i.e. rockslides) often is controlled by geological structures such as foliation and bedding planes, brittle fault zones, meso-scale shear and tensile fractures. Discontinuity properties, i.e. orientation, size, density, persistence and infilling, and their spatial interrelationship to the slope controls the failure mechanism and the failure geometry at scales from meters to kilometers. Furthermore it was observed that in similar rock types slope instabilities often occur in less steeply inclined slopes, whereas the steeper slopes nearby remain stable over a time span of thousands of years. This anomalous slope stability behavior may result from a mechanical rock mass anisotropy which is caused by geological discontinuities. The highly relevant interrelationship between rockslide failure and structural geology was observed in several case studies in the past [1-3]. In this paper a well exposed crystalline rockslide referred to as "Kreuzkopf rockslide" was investigated by means of detailed field mapping, surface deformation measurements and numerical simulations. The Kreuzkopf rockslide comprises a volume of 3.2 million M3 and is part of several deep-seated landslides in the region of the Kaunertal (Austria). For example, one of them is the large-scale deep-seated creeping slope Hochmais-Atemkopf [4-7] nearby, comprising a volume of 264 million m3 and reaching a slope height of about 1000 meters. Field investigations and numerical modeling were performed to study processes and mechanisms of rock slide failure and sliding.

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