ABSTRACT

Glacial erosion brings many alpine valleys to limit equilibrium because of the steepened valley flanks. Instable slopes hold potential dangers, even though slowly creeping landslides as treated below can be observed and do seldom show surprising behavior as long as they do not accelerate. Nevertheless a sudden loss he stability can lead to uncontrollable movements with much higher velocities. Aim of the project presented here is to gain a better understanding of mass movements and the influences of different boundary conditions on slope displacement. A well monitored example for a slowly creeping landslide is the mass movement Hochmais- Atemkopf, situated in the Kaunertal, Tyrol, Austria. Based on a geological model a calculation model including four sliding masses has been developed. In addition, external effects were also taken into account to get a idea of their possible influences on stability and deformation. This paper presents a limit equilibrium analysis and the preliminary results of a numerical finite element study.

INTRODUCTION

The more than 1000 m high mass movement Hochmais- Atemkopf in the Kaunertal. Tyrol. Austria, see Figure 1 (BEV 1996) is one of several site under investigation The availability for over 40 years of a long-term monitoring program, including geodetical, geophysical, meteorological, geotechnical and geological data, provides a good data basis for a numerical analysis. Beneath the mass movement a hydropower reservoir is located, underlining the importance of the investigations. From geological and geophysical field investigations a geological model was developed (Bruckl et al. 2004). The slope is situated in a foliated, paragneissic rock unit of the Otztal-crystalline basement. Four individual sliding masses, bounded by

(Figure in full paper)

sliding zones were verified (Fig. 2). Resulting from postglacial sliding of a fractured paragneiss slab, the sliding zone between sliding mass 3 and 4 is situated in moraine deposit. The other sliding zones are located in fractured paragneiss, assuming a zone of densely fractured and crushed material. Below the four sliding masses stable fractured paragneiss is located, treated here as bedrock. The slope movements range from 3 to 4 per year in the lower part. The displacement vectors are mainly aligned parallel downwards the slope suggesting a translational sliding mechanism. Based on these results a computer model was build (Fig. 3). The two lines display the two different groundwater levels in the calculations. Even though a horizontal groundwater level is not a realistic assumption in this case, it allows to demonstrate the effects of

(Figure in full paper)

MEASUREMENTS

The slope is observed geodetically, five of the geodetic points (spheres), see Figure 4 and the storage water level are measured frequently. For each of the recorded points a monthly moving mean is calculated. In the exploration adit (gray line, Fig. 4) deformation between the underlying, more stable rock (sliding mass 3) and sliding mass 4 is measured with a wire extensometer and recorded automatically (Tentschert 1998). Also the initiations of the different sliding zones are displayed in the figure.

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