The Larchberg-Galgenwald landslide is situated in the southwestern slope of the Rantenbach valley 2 km upstream of the city of Murau. The slope is built up by marbles some 300 m thick lying on a phyllite base some 140 m thick. The cracks of the main scarp, which are some 10m wide, are monitored by wire extensometers and are opening with a precipitation dependent velocity of 30 cm per year. Monitoring by an automatic servo-theodolit revealed that the lowest base area is moving very slowly at the moment. Numerical analyses of possible rock mass falls up to 50,000 m3 by the Particle Flow Code (PFC) showed that rock blocks are not able to cross the dam protecting the road. However, the unprotected farm "Fritz" is endangered by single rock blocks. Further simulations showed that an overall volume of up to 10,000,000 m3 might bury the road in the valley and dam up the Rantenbach river causing a debris flow. Measures reducing the amount of precipitation seeping into the ground seem to be most promising due to the high degree of rock fracturing and the dependence of displacements on precipitation.
The Larchberg-Galgenwald landslide (Fig. 1) is part of the southwestern slope of the Rantenbach valley about 2 km upstream of the city of Murau. The main scarp at 1060 m above sea level of some 70 m length and up to 10m width has repeatedly given reason for investigations (Pohl 1983, Becker 2001). In April 2001 some rock blocks the size of some cubic meters hit the 50 to 70 m broad zone next to the main road to Murau. Consequently this zone was declared prohibited by the local authorities allowing only residents of the farm "Fritz" to pass on special terms.
As a first step of monitoring 10 wire extensometers and a precipitation measuring device were installed in the early summer of 2001. Additionally, following a coordination meeting between the provincial authorities and the service for torrent, erosion and avalanche control, the federal road administration had a dam of e length of 300 m and the height of 5 m constructed for the protection of the main road against rock falls. Due to the results brought forth by the wire extensometers a monitoring and investigation project was initiated by the administration of the Land Styria, department 19B.
Parts of the Rantenbach valley including the slopes of Mount Larchberg indicate an area prone to landslides due to erosion and melting down of a Quaternary glacier. The slope is built up by a 300 m thick formation of mylonitic and strongly foliated marbles resting on phyllites interlayered by graphitic schists.
At Mount Larchberg, three areas of different landslide morphology were mapped (Hermann, 2004).
(Figure in full paper)
These observations indicate that the slope is a system of hard rock lying on a soft base. The competent part of the system consists of a 300 m thick formation of mylonitic and strongly foliated marbles.