Numerical modelling of rock mass falls at the Lärchberg-Galgenwald landslide (Austria)

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 10 m 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 m 3 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 m 3 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.