THE 2010 ROCK FALL EVENT IN STEIN AN DER TRAUN – CAUSES AND TRIGGERS

On January 25^th, 2010 at the village of Stein at the river Traun, at about 8 p.m. a 500 m³ large and 1,200 t heavy block was released from a conglomerate face obviously without warning, destroying the family home below. Only two of four inhabitants could be saved out of the debris by means of a spectacular rescue operation through the local fire brigade.

After this event the question arose, if the rock fall could have been foreseen or if such spontaneous incidents are abrupt and unpredictable.

In this paper the conducted studies to reconstruct the processes leading to this event will be presented. These investigations included field mapping, geodetic survey, laserscanning of the rupture face, mineralogical analysis of sinter crust thin sections, inventory of the block dimensions and reconstruction of the collapse kinematics, analysis of the weather data prior to the event and a 2d finite element calculation (Phase2, rocscience) using the geometry of the overhanging conglomerate strata.

In this case, it seems like there was no clear triggering event prior to the wall collapse. Instead, it could be proved by engineering geology mapping, mineralogical analysis of the sinter crusts and numerical modelling, that the back scarp connected with a set of discontinuities started to propagate several years ago already. Also supported by early photographs of the cracks in the brick walls of the endangered house in 1993 and 2006 together with eye wittnesses, it could be shown, that the fracture propagation started tens of years beforehand and the rock topple – rock fall took place after the last rock bond bridges finally were sheared through.

Therefore it can be stated, that the rock fall of Stein could have been predicted, if the clear signs of damage would have been interpreted correctly and the failure process would have been fully understood.