mass falls in the Wachau-Danube Valley (Bohemian Massif

There is not much information on further rock-mass fall events over the following decades. However, a
comparison of photographs taken in 1909 and 100 years later, just before the rock-mass fall in 2009, clearly
indicates that further rockfalls must have occurred in the time between. There is only one report of a rockfall,
which took place in the winter of 2002/2003, after an unusually rainy autumn. The latest rock-mass fall
occurred in July 2009, yet again preceded by a period of heavy rainfall. The rail infrastructure was destroyed by
this event, and some smaller boulders even reached the main road B3 “Donauuferstraße” (Fig. 7).
Fig. 7 Damage to rail infrastructure in 2009 (view towards NNW, photo: Laimer, July 2009).
Methods
Failure model analysis
To determine the mode of failure, which caused the rock-mass falls/rock slides, a kinematic failure analysis was
performed, based upon the principals of “block theory” (Goodman and Shi 1985).
The application of this method is practicable for both sites (Spitz and Dürnstein), since the exceeding of rock
mass strength can be excluded as a cause of failure, because of the fact, that in both cases, the uniaxial
compressive strength of the rock mass is by far higher than the overburden stress (subchapter Lithology).
Therefore, the rock mass can be seen as a system of rigid blocks separated by discontinuities. Large scale
displacements within the rock mass can only occur if blocks are moved relative to each other along distinct
shear planes, an effect known as “block failure” in geotechnical engineering (Hoek and Bray 1974). For the
analysis, discontinuity orientation data acquired by detailed geotechnical mapping of the rock face were used
and put in relation to the spatial orientation of the rock face, using stereographic projection. The main failure
modes block sliding (sliding of a block on a single plane), wedge sliding (sliding of a block on two planes in a
direction along the line of intersection) and toppling (rotational failure of thin columns caused by
discontinuities striking +/- parallel and dipping steeply contrary to the rock face) were examined, using the
computer program dips ® (ROCSCIENCE).
Kinematically, a block can slide on a single plane, if the following preconditions are met: firstly, the strike of the
potential sliding plane must be approximately parallel to the rock face (maximum deviation of about 30°).
Secondly, the dip angle of the potential sliding plane must be lower than the dip of the rock face and, thirdly,
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