mass falls in the Wachau-Danube Valley (Bohemian Massif

strength of eight MPa, and a friction angle of 25° for the biotite schists and joint roughness coefficients (JRC) of
five to 10 respectively, the effective friction angles for a 20 m thick marble complex are in the range 30.8° -
36.7°. With increasing layer thickness, the effective friction angle diminishes as does the influence of cohesion
on the safety coefficient. As a consequence, all major rockslides in the former quarry of Spitz were limited to
marble layers thicker than 10 m and underlaid by biotite schists. Future rockslide events are most likely to be
tied to this failure model.
Results from monitoring system
In the former quarry at Spitz no measurable movements were observed since the implementation of the
monitoring system.
At the Dürnstein quarry, movements were observed in six out of seven fissurometers. The total deformation
rates in five fissurometers were < five mm and therefore of minor geotechnical relevance. Larger deformations,
however were observed at fissurometer number three, installed at the foot of a 1000 m 3 wedge-shaped rock,
sliding on a single plane in the central part of the rock face (Fig. 4), especially following heavy rainfalls. The daily
volumes of rainfall recorded against movements at fissurometer number three, in the time between
September 2009 and October 2010, are shown in the following diagram (Fig. 13).
Fig. 13 Movement measured by fissurometer number 3 versus rainfall (daily sums), time span: September 2009-
October 2010.
The largest rate of deformation was measured in the period from the middle of April until the end of August
2010, exactly the same period when the most rainfall was logged. The diagram shows in great detail that single
heavy rainfalls with daily amounts of > 10 mm (as in April 2009) led to a sudden increase in the deformation
curve with a time delay of approximately two days. In total, the period of accelerated movement lasted for
approximately 14 days. Within this time, the curve flattened to a hyperbolic decline, after the first steep ascent.
However, an obvious analogy between frequent frost alternating with thaw (e.g. in the period between
December 2009 and March 2010) cannot be drawn from the diagram in Fig. 14, in which movement of
fissurometer number three is plotted against temperature.
Results from DTM comparison
By intersecting two digital terrain models before and after the rock-mass fall of 2009, the total volume of the
sliding mass could be estimated at 13 000 +/- 2000 m³ (Fig. 15). The inaccuracy in this volume calculation can
be explained by the following facts: In the DTM provided by the federal government of Lower Austria,
overhangs were not considered. Minor rockfalls could have occurred in the time span between the acquisition
of ALS data and the 2009 rock-mass fall. Further on the loosening factor between intact rock and debris could
not be considered sufficiently.
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