mass falls in the Wachau-Danube Valley (Bohemian Massif

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Results Rock mechanical failure analysis Rock mechanical failure analysis performed on the rockslides/rock-mass falls of Spitz and Dürnstein clearly show, that block sliding along failure planes, dipping out of the slope is kinematically possible and very likely. Figure 12a shows the results of the Markland test for the bedding planes in Spitz and Fig. 12b for the foliation planes in Dürnstein. In the diagram, poles of single discontinuities are represented by white squares, the major planes are marked with a black spot. The inner circle represents a friction angle along discontinuities of 35°. Since the major bedding planes in Spitz and the foliation planes in Dürnstein lie within the grey shaded region, single plane sliding is kinematically possible in both cases. A higher friction angle represented by a larger inner circle in the stereonet, thus reducing the grey-shaded region, would reduce the number of poles within the grey shaded region, which means, in practice, that the number of potential sliding planes that are kinematically free to slide would be decreased. With a friction angle of 40° for example, the major plane (black spot) borders exactly on the shaded region, indicating a critical state of equilibrium. In practical terms, planes dipping 40° out of the rock face and undercut by man-made morphology owing to mining are in a state of critical equilibrium. Planes dipping at a lower angle of e.g. 30° can be expected to be stable due to friction. Lower friction angles (e.g. due to wet sliding surfaces) represented by smaller inner circles in the stereonet, lead to an enlargement of the grey-shaded region, which signifies an increasing number of potential sliding planes. Sliding blocks are terminated either by slope morphology, or by two sets of steeply dipping joints. Fig. 12 (a) Markland test for single plane sliding at Spitz. (b) Markland test for single plane sliding at Dürnstein. (equal angle overlay, lower hemisphere, friction angle along discontinuities 35°) Regarding the potential of wedge failure, the majority of the analysed intersection lines of the present discontinuity sets at Spitz as well as at Dürnstein are either too steep, or too flat, so that wedge sliding is unlikely in both cases. Similar results were achieved concerning toppling failure. The basic friction angle estimated from tilt tests at Dürnstein is in the magnitude of 35-40° for dry, flat and smooth slickensides with limonitic coating without any infillings. Tests under wet conditions showed that friction angles decreased at a magnitude of five degrees. In the case of the former quarry at Spitz, similar conditions can be assumed. At Spitz, block sliding is further favoured by sheet silicates sandwiched between the marble layers. A 0.5 m thick layer of biotite schist formed the sliding plane for a 15 m thick marble complex during the rockslide in 2002. While fissured and partly karstified marbles drain very fast, the mica-rich layers function as an aquiclude. Penetrating water softens the rock and the friction angle decreases with increasing water content. In his back analysis of the 2002 rockslide/rock-mass fall, Wagner (2006, unpubl.) investigated the relationship between the failure surface´s shear strength and layer thickness according to Barton (1971). Assuming a 14
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. 15
Page 1 and 2: Geomorphological and geotechnical c
Page 3 and 4: Fig. 3 Rock-mass fall near the town
Page 5 and 6: Tectonically, the whole Bohemian Ma
Page 7 and 8: There is not much information on fu
Page 9 and 10: Fig. 8 Monitoring devices in the fo
Page 11 and 12: Fig. 9 (a) Orientation of bedding p
Page 13: Fig. 11 (a) Orientation of joints a
Page 17 and 18: Discussion Interpretation of failur
Page 19 and 20: Conclusions There are two causes of

mass falls in the Wachau-Danube Valley (Bohemian Massif

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