mass falls in the Wachau-Danube Valley (Bohemian Massif
mass falls in the Wachau-Danube Valley (Bohemian Massif mass falls in the Wachau-Danube Valley (Bohemian Massif
Fig. 14 Movement measured by fissurometer number 3 versus temperature (daily 05:00 a.m.), time span: September 2009-October 2010. a x = E y = N z = vertical axis b Fig. 15 Morphometric evaluation of rock-mass fall deposits by means of the intersection of digital terrain models. (a) Perspective view before (black) and after (grey) rock-mass fall event. (b) Differential DEM (top view) showing surface with surface loss (black) and surface gain (white) including +5 m and +10 m isolines. Insert 16 +10 m +5 m +5 m
Discussion Interpretation of failure analysis While GIS-based methods (Meentemeyer and Moody 2000 or Günther, Carstensen and Pohl 2002) are a useful tool for mapping conformities between topographic and geological surfaces at a regional scale, a geotechnical approach using the stereonet was necessary in these projects, where detailed analyses were required for the design of local preventive measures. The used method on the one hand allows to process not only joint orientation data, but also considers joint characteristics, and on the other hand enables a probabilistic kinematical failure analysis for all three major failure modes (block sliding, wedge sliding and toppling). The results of the failure analysis can be interpreted in a way that block sliding is the major failure mode, since wedge sliding and toppling are unlikely. As shown in Fig. 12, the variation in the dip angle of potential failure planes is high. Practically this means that an increase of the basic friction angle does not guarantees stable conditions, though the number of potential failure planes is reduced (cf. subchapter results of failure analysis). Interpretation of monitoring data and trigger of failure The monitoring data illustrate that heavy rainfalls triggered the failure of the 2009 rock-mass fall near Dürnstein. It is worth discussing how rain influenced the slope equilibrium and what led to failure. Either water reduced friction along the sliding planes, or a joint water pressure was built up, or both. We suggest that, in this case, joint water pressure was of minor relevance for triggering rock-mass falls, since the very widely open joints drain very fast. Two observations support this thesis: On the one hand no water-leakages along open joints were observed after rainfalls and on the other hand, increasing movement rates occurred with a delay of days. The fact, that no correlation between frost and deformation was observed is also worth discussing. Assuming that congelifraction can only take effect in completely water-filled cracks, the high aperture of joints combined with large block sizes could be an explanation. In the studied areas congelifraction might be an important trigger for small-scale rockfalls (cf. Krähenbühl 2004), but not for rock-mass falls and rockslides. The proximity of the study area to the Diendorf fault suggests, that seismic activities could play a role in triggering rock-mass falls. A correlation of seismic data from the study area (ZAMG unpubl.) with known rockmass falls within the last century however did not show any link between earthquakes and mass movements. According to Lenhardt (2007) the probability for triggering landslides by ground vibrations induced by seismic activity is generally low in the study area. Preventive Measures Preventive measures to protect the local infrastructure from further possible rock-mass falls are difficult to accomplish, both in Spitz as well as in Dürnstein, as neither the discontinuity situation, which is a given, nor the artificial modification of the slope geometry can be changed or undone. A long-term stable equilibrium can only be achieved by removing all potentially unstable falls and by cutting the rock back extensively, which, however, means further, possibly harmful interference to slope morphology and landscape by man. Since the implementation of these measures is a long-term project and also dependent on the political, economic and environmental situation, passive protection measures (protection dams) in combination with monitoring systems were carried out. A similar approach was already applied to the 150 000 m 3 Eiblschrofen rock-mass fall (Scheikl et al. 2000; Roth et al. 2002). In the former quarry at Spitz, a 130 m long earth dam, running parallel to the railway line, was built in 2004 (Fig. 9). The energy-absorbing capacity of the dam, however, is limited. Major combined rockslides/rock-mass falls could easily exceed its energy-absorbing capacity. In order to protect the infrastructure against such events, a rockfall barrier with an integrated warning system was installed on the crest of the dam in 2006. Any deformation of the lightly supported posts triggers an alarm system, activating red traffic lights on the railway track as well as on the main road and sending an alarm signal to the authorities of the federal government of Lower Austria via SMS. In addition, an automatic 3D survey system has been used since 2008 to identify movements in potentially unstable regions at an early stage. There are 37 prisms installed in the rock face that are automatically measured by a permanently mounted laser theodolite at an interval of two hours. After a test phase of one year, parameters for alarm triggering were defined and implemented in the alarm system. It has 17
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- Page 19 and 20: Conclusions There are two causes of
Fig. 14 Movement measured by fissurometer number 3 versus temperature (daily 05:00 a.m.), time span:<br />
September 2009-October 2010.<br />
a<br />
x = E<br />
y = N<br />
z = vertical axis<br />
b<br />
Fig. 15 Morphometric evaluation of rock-<strong>mass</strong> fall deposits by means of <strong>the</strong> <strong>in</strong>tersection of digital terra<strong>in</strong><br />
models. (a) Perspective view before (black) and after (grey) rock-<strong>mass</strong> fall event. (b) Differential DEM (top view)<br />
show<strong>in</strong>g surface with surface loss (black) and surface ga<strong>in</strong> (white) <strong>in</strong>clud<strong>in</strong>g +5 m and +10 m isol<strong>in</strong>es.<br />
Insert<br />
16<br />
+10 m<br />
+5 m<br />
+5 m
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