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
the dip angle of the sliding plane must be higher than the angle of friction on the sliding surface, since friction works against sliding. Using stereographic projection, this testing method, first published by Markland (1972), is a simple technique that can be employed to ascertain, if the potential sliding plane complies with the requirements mentioned above. This test distinguishes between discontinuities which daylight into free space and those which do not daylight. For a certain slope orientation, a region can be defined as bounded by the daylighting envelope shown as a small circle in the stereonet. If the pole of a discontinuity is situated within this region it daylights. It is, then, kinematically prone to single plane failure if it daylights and at the same time lies outside the friction cone represented by the inner circle in the stereonet (grey-shaded region). Likewise the kinematic possibility of wedge sliding or toppling can be tested using the stereonet. Estimation of friction angle The friction angle along discontinuities was estimated by simple tilt tests in the field, performed in the former quarry near Dürnstein. Tilt tests are a common method used in geotechnical engineering, to investigate the basic friction angle of rock joints (Cawsey and Farrar 1976 or Bruce, Cruden and Eaton 1989). Two pieces of rock containing a discontinuity are held in hand with the discontinuity horizontal. The sample is slowly tilted until the top block moves. The angle with the horizontal at onset of movement is the so-called tilt-angle. The tilt-angle equals the material friction of the discontinuity wall (φ) plus the roughness angle (i), if no real cohesion is present (i.e. no cementing or gluing material between the two blocks), no infill material is present, the asperities do not break, and the walls of the discontinuity are completely fitting at the start of the test (tiltangle = φwall material + i). If the walls of the discontinuity are completely non-fitting, the tilt-angle equals the friction of the material of the discontinuity walls (tilt-angle = φwall material). If cementation or gluing material is present or asperities break, the tilt-angle represents a combination of the (apparent or real) cohesion and the friction along the discontinuity. If infill material is present, the tilt-angle is governed partially or completely by the infill, depending on the thickness of the infill and height of asperities (Hoek and Bray 1974). In the case of Dürnstein, open discontinuities without infillings and hard side walls are present (cf. subchapter discontinuity characteristics). The tests were performed with non-fitting walls, so that obtained tilt-angles are equal to the basic friction angle along discontinuities. Monitoring system In both quarries, monitoring systems including fissurometers measuring the width of cracks of potential failure planes and geophones registering ground vibrations, were installed to observe the rock face. Moreover 3D prism targets measured by a laser theodolite were installed at both locations providing a redundant measuring tool. Since rock-mass falls in Spitz as well as in Dürnstein have occurred especially after lengthy periods of heavy rain and historical evidence (Austrian Federal Railways unpubl.) also strongly suggests that heavy rainfall acts as a trigger for these rock-mass falls, additionally, a pluviometer (tipping bucket rain gauge, MICRO STEP-MIS MR2) was included in the monitoring system of the rock face in the former quarry at Dürnstein. At Spitz the system was installed by the geological service of Lower Austria as a consequence of the 2002 rockslide/rock-mass fall and was used as a permanent monitoring device, until it was replaced by an automatic 3D survey system (cf. Fig. 2 and subchapter Preventive Measures further below). At Dürnstein, the monitoring system was configured as a temporary system to ensure safe working conditions during restoring traffic infrastructure. The system was installed in September 2009 and maintained until November 2010 by a system operator, who provided an online data service. In total seven fissurometers and four geophones were installed. Two geophones were installed on both sides of two major discontinuities observed by fissurometers number three and number two. The exact positions of the measuring devices in the rock face at Dürnstein are shown in Fig. 8. 8
Fig. 8 Monitoring devices in the former quarry at Dürnstein (view towards E, photo: Satzl, July 2009). DTM generation At the quarry near Dürnstein, an existing DTM (Digital Terrain Model) from 2008, allowed to calculate the volume of the 2009 rock-mass fall, by comparing these data with a DTM generated from data, collected shortly after the event. The existing DTM was provided by the provincial government of Lower Austria, based on data collected by ALS (Airborne Laser Scanning), performed in January-February 2008, using a RIEGL LMS-Q 560 scanner, with a density of 15-20 points per m² and a spatial resolution of 1 m x 1 m (NÖGIS, http://www.intermap1.noel.gv.at/webgisatlas/init.aspx, 18-Jul-11). The DTM which was generated in 2009 after the rock-mass fall, was based on TLS (Terrestric Laser Scanning) performed by a local surveying engineer (AVT – ZT-GmbH), using a RIEGL LMS-Z420i combined with a GPS LEICA System 1200. For generating this laser scan model, seven scans from different positions were performed, each generating a point cloud with a number of 1 900 000 points. The accuracy of the scans was stated 10 mm by the surveying engineer. Each scan was referenced using D-GPS LEICA System 1200 via temporarily installed targets and combined with each other. The relative accuracy of the GPS measurements is +/- two cm. The single scans were transformed in the local reference coordinate system and merged to a single DSM (Digital Surface Model) with an accuracy of approximately three cm. The DSM was then filtered to produce the final DTM used for all further calculations. Terrain modelling was in both cases done by triangulation in consideration of breaking edges. Overhanging areas were modelled separately to gain more realistic data. Results Lithology at the former quarries In the former quarry near Spitz, marbles are predominant in the hanging wall. The foot wall is composed of calc- silicate gneiss and granodiorite gneiss. Mica-rich layers (biotite schist) between the marble beds occur frequently within the Spitz marbles. Lentoid bodies of amphibolites and pegmatitic inclusions can also be found. 9
- 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: There is not much information on fu
- Page 11 and 12: Fig. 9 (a) Orientation of bedding p
- Page 13 and 14: Fig. 11 (a) Orientation of joints a
- Page 15 and 16: strength of eight MPa, and a fricti
- Page 17 and 18: Discussion Interpretation of failur
- Page 19 and 20: Conclusions There are two causes of
Fig. 8 Monitor<strong>in</strong>g devices <strong>in</strong> <strong>the</strong> former quarry at Dürnste<strong>in</strong> (view towards E, photo: Satzl, July 2009).<br />
DTM generation<br />
At <strong>the</strong> quarry near Dürnste<strong>in</strong>, an exist<strong>in</strong>g DTM (Digital Terra<strong>in</strong> Model) from 2008, allowed to calculate <strong>the</strong><br />
volume of <strong>the</strong> 2009 rock-<strong>mass</strong> fall, by compar<strong>in</strong>g <strong>the</strong>se data with a DTM generated from data, collected shortly<br />
after <strong>the</strong> event. The exist<strong>in</strong>g DTM was provided by <strong>the</strong> prov<strong>in</strong>cial government of Lower Austria, based on data<br />
collected by ALS (Airborne Laser Scann<strong>in</strong>g), performed <strong>in</strong> January-February 2008, us<strong>in</strong>g a RIEGL LMS-Q 560<br />
scanner, with a density of 15-20 po<strong>in</strong>ts per m² and a spatial resolution of 1 m x 1 m (NÖGIS,<br />
http://www.<strong>in</strong>termap1.noel.gv.at/webgisatlas/<strong>in</strong>it.aspx, 18-Jul-11).<br />
The DTM which was generated <strong>in</strong> 2009 after <strong>the</strong> rock-<strong>mass</strong> fall, was based on TLS (Terrestric Laser Scann<strong>in</strong>g)<br />
performed by a local survey<strong>in</strong>g eng<strong>in</strong>eer (AVT – ZT-GmbH), us<strong>in</strong>g a RIEGL LMS-Z420i comb<strong>in</strong>ed with a GPS LEICA<br />
System 1200. For generat<strong>in</strong>g this laser scan model, seven scans from different positions were performed, each<br />
generat<strong>in</strong>g a po<strong>in</strong>t cloud with a number of 1 900 000 po<strong>in</strong>ts. The accuracy of <strong>the</strong> scans was stated 10 mm by<br />
<strong>the</strong> survey<strong>in</strong>g eng<strong>in</strong>eer. Each scan was referenced us<strong>in</strong>g D-GPS LEICA System 1200 via temporarily <strong>in</strong>stalled<br />
targets and comb<strong>in</strong>ed with each o<strong>the</strong>r. The relative accuracy of <strong>the</strong> GPS measurements is +/- two cm. The<br />
s<strong>in</strong>gle scans were transformed <strong>in</strong> <strong>the</strong> local reference coord<strong>in</strong>ate system and merged to a s<strong>in</strong>gle DSM (Digital<br />
Surface Model) with an accuracy of approximately three cm. The DSM was <strong>the</strong>n filtered to produce <strong>the</strong> f<strong>in</strong>al<br />
DTM used for all fur<strong>the</strong>r calculations. Terra<strong>in</strong> modell<strong>in</strong>g was <strong>in</strong> both cases done by triangulation <strong>in</strong><br />
consideration of break<strong>in</strong>g edges. Overhang<strong>in</strong>g areas were modelled separately to ga<strong>in</strong> more realistic data.<br />
Results<br />
Lithology at <strong>the</strong> former quarries<br />
In <strong>the</strong> former quarry near Spitz, marbles are predom<strong>in</strong>ant <strong>in</strong> <strong>the</strong> hang<strong>in</strong>g wall. The foot wall is composed of<br />
calc- silicate gneiss and granodiorite gneiss. Mica-rich layers (biotite schist) between <strong>the</strong> marble beds occur<br />
frequently with<strong>in</strong> <strong>the</strong> Spitz marbles. Lentoid bodies of amphibolites and pegmatitic <strong>in</strong>clusions can also be found.<br />
9
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