mass falls in the Wachau-Danube Valley (Bohemian Massif

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Fig. 4 Geological-tectonic map of the Wachau-Danube Valley (Matura 1983, modified) and study areas near Spitz (1) and Dürnstein (2). Geological and tectonic overview From a tectonic point of view, the crystalline basement of the Wachau region is part of the mainly NE-SW trending gneiss massifs of the Moldanubian Complex, which is a major tectonic unit of the Bohemian Massif, representing Austria´s share of the European Variscides (Fuchs and Matura 1980). The Moldanubian zone is further divided into several thrusts, of which the Gföhl and the Raabs units are exposed in the study area, according to the latest tectonic classification by Matura (2003 and 2006). The high temperature/high pressure metamorphic Gföhl unit is generally composed of migmatitic granite type gneisses and granulite in its south-eastern part and is belted by amphibolites. The Raabs unit contains mostly para- and orthogneisses, as well as different alkaline meta-magmatites. The marble and the Spitz calc silicate gneiss-formation had previously been attributed to the Drosendorf unit (Fuchs and Matura 1980 and Schnabel et al. 2002). Recently, however, this unit has been included in the Raabs unit. The former Drosendorf unit (more recently Drosendorf formation) has been subsumed under the Bíteš unit, which forms the uppermost tectonic unit of the Moravo-Silesian nappe complex. Since Matura (2003) defines the border between the Spitz marbles in the hanging wall and the granodiorite gneiss in the foot wall as the contact between the Moravian and the Moldanubian zones, this formation is of major tectonic significance. 4

Tectonically, the whole Bohemian Massif is cut by subvertical, NW-SE and NE-SW trending strike-slip faults. Their kinematics have already been extensively investigated by Wallbrecher, Brandmayr and Handler (1990) and Brandmayr et al. (1995). They have interpreted these faults as a conjugated system of slip-lines, induced by indentation, elongated in the E-W direction and moved from S to N. During the alpine orogenesis, these structures dating back to the late Variscian were reactivated and their tectonic offset has not yet come to the end (Scheidegger 1976). In the case of both study areas, the subvertical NE-SW trending Diendorf fault is of central importance. The offset along this major, 160 km long, sinistral strike slip fault, covers a range of up to 25 km. Hence, the surrounding rocks are extremely folded, faulted and fractured (Tollmann 1985). The Diendorf fault also marks the boundary between the granulite and the granite-gneiss within the Gföhl unit. A secondary fault near Spitz, parallel to the master fault, indicates the NW border of the seismically active fault zone (Figdor and Scheidegger 1977). In both quarries, the main conjugated fault orientations, NE-SW as well as NW-SE striking, are dominant (Fig. 5). The locations of the former quarries near Spitz and Dürnstein are labelled as number 1 (Spitz) and 2 (Dürnstein, approximately nine kilometres NE of Spitz) in Fig. 4. Historical mining activities and previous rock-mass falls In both quarries, rubble, mainly used for construction, has been mined since the nineteenth century. The Spitz marble was also used for decoration. The high quality of the Spitz marble and the Gföhl gneiss of Dürnstein in terms of technical properties, such as high uniaxial compressive strength, high wear and erosion resistance, regular shape of the broken stone and the quarries’ favourable position directly along the banks of the Danube River, a major traffic route at that time, made the quarries ideal production sites. Natural slip planes were deliberately activated by blasting, thus inducing controlled failure, which, in turn, produced a maximum of rubble using a minimum of explosives. This was a common mining method in the nineteenth century, and was employed in both quarries. Matura (1989) was the first geologist to consider these locational advantages in terms of landscape degradation. The Spitz marble quarry was first opened around 1800, starting from the Danube, at the foot of a typical cataclinal slope. The open pit consisted of one single mining face of great height, lacking any benches. Decades ago, Stiny (1940, unpubl.) already critically remarked that this mining method did not meet the regulations of the authorities. He did, however, disregard the accidental triggering of rockslides. On the contrary, he considered the dip of the bedding planes in relation to the mining face as favourable, since it facilitated mining. Extreme undercutting of the foot of the beds resulted in the failure of a huge rock mass of 70 000 m³ in March 1961. It first slid along a bedding plane (rockslide) and then plunged down the rock face to the base of the open pit mine (rock-mass fall). In 1975, a new mining concept was devised, developing several benches from the South to the North, to make it blend in better with the geological and geomorphological conditions. Further undercutting of the beds was avoided and the bench geometry was adjusted to meet the natural joint system. In May 1982, cracks along the crest of the mining face developed, which resulted in another rock-mass fall of 10 000 m³ mass in October 1984. This event marked the start of geotechnical monitoring of the mine. Movements above the mining face were observed, yet again, in April 1996, which led to the termination of mining activities. A remedial mine design had to be planned. Before the realisation of this design, the worstever rockslide so far occurred in November 2002, after a rainy summer season causing several floods in Lower Austria, triggering a combined rockslide/rock-mass fall of 60 000-80 000 m³ in volume. Fig. 5 shows the present geomorphological situation in the former quarry. The Dürnstein quarry is situated in a steep rock cliff, which faces W towards the Danube River. It was mined until the year 1903 using mining methods similar to those employed in the quarry at Spitz, leaving a 130 m high, partly up to seven metres overhanging mining face directly above the provincial road (Fig. 6a). According to historical sources (Stary 1972), a rock-mass fall completely devastated and buried the local road as early as 1899. In the winter of 1909, another rock-mass fall destroyed the already existing alignment of rail road tracks under construction. As a result, it was decided to remove by blasting potentially instable parts of the rock face. The plan was to remove the bigger part of the overhanging rock mass with one blast only. In order to realise this, three caverns inside the mountain, accessed via galleries, were excavated and charged with 825, 650 and 2200 kg of dynamite. This engineering work was planned and carried out by a private contractor, supported by blasting experts of the imperial-royal army. The payload was ignited on 4-May-1909. The information provided by historical sources (Stary 1972; Esop unpubl.; Mayreder unpubl.) concerning the blasted volume of rock is contradictory, ranging between 60 000-80 000 m³. 5

Page 1 and 2: Geomorphological and geotechnical c

Page 3: Fig. 3 Rock-mass fall near the town

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 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

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mass falls in the Wachau-Danube Valley (Bohemian Massif

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