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
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Tectonically, <strong>the</strong> whole <strong>Bohemian</strong> <strong>Massif</strong> is cut by subvertical, NW-SE and NE-SW trend<strong>in</strong>g strike-slip faults.<br />
Their k<strong>in</strong>ematics have already been extensively <strong>in</strong>vestigated by Wallbrecher, Brandmayr and Handler (1990)<br />
and Brandmayr et al. (1995). They have <strong>in</strong>terpreted <strong>the</strong>se faults as a conjugated system of slip-l<strong>in</strong>es, <strong>in</strong>duced by<br />
<strong>in</strong>dentation, elongated <strong>in</strong> <strong>the</strong> E-W direction and moved from S to N. Dur<strong>in</strong>g <strong>the</strong> alp<strong>in</strong>e orogenesis, <strong>the</strong>se<br />
structures dat<strong>in</strong>g back to <strong>the</strong> late Variscian were reactivated and <strong>the</strong>ir tectonic offset has not yet come to <strong>the</strong><br />
end (Scheidegger 1976).<br />
In <strong>the</strong> case of both study areas, <strong>the</strong> subvertical NE-SW trend<strong>in</strong>g Diendorf fault is of central importance. The<br />
offset along this major, 160 km long, s<strong>in</strong>istral strike slip fault, covers a range of up to 25 km. Hence, <strong>the</strong><br />
surround<strong>in</strong>g rocks are extremely folded, faulted and fractured (Tollmann 1985). The Diendorf fault also marks<br />
<strong>the</strong> boundary between <strong>the</strong> granulite and <strong>the</strong> granite-gneiss with<strong>in</strong> <strong>the</strong> Gföhl unit. A secondary fault near Spitz,<br />
parallel to <strong>the</strong> master fault, <strong>in</strong>dicates <strong>the</strong> NW border of <strong>the</strong> seismically active fault zone (Figdor and<br />
Scheidegger 1977). In both quarries, <strong>the</strong> ma<strong>in</strong> conjugated fault orientations, NE-SW as well as NW-SE strik<strong>in</strong>g,<br />
are dom<strong>in</strong>ant (Fig. 5). The locations of <strong>the</strong> former quarries near Spitz and Dürnste<strong>in</strong> are labelled as number 1<br />
(Spitz) and 2 (Dürnste<strong>in</strong>, approximately n<strong>in</strong>e kilometres NE of Spitz) <strong>in</strong> Fig. 4.<br />
Historical m<strong>in</strong><strong>in</strong>g activities and previous rock-<strong>mass</strong> <strong>falls</strong><br />
In both quarries, rubble, ma<strong>in</strong>ly used for construction, has been m<strong>in</strong>ed s<strong>in</strong>ce <strong>the</strong> n<strong>in</strong>eteenth century. The Spitz<br />
marble was also used for decoration. The high quality of <strong>the</strong> Spitz marble and <strong>the</strong> Gföhl gneiss of Dürnste<strong>in</strong> <strong>in</strong><br />
terms of technical properties, such as high uniaxial compressive strength, high wear and erosion resistance,<br />
regular shape of <strong>the</strong> broken stone and <strong>the</strong> quarries’ favourable position directly along <strong>the</strong> banks of <strong>the</strong> <strong>Danube</strong><br />
River, a major traffic route at that time, made <strong>the</strong> quarries ideal production sites. Natural slip planes were<br />
deliberately activated by blast<strong>in</strong>g, thus <strong>in</strong>duc<strong>in</strong>g controlled failure, which, <strong>in</strong> turn, produced a maximum of<br />
rubble us<strong>in</strong>g a m<strong>in</strong>imum of explosives. This was a common m<strong>in</strong><strong>in</strong>g method <strong>in</strong> <strong>the</strong> n<strong>in</strong>eteenth century, and was<br />
employed <strong>in</strong> both quarries. Matura (1989) was <strong>the</strong> first geologist to consider <strong>the</strong>se locational advantages <strong>in</strong><br />
terms of landscape degradation.<br />
The Spitz marble quarry was first opened around 1800, start<strong>in</strong>g from <strong>the</strong> <strong>Danube</strong>, at <strong>the</strong> foot of a typical<br />
catacl<strong>in</strong>al slope. The open pit consisted of one s<strong>in</strong>gle m<strong>in</strong><strong>in</strong>g face of great height, lack<strong>in</strong>g any benches. Decades<br />
ago, St<strong>in</strong>y (1940, unpubl.) already critically remarked that this m<strong>in</strong><strong>in</strong>g method did not meet <strong>the</strong> regulations of<br />
<strong>the</strong> authorities. He did, however, disregard <strong>the</strong> accidental trigger<strong>in</strong>g of rockslides. On <strong>the</strong> contrary, he<br />
considered <strong>the</strong> dip of <strong>the</strong> bedd<strong>in</strong>g planes <strong>in</strong> relation to <strong>the</strong> m<strong>in</strong><strong>in</strong>g face as favourable, s<strong>in</strong>ce it facilitated m<strong>in</strong><strong>in</strong>g.<br />
Extreme undercutt<strong>in</strong>g of <strong>the</strong> foot of <strong>the</strong> beds resulted <strong>in</strong> <strong>the</strong> failure of a huge rock <strong>mass</strong> of 70 000 m³ <strong>in</strong> March<br />
1961. It first slid along a bedd<strong>in</strong>g plane (rockslide) and <strong>the</strong>n plunged down <strong>the</strong> rock face to <strong>the</strong> base of <strong>the</strong> open<br />
pit m<strong>in</strong>e (rock-<strong>mass</strong> fall). In 1975, a new m<strong>in</strong><strong>in</strong>g concept was devised, develop<strong>in</strong>g several benches from <strong>the</strong><br />
South to <strong>the</strong> North, to make it blend <strong>in</strong> better with <strong>the</strong> geological and geomorphological conditions. Fur<strong>the</strong>r<br />
undercutt<strong>in</strong>g of <strong>the</strong> beds was avoided and <strong>the</strong> bench geometry was adjusted to meet <strong>the</strong> natural jo<strong>in</strong>t system.<br />
In May 1982, cracks along <strong>the</strong> crest of <strong>the</strong> m<strong>in</strong><strong>in</strong>g face developed, which resulted <strong>in</strong> ano<strong>the</strong>r rock-<strong>mass</strong> fall of<br />
10 000 m³ <strong>mass</strong> <strong>in</strong> October 1984. This event marked <strong>the</strong> start of geotechnical monitor<strong>in</strong>g of <strong>the</strong> m<strong>in</strong>e.<br />
Movements above <strong>the</strong> m<strong>in</strong><strong>in</strong>g face were observed, yet aga<strong>in</strong>, <strong>in</strong> April 1996, which led to <strong>the</strong> term<strong>in</strong>ation of<br />
m<strong>in</strong><strong>in</strong>g activities. A remedial m<strong>in</strong>e design had to be planned. Before <strong>the</strong> realisation of this design, <strong>the</strong> worstever<br />
rockslide so far occurred <strong>in</strong> November 2002, after a ra<strong>in</strong>y summer season caus<strong>in</strong>g several floods <strong>in</strong> Lower<br />
Austria, trigger<strong>in</strong>g a comb<strong>in</strong>ed rockslide/rock-<strong>mass</strong> fall of 60 000-80 000 m³ <strong>in</strong> volume. Fig. 5 shows <strong>the</strong> present<br />
geomorphological situation <strong>in</strong> <strong>the</strong> former quarry.<br />
The Dürnste<strong>in</strong> quarry is situated <strong>in</strong> a steep rock cliff, which faces W towards <strong>the</strong> <strong>Danube</strong> River. It was m<strong>in</strong>ed<br />
until <strong>the</strong> year 1903 us<strong>in</strong>g m<strong>in</strong><strong>in</strong>g methods similar to those employed <strong>in</strong> <strong>the</strong> quarry at Spitz, leav<strong>in</strong>g a 130 m high,<br />
partly up to seven metres overhang<strong>in</strong>g m<strong>in</strong><strong>in</strong>g face directly above <strong>the</strong> prov<strong>in</strong>cial road (Fig. 6a). Accord<strong>in</strong>g to<br />
historical sources (Stary 1972), a rock-<strong>mass</strong> fall completely devastated and buried <strong>the</strong> local road as early as<br />
1899. In <strong>the</strong> w<strong>in</strong>ter of 1909, ano<strong>the</strong>r rock-<strong>mass</strong> fall destroyed <strong>the</strong> already exist<strong>in</strong>g alignment of rail road tracks<br />
under construction. As a result, it was decided to remove by blast<strong>in</strong>g potentially <strong>in</strong>stable parts of <strong>the</strong> rock face.<br />
The plan was to remove <strong>the</strong> bigger part of <strong>the</strong> overhang<strong>in</strong>g rock <strong>mass</strong> with one blast only. In order to realise<br />
this, three caverns <strong>in</strong>side <strong>the</strong> mounta<strong>in</strong>, accessed via galleries, were excavated and charged with 825, 650 and<br />
2200 kg of dynamite. This eng<strong>in</strong>eer<strong>in</strong>g work was planned and carried out by a private contractor, supported by<br />
blast<strong>in</strong>g experts of <strong>the</strong> imperial-royal army. The payload was ignited on 4-May-1909.<br />
The <strong>in</strong>formation provided by historical sources (Stary 1972; Esop unpubl.; Mayreder unpubl.) concern<strong>in</strong>g <strong>the</strong><br />
blasted volume of rock is contradictory, rang<strong>in</strong>g between 60 000-80 000 m³.<br />
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