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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Geomorphological and geotechnical causes of anthropogenic-<strong>in</strong>duced rock<strong>mass</strong><br />
<strong>falls</strong> <strong>in</strong> <strong>the</strong> <strong>Wachau</strong>-<strong>Danube</strong> <strong>Valley</strong> (<strong>Bohemian</strong> <strong>Massif</strong>, Lower Austria)<br />
Hans J. Laimer 1 and Mart<strong>in</strong> Müllegger 2<br />
1 Austrian Federal Railways (ÖBB), Infrastruktur AG, Salzburg, Austria<br />
2 iC consulenten Ziviltechniker GesmbH, Bergheim, Austria<br />
Laimer, H.J. and Müllegger, M., 2012. Geomorphological and geotechnical causes of anthropogenic-<strong>in</strong>duced rock-<strong>mass</strong> <strong>falls</strong><br />
<strong>in</strong> <strong>the</strong> <strong>Wachau</strong>-<strong>Danube</strong> <strong>Valley</strong> (<strong>Bohemian</strong> <strong>Massif</strong>, Lower Austria). Geografiska Annaler Series A Volume 94, Issue 1, p.157-<br />
174. doi:10.1111/j.1468-0459.2012.00451.x<br />
ABSTRACT<br />
The <strong>Wachau</strong>-<strong>Danube</strong> <strong>Valley</strong> represents a transverse valley, <strong>in</strong>tersect<strong>in</strong>g <strong>the</strong> Variscian <strong>Bohemian</strong> <strong>Massif</strong>.<br />
Weakened rocks along fault structures led to accelerated river erosion, form<strong>in</strong>g relatively steep rock slopes.<br />
The exceptional cultural position of <strong>the</strong> region generated an <strong>in</strong>creas<strong>in</strong>g demand for build<strong>in</strong>g materials. Over <strong>the</strong><br />
centuries quarry<strong>in</strong>g had a sizeable impact on slope morphology. Interdependences between quarry<strong>in</strong>g and<br />
construction caused unstable rock slopes and four rock-<strong>mass</strong> <strong>falls</strong> have occurred at two quarries near Spitz<br />
(1961, 1984, 2002) and Dürnste<strong>in</strong> (2009). Rock mechanical analysis at <strong>the</strong>se quarries has shown that <strong>the</strong><br />
comb<strong>in</strong>ation of exist<strong>in</strong>g geological discont<strong>in</strong>uities and artificially modified morphology is fatal <strong>in</strong> terms of slope<br />
stability. In Spitz <strong>the</strong> bedd<strong>in</strong>g planes with<strong>in</strong> <strong>the</strong> marble had been undercut by <strong>the</strong> m<strong>in</strong><strong>in</strong>g face. Additionally, two<br />
conjugated, steeply dipp<strong>in</strong>g jo<strong>in</strong>t sets formed large scale blocks slid<strong>in</strong>g on bedd<strong>in</strong>g planes. In three major<br />
rockslides/rock-<strong>mass</strong> <strong>falls</strong>, each triggered by heavy ra<strong>in</strong><strong>falls</strong>, a total <strong>mass</strong> of 170 000 m³ of rock failed. At <strong>the</strong><br />
quarry near Dürnste<strong>in</strong> <strong>the</strong> geotechnical characteristics of <strong>the</strong> gneiss are also unfavourable <strong>in</strong> relation to <strong>the</strong><br />
exposition of <strong>the</strong> m<strong>in</strong><strong>in</strong>g face. After several rock<strong>falls</strong>, 65 000 m³ were blasted away <strong>in</strong> 1909 to remove unstable<br />
rock slopes. The residual rock face was destabilized and rockfall activities culm<strong>in</strong>ated <strong>in</strong> an event with a total<br />
volume of approximately 15 000 m³. Remedial measures for both locations are essential to ma<strong>in</strong>ta<strong>in</strong> transport<br />
<strong>in</strong>frastructure. Sufficiently stable conditions can only be achieved by extensive reshap<strong>in</strong>g of <strong>the</strong> m<strong>in</strong><strong>in</strong>g faces,<br />
which <strong>in</strong>volves adapt<strong>in</strong>g slope geometries to naturally stable jo<strong>in</strong>t faces.<br />
Key words: <strong>Bohemian</strong> <strong>Massif</strong>, rock-<strong>mass</strong> <strong>falls</strong>, rock mechanics, slope geometry, rock eng<strong>in</strong>eer<strong>in</strong>g, protective<br />
measures<br />
Introduction<br />
In contrast to alp<strong>in</strong>e regions, where rock slope failures are frequent, e.g. <strong>in</strong> <strong>the</strong> steep side walls of U-shaped<br />
valleys formed by glacial erosion <strong>in</strong> <strong>the</strong> Central Alps (Abele 1974), present day large-scale rock<strong>falls</strong> <strong>in</strong> <strong>the</strong> slopes<br />
of <strong>the</strong> <strong>Bohemian</strong> <strong>Massif</strong> are rare. Known events of <strong>the</strong> last decades were man <strong>in</strong>duced and took place <strong>in</strong> former<br />
quarries as described below. In this paper <strong>the</strong> term rock-<strong>mass</strong> fall will be used to describe failures of large<br />
bodies of material at very steep, mostly undercut slopes (cf. Selby 1993). Rock-<strong>mass</strong> <strong>falls</strong> differ from fragmental<br />
rockfall (<strong>falls</strong> of s<strong>in</strong>gle blocks) <strong>in</strong> frequency and volume of <strong>the</strong> mov<strong>in</strong>g material, while <strong>the</strong> term rockslide is used<br />
for a different type of rock-slope failure (mov<strong>in</strong>g of a rock <strong>mass</strong> along a slid<strong>in</strong>g plane).<br />
The former quarries of Dürnste<strong>in</strong> and Spitz are located <strong>in</strong> <strong>the</strong> so-called <strong>Wachau</strong> Cultural Landscape, stretch<strong>in</strong>g<br />
from Melk to Krems along <strong>the</strong> <strong>Danube</strong> <strong>Valley</strong>, approximately 80 km west of <strong>the</strong> City of Vienna. This UNESCO<br />
protected region represents a typical transverse valley, <strong>in</strong>tersect<strong>in</strong>g <strong>the</strong> sou<strong>the</strong>rnmost part of <strong>the</strong> Variscan<br />
<strong>Bohemian</strong> <strong>Massif</strong>. Archaeological evidence such as <strong>the</strong> "Venus of Willendorf” shows that this region has been<br />
<strong>in</strong>habited by man s<strong>in</strong>ce <strong>the</strong> Upper Palaeolithic (20 000-30 000 BP). Hence, this section of <strong>the</strong> <strong>Danube</strong> <strong>Valley</strong> can<br />
be considered one of Austria´s oldest cultural landscapes. Until <strong>the</strong> Early Middle Ages, <strong>the</strong> region was primarily<br />
relevant as a traffic route. The ma<strong>in</strong> build<strong>in</strong>g period started <strong>in</strong> <strong>the</strong> tenth century under Bavarian rule (Lechner<br />
1983). The exceptional cultural position of <strong>the</strong> <strong>Wachau</strong> led to an ever-<strong>in</strong>creas<strong>in</strong>g demand for natural resources<br />
to support large scale build<strong>in</strong>g activities (religious and secular build<strong>in</strong>gs toge<strong>the</strong>r with <strong>the</strong> characteristic w<strong>in</strong>e<br />
terraces). Gneiss and marble were m<strong>in</strong>ed <strong>in</strong> small quarries, which even <strong>the</strong>n had m<strong>in</strong>or effects on slope<br />
morphology. Quarry<strong>in</strong>g peaked between <strong>the</strong> sixteenth and n<strong>in</strong>eteenth centuries as a consequence of transport<br />
1
<strong>in</strong>frastructure construction (roads, railways), which eventually had a dual effect on slope morphology: on <strong>the</strong><br />
one hand, exist<strong>in</strong>g quarries were expanded and/or reactivated and, on <strong>the</strong> o<strong>the</strong>r hand, traffic <strong>in</strong>frastructure<br />
construction itself had an enormous impact on <strong>the</strong> area. With <strong>the</strong> advent of railway eng<strong>in</strong>eer<strong>in</strong>g, fur<strong>the</strong>r<br />
difficulties were encountered especially at sites with work<strong>in</strong>g conditions comparable to alp<strong>in</strong>e construction<br />
sites, as <strong>the</strong> planned routes led along steep rock slopes over long distances. Route construction often required<br />
high slope cuts or subtle eng<strong>in</strong>eer<strong>in</strong>g structures such as tunnels, bridges and reta<strong>in</strong><strong>in</strong>g walls.<br />
Fig. 1 The extent of <strong>the</strong> <strong>Bohemian</strong> <strong>Massif</strong> <strong>in</strong> Austria. The box refers to <strong>the</strong> wider study area as shown <strong>in</strong> Fig. 4<br />
Interdependences between quarry<strong>in</strong>g and transport <strong>in</strong>frastructure construction eventually resulted <strong>in</strong> <strong>the</strong><br />
development of unstable slopes. Besides several documented events with<strong>in</strong> <strong>the</strong> 20th century <strong>the</strong> latest rock<strong>mass</strong><br />
<strong>falls</strong> occurred <strong>in</strong> 2002 (Spitz, Fig. 2) and <strong>in</strong> 2009 (Dürnste<strong>in</strong>, Fig. 3).<br />
30 m<br />
Fig. 2 Rock-<strong>mass</strong> fall near <strong>the</strong> village of Spitz (view towards WNW, photo: Müllegger, May 2010).<br />
2
Fig. 3 Rock-<strong>mass</strong> fall near <strong>the</strong> town of Dürnste<strong>in</strong> (view towards SSE, photo: Müllegger, August 2009).<br />
Contrary to many alp<strong>in</strong>e rock-<strong>mass</strong> <strong>falls</strong> <strong>in</strong> un<strong>in</strong>habited areas (cf. Weid<strong>in</strong>ger and Vortisch 2005) <strong>the</strong>se two<br />
events <strong>in</strong> abandoned quarries caused major damage and still endanger transport <strong>in</strong>frastructure, which thus<br />
made geo-scientific <strong>in</strong>vestigations and protective measures necessary. Aim of this paper is to comment on<br />
geomorphological and geotechnical causes of <strong>the</strong>se rockslides/rock-<strong>mass</strong> <strong>falls</strong>.<br />
Study area<br />
Geomorphic overview<br />
40 m<br />
Between Passau and Krems, <strong>the</strong> <strong>Danube</strong> River runs along <strong>the</strong> sou<strong>the</strong>rn edge of <strong>the</strong> <strong>Bohemian</strong> <strong>Massif</strong>, most of<br />
which is overlaid by Tertiary and Quaternary sediments. The exposed edge of <strong>the</strong> hilly fault residual plateau<br />
takes <strong>the</strong> form of a 500-600 m high piedmont stairway. Where higher mounta<strong>in</strong> ridges reach <strong>the</strong> plateau’s<br />
marg<strong>in</strong>, <strong>the</strong> river cuts down <strong>in</strong>to <strong>the</strong> granites and gneisses of <strong>the</strong> crystall<strong>in</strong>e basement, and carves out<br />
transverse valleys, such as <strong>the</strong> <strong>Wachau</strong>-<strong>Danube</strong> <strong>Valley</strong> (Kohl 1966). The morphogenesis of this river section was<br />
last analysed by Nagl and Verg<strong>in</strong>is (1987), whose research focused on <strong>the</strong> palaeogeographic development of<br />
<strong>the</strong> catchment area. They suppose that <strong>the</strong> valley between Melk and Spitz (cf. Figs 1 and 4)was dra<strong>in</strong>ed by <strong>the</strong><br />
Enns River dur<strong>in</strong>g <strong>the</strong> Tertiary, while <strong>the</strong> <strong>Danube</strong> River at that time run fur<strong>the</strong>r north. In consequence of Late<br />
Tertiary tectonic processes <strong>the</strong> <strong>Danube</strong> River was captured by <strong>the</strong> <strong>Wachau</strong>-Enns River.<br />
The geomorphological appearance is characterised by <strong>the</strong> fact that <strong>the</strong> orientation of <strong>the</strong> river bed follows two<br />
conjugated major tectonic l<strong>in</strong>eaments, strik<strong>in</strong>g approximately NE-SW and NW-SE. Wide bas<strong>in</strong> and terraced<br />
landscapes are <strong>the</strong>refore non-existent <strong>in</strong> contrast to adjacent areas with Tertiary and Quaternary cover<br />
sediments. The tectonically fragmented and weakened rock <strong>mass</strong> along fault structures caused accelerated<br />
river erosion form<strong>in</strong>g V-shaped valleys with relatively steep rock slopes and trigger<strong>in</strong>g large rockslides dur<strong>in</strong>g<br />
<strong>the</strong> Oligocene. Matura (1983 and 1989) mapped a fossil slide <strong>mass</strong> deposited <strong>in</strong> <strong>the</strong> <strong>Wachau</strong>-<strong>Danube</strong> <strong>Valley</strong><br />
near <strong>the</strong> village of Weißenkirchen (Fig. 4). He also <strong>in</strong>terprets deposits of gneiss boulders <strong>in</strong> Weißenkirchen as<br />
rockslide sediments of <strong>the</strong> same age.<br />
In <strong>the</strong> present day, <strong>the</strong> rock slopes of <strong>the</strong> <strong>Bohemian</strong> <strong>Massif</strong> are usually considered to be stable. Gatt<strong>in</strong>ger (1980)<br />
assumes that <strong>mass</strong> movements are of m<strong>in</strong>or relevance to <strong>the</strong> <strong>Bohemian</strong> <strong>Massif</strong>, ow<strong>in</strong>g to its relatively small<br />
relief ratio, and are limited to s<strong>in</strong>gle rock<strong>falls</strong> or local slope ruptures.<br />
3<br />
Dürnste<strong>in</strong><br />
<strong>Danube</strong> river
Fig. 4 Geological-tectonic map of <strong>the</strong> <strong>Wachau</strong>-<strong>Danube</strong> <strong>Valley</strong> (Matura 1983, modified) and study areas near<br />
Spitz (1) and Dürnste<strong>in</strong> (2).<br />
Geological and tectonic overview<br />
From a tectonic po<strong>in</strong>t of view, <strong>the</strong> crystall<strong>in</strong>e basement of <strong>the</strong> <strong>Wachau</strong> region is part of <strong>the</strong> ma<strong>in</strong>ly NE-SW<br />
trend<strong>in</strong>g gneiss <strong>mass</strong>ifs of <strong>the</strong> Moldanubian Complex, which is a major tectonic unit of <strong>the</strong> <strong>Bohemian</strong> <strong>Massif</strong>,<br />
represent<strong>in</strong>g Austria´s share of <strong>the</strong> European Variscides (Fuchs and Matura 1980). The Moldanubian zone is<br />
fur<strong>the</strong>r divided <strong>in</strong>to several thrusts, of which <strong>the</strong> Gföhl and <strong>the</strong> Raabs units are exposed <strong>in</strong> <strong>the</strong> study area,<br />
accord<strong>in</strong>g to <strong>the</strong> latest tectonic classification by Matura (2003 and 2006).<br />
The high temperature/high pressure metamorphic Gföhl unit is generally composed of migmatitic granite type<br />
gneisses and granulite <strong>in</strong> its south-eastern part and is belted by amphibolites. The Raabs unit conta<strong>in</strong>s mostly<br />
para- and orthogneisses, as well as different alkal<strong>in</strong>e meta-magmatites. The marble and <strong>the</strong> Spitz calc silicate<br />
gneiss-formation had previously been attributed to <strong>the</strong> Drosendorf unit (Fuchs and Matura 1980 and Schnabel<br />
et al. 2002). Recently, however, this unit has been <strong>in</strong>cluded <strong>in</strong> <strong>the</strong> Raabs unit. The former Drosendorf unit<br />
(more recently Drosendorf formation) has been subsumed under <strong>the</strong> Bíteš unit, which forms <strong>the</strong> uppermost<br />
tectonic unit of <strong>the</strong> Moravo-Silesian nappe complex. S<strong>in</strong>ce Matura (2003) def<strong>in</strong>es <strong>the</strong> border between <strong>the</strong> Spitz<br />
marbles <strong>in</strong> <strong>the</strong> hang<strong>in</strong>g wall and <strong>the</strong> granodiorite gneiss <strong>in</strong> <strong>the</strong> foot wall as <strong>the</strong> contact between <strong>the</strong> Moravian<br />
and <strong>the</strong> Moldanubian zones, this formation is of major tectonic significance.<br />
4
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 />
5
Fig. 5 Geomorphological situation <strong>in</strong> <strong>the</strong> former quarry at Spitz. The square shows <strong>the</strong> ru<strong>in</strong> of a service block.<br />
Be that as it may, <strong>the</strong> railroad track alignment as well as <strong>the</strong> prov<strong>in</strong>cial road were aga<strong>in</strong> completely buried, with<br />
<strong>the</strong> debris cone even reach<strong>in</strong>g <strong>the</strong> <strong>Danube</strong> River (Fig. 6b). Despite <strong>the</strong> blast, ano<strong>the</strong>r rockfall happened close to<br />
<strong>the</strong> nor<strong>the</strong>rn portal of <strong>the</strong> Dürnste<strong>in</strong> tunnel follow<strong>in</strong>g a period of heavy ra<strong>in</strong>fall <strong>in</strong> September 1909. Two people<br />
were killed and six severely <strong>in</strong>jured by this event, which also buried workers’ barracks (regional newspaper<br />
article, Niederösterreichische Presse Nr. 28, 18-Sept-1909).<br />
Fig. 6 (a) Rock face at <strong>the</strong> former quarry near Dürnste<strong>in</strong> before and (b) after <strong>the</strong> blast of 1909. The height of <strong>the</strong><br />
rock wall is about 130 m (view towards E, photo: Mayreder, May 1909).<br />
6
There is not much <strong>in</strong>formation on fur<strong>the</strong>r rock-<strong>mass</strong> fall events over <strong>the</strong> follow<strong>in</strong>g decades. However, a<br />
comparison of photographs taken <strong>in</strong> 1909 and 100 years later, just before <strong>the</strong> rock-<strong>mass</strong> fall <strong>in</strong> 2009, clearly<br />
<strong>in</strong>dicates that fur<strong>the</strong>r rock<strong>falls</strong> must have occurred <strong>in</strong> <strong>the</strong> time between. There is only one report of a rockfall,<br />
which took place <strong>in</strong> <strong>the</strong> w<strong>in</strong>ter of 2002/2003, after an unusually ra<strong>in</strong>y autumn. The latest rock-<strong>mass</strong> fall<br />
occurred <strong>in</strong> July 2009, yet aga<strong>in</strong> preceded by a period of heavy ra<strong>in</strong>fall. The rail <strong>in</strong>frastructure was destroyed by<br />
this event, and some smaller boulders even reached <strong>the</strong> ma<strong>in</strong> road B3 “Donauuferstraße” (Fig. 7).<br />
Fig. 7 Damage to rail <strong>in</strong>frastructure <strong>in</strong> 2009 (view towards NNW, photo: Laimer, July 2009).<br />
Methods<br />
Failure model analysis<br />
To determ<strong>in</strong>e <strong>the</strong> mode of failure, which caused <strong>the</strong> rock-<strong>mass</strong> <strong>falls</strong>/rock slides, a k<strong>in</strong>ematic failure analysis was<br />
performed, based upon <strong>the</strong> pr<strong>in</strong>cipals of “block <strong>the</strong>ory” (Goodman and Shi 1985).<br />
The application of this method is practicable for both sites (Spitz and Dürnste<strong>in</strong>), s<strong>in</strong>ce <strong>the</strong> exceed<strong>in</strong>g of rock<br />
<strong>mass</strong> strength can be excluded as a cause of failure, because of <strong>the</strong> fact, that <strong>in</strong> both cases, <strong>the</strong> uniaxial<br />
compressive strength of <strong>the</strong> rock <strong>mass</strong> is by far higher than <strong>the</strong> overburden stress (subchapter Lithology).<br />
Therefore, <strong>the</strong> rock <strong>mass</strong> can be seen as a system of rigid blocks separated by discont<strong>in</strong>uities. Large scale<br />
displacements with<strong>in</strong> <strong>the</strong> rock <strong>mass</strong> can only occur if blocks are moved relative to each o<strong>the</strong>r along dist<strong>in</strong>ct<br />
shear planes, an effect known as “block failure” <strong>in</strong> geotechnical eng<strong>in</strong>eer<strong>in</strong>g (Hoek and Bray 1974). For <strong>the</strong><br />
analysis, discont<strong>in</strong>uity orientation data acquired by detailed geotechnical mapp<strong>in</strong>g of <strong>the</strong> rock face were used<br />
and put <strong>in</strong> relation to <strong>the</strong> spatial orientation of <strong>the</strong> rock face, us<strong>in</strong>g stereographic projection. The ma<strong>in</strong> failure<br />
modes block slid<strong>in</strong>g (slid<strong>in</strong>g of a block on a s<strong>in</strong>gle plane), wedge slid<strong>in</strong>g (slid<strong>in</strong>g of a block on two planes <strong>in</strong> a<br />
direction along <strong>the</strong> l<strong>in</strong>e of <strong>in</strong>tersection) and toppl<strong>in</strong>g (rotational failure of th<strong>in</strong> columns caused by<br />
discont<strong>in</strong>uities strik<strong>in</strong>g +/- parallel and dipp<strong>in</strong>g steeply contrary to <strong>the</strong> rock face) were exam<strong>in</strong>ed, us<strong>in</strong>g <strong>the</strong><br />
computer program dips ® (ROCSCIENCE).<br />
K<strong>in</strong>ematically, a block can slide on a s<strong>in</strong>gle plane, if <strong>the</strong> follow<strong>in</strong>g preconditions are met: firstly, <strong>the</strong> strike of <strong>the</strong><br />
potential slid<strong>in</strong>g plane must be approximately parallel to <strong>the</strong> rock face (maximum deviation of about 30°).<br />
Secondly, <strong>the</strong> dip angle of <strong>the</strong> potential slid<strong>in</strong>g plane must be lower than <strong>the</strong> dip of <strong>the</strong> rock face and, thirdly,<br />
7
<strong>the</strong> dip angle of <strong>the</strong> slid<strong>in</strong>g plane must be higher than <strong>the</strong> angle of friction on <strong>the</strong> slid<strong>in</strong>g surface, s<strong>in</strong>ce friction<br />
works aga<strong>in</strong>st slid<strong>in</strong>g.<br />
Us<strong>in</strong>g stereographic projection, this test<strong>in</strong>g method, first published by Markland (1972), is a simple technique<br />
that can be employed to ascerta<strong>in</strong>, if <strong>the</strong> potential slid<strong>in</strong>g plane complies with <strong>the</strong> requirements mentioned<br />
above. This test dist<strong>in</strong>guishes between discont<strong>in</strong>uities which daylight <strong>in</strong>to free space and those which do not<br />
daylight. For a certa<strong>in</strong> slope orientation, a region can be def<strong>in</strong>ed as bounded by <strong>the</strong> daylight<strong>in</strong>g envelope<br />
shown as a small circle <strong>in</strong> <strong>the</strong> stereonet. If <strong>the</strong> pole of a discont<strong>in</strong>uity is situated with<strong>in</strong> this region it daylights. It<br />
is, <strong>the</strong>n, k<strong>in</strong>ematically prone to s<strong>in</strong>gle plane failure if it daylights and at <strong>the</strong> same time lies outside <strong>the</strong> friction<br />
cone represented by <strong>the</strong> <strong>in</strong>ner circle <strong>in</strong> <strong>the</strong> stereonet (grey-shaded region). Likewise <strong>the</strong> k<strong>in</strong>ematic possibility of<br />
wedge slid<strong>in</strong>g or toppl<strong>in</strong>g can be tested us<strong>in</strong>g <strong>the</strong> stereonet.<br />
Estimation of friction angle<br />
The friction angle along discont<strong>in</strong>uities was estimated by simple tilt tests <strong>in</strong> <strong>the</strong> field, performed <strong>in</strong> <strong>the</strong> former<br />
quarry near Dürnste<strong>in</strong>. Tilt tests are a common method used <strong>in</strong> geotechnical eng<strong>in</strong>eer<strong>in</strong>g, to <strong>in</strong>vestigate <strong>the</strong><br />
basic friction angle of rock jo<strong>in</strong>ts (Cawsey and Farrar 1976 or Bruce, Cruden and Eaton 1989). Two pieces of<br />
rock conta<strong>in</strong><strong>in</strong>g a discont<strong>in</strong>uity are held <strong>in</strong> hand with <strong>the</strong> discont<strong>in</strong>uity horizontal. The sample is slowly tilted<br />
until <strong>the</strong> top block moves. The angle with <strong>the</strong> horizontal at onset of movement is <strong>the</strong> so-called tilt-angle. The<br />
tilt-angle equals <strong>the</strong> material friction of <strong>the</strong> discont<strong>in</strong>uity wall (φ) plus <strong>the</strong> roughness angle (i), if no real<br />
cohesion is present (i.e. no cement<strong>in</strong>g or glu<strong>in</strong>g material between <strong>the</strong> two blocks), no <strong>in</strong>fill material is present,<br />
<strong>the</strong> asperities do not break, and <strong>the</strong> walls of <strong>the</strong> discont<strong>in</strong>uity are completely fitt<strong>in</strong>g at <strong>the</strong> start of <strong>the</strong> test (tiltangle<br />
= φwall material + i). If <strong>the</strong> walls of <strong>the</strong> discont<strong>in</strong>uity are completely non-fitt<strong>in</strong>g, <strong>the</strong> tilt-angle equals <strong>the</strong><br />
friction of <strong>the</strong> material of <strong>the</strong> discont<strong>in</strong>uity walls (tilt-angle = φwall material). If cementation or glu<strong>in</strong>g material is<br />
present or asperities break, <strong>the</strong> tilt-angle represents a comb<strong>in</strong>ation of <strong>the</strong> (apparent or real) cohesion and <strong>the</strong><br />
friction along <strong>the</strong> discont<strong>in</strong>uity. If <strong>in</strong>fill material is present, <strong>the</strong> tilt-angle is governed partially or completely by<br />
<strong>the</strong> <strong>in</strong>fill, depend<strong>in</strong>g on <strong>the</strong> thickness of <strong>the</strong> <strong>in</strong>fill and height of asperities (Hoek and Bray 1974).<br />
In <strong>the</strong> case of Dürnste<strong>in</strong>, open discont<strong>in</strong>uities without <strong>in</strong>fill<strong>in</strong>gs and hard side walls are present (cf. subchapter<br />
discont<strong>in</strong>uity characteristics). The tests were performed with non-fitt<strong>in</strong>g walls, so that obta<strong>in</strong>ed tilt-angles are<br />
equal to <strong>the</strong> basic friction angle along discont<strong>in</strong>uities.<br />
Monitor<strong>in</strong>g system<br />
In both quarries, monitor<strong>in</strong>g systems <strong>in</strong>clud<strong>in</strong>g fissurometers measur<strong>in</strong>g <strong>the</strong> width of cracks of potential failure<br />
planes and geophones register<strong>in</strong>g ground vibrations, were <strong>in</strong>stalled to observe <strong>the</strong> rock face. Moreover 3D<br />
prism targets measured by a laser <strong>the</strong>odolite were <strong>in</strong>stalled at both locations provid<strong>in</strong>g a redundant measur<strong>in</strong>g<br />
tool.<br />
S<strong>in</strong>ce rock-<strong>mass</strong> <strong>falls</strong> <strong>in</strong> Spitz as well as <strong>in</strong> Dürnste<strong>in</strong> have occurred especially after lengthy periods of heavy ra<strong>in</strong><br />
and historical evidence (Austrian Federal Railways unpubl.) also strongly suggests that heavy ra<strong>in</strong>fall acts as a<br />
trigger for <strong>the</strong>se rock-<strong>mass</strong> <strong>falls</strong>, additionally, a pluviometer (tipp<strong>in</strong>g bucket ra<strong>in</strong> gauge, MICRO STEP-MIS MR2)<br />
was <strong>in</strong>cluded <strong>in</strong> <strong>the</strong> monitor<strong>in</strong>g system of <strong>the</strong> rock face <strong>in</strong> <strong>the</strong> former quarry at Dürnste<strong>in</strong>.<br />
At Spitz <strong>the</strong> system was <strong>in</strong>stalled by <strong>the</strong> geological service of Lower Austria as a consequence of <strong>the</strong> 2002<br />
rockslide/rock-<strong>mass</strong> fall and was used as a permanent monitor<strong>in</strong>g device, until it was replaced by an automatic<br />
3D survey system (cf. Fig. 2 and subchapter Preventive Measures fur<strong>the</strong>r below).<br />
At Dürnste<strong>in</strong>, <strong>the</strong> monitor<strong>in</strong>g system was configured as a temporary system to ensure safe work<strong>in</strong>g conditions<br />
dur<strong>in</strong>g restor<strong>in</strong>g traffic <strong>in</strong>frastructure. The system was <strong>in</strong>stalled <strong>in</strong> September 2009 and ma<strong>in</strong>ta<strong>in</strong>ed until<br />
November 2010 by a system operator, who provided an onl<strong>in</strong>e data service. In total seven fissurometers and<br />
four geophones were <strong>in</strong>stalled. Two geophones were <strong>in</strong>stalled on both sides of two major discont<strong>in</strong>uities<br />
observed by fissurometers number three and number two. The exact positions of <strong>the</strong> measur<strong>in</strong>g devices <strong>in</strong> <strong>the</strong><br />
rock face at Dürnste<strong>in</strong> are shown <strong>in</strong> Fig. 8.<br />
8
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
The rock face of <strong>the</strong> former quarry near Dürnste<strong>in</strong> is composed of Gföhl gneiss, a f<strong>in</strong>e-gra<strong>in</strong>ed gneiss of granitic<br />
composition, which constitutes a typical rock type of <strong>the</strong> Moldanubian gneiss complex. The rock texture is<br />
characterised by schlieren, bumpy fold<strong>in</strong>g and a knobbly structure, which gives <strong>the</strong> gneiss a ra<strong>the</strong>r migmatitic<br />
appearance (Fuchs and Matura 1980). Partly slab- or lens-shaped <strong>in</strong>clusions of amphibolite with a diameter of<br />
several decimetres appear with<strong>in</strong> <strong>the</strong> gneiss. These bodies are accumulated <strong>in</strong> several horizons; <strong>the</strong>ir<br />
longitud<strong>in</strong>al axes are orientated parallel to <strong>the</strong> well-developed foliation of <strong>the</strong> gneiss. Coarse gra<strong>in</strong>ed,<br />
pegmatitic layers (muscovite) occur rarely. In some areas, <strong>the</strong> gneiss is closely folded. The strik<strong>in</strong>g directions of<br />
<strong>the</strong> fold axes are orientated NNE-SSW.<br />
Based on field tests accord<strong>in</strong>g to EN ISO 14689-1:2003 (ON 2004), <strong>the</strong> uniaxial compressive strength (UCS) of<br />
<strong>the</strong> rocks <strong>in</strong> Spitz and Dürnste<strong>in</strong>, respectively, can be estimated as “very high”, which means UCS is <strong>in</strong> a range<br />
of 100-250 MPa.<br />
Discont<strong>in</strong>uity orientation<br />
In both former quarries, <strong>the</strong> system of discont<strong>in</strong>uities is typified by well-developed bedd<strong>in</strong>g and foliation planes<br />
as well as steeply dipp<strong>in</strong>g jo<strong>in</strong>ts and slickensides. In <strong>the</strong> metamorphic rocks of Spitz, sedimentary structures<br />
such as bedd<strong>in</strong>g planes are well preserved. In contrast to <strong>the</strong> W-dipp<strong>in</strong>g foliation planes <strong>in</strong> Dürnste<strong>in</strong>, <strong>the</strong> strike<br />
of bedd<strong>in</strong>g planes <strong>in</strong> <strong>the</strong> Spitz marbles is orientated N-S to NNE-SSW (subparallel to <strong>the</strong> <strong>Danube</strong> River) and dip E<br />
to ESE, <strong>in</strong>cl<strong>in</strong>ed by 28-45°(mean: 35-40°) (Fig. 9a). The thickness of s<strong>in</strong>gle marble beds is up to 10 m; <strong>the</strong><br />
bedd<strong>in</strong>g planes are not planar, but uneven and wavy with syncl<strong>in</strong>als plung<strong>in</strong>g towards <strong>the</strong> <strong>Danube</strong> River.<br />
The foliation planes with<strong>in</strong> <strong>the</strong> Gföhl gneiss <strong>in</strong> Dürnste<strong>in</strong> and <strong>the</strong> jo<strong>in</strong>ts and slickensides orientated parallel are<br />
strik<strong>in</strong>g N-S, and dip to <strong>the</strong> W at a mean angle of 40° (Fig. 9b). The rock face of <strong>the</strong> former quarry is also<br />
exposed towards W (towards <strong>the</strong> <strong>Danube</strong> River), <strong>the</strong>refore <strong>the</strong>se discont<strong>in</strong>uities strike parallel to <strong>the</strong> rock face<br />
but dip at a lower angle.<br />
Apart from bedd<strong>in</strong>g and foliation planes dipp<strong>in</strong>g out of <strong>the</strong> rock face, at least two approximately perpendicular<br />
sets of steep or even vertical discont<strong>in</strong>uities can be found <strong>in</strong> both locations. The Spitz marbles are cut by ESE-<br />
WNW strik<strong>in</strong>g subvertical fault planes, accompanied by jo<strong>in</strong>ts perpendicular to <strong>the</strong> bedd<strong>in</strong>g planes.<br />
Fur<strong>the</strong>rmore <strong>the</strong>re are two steeply WNW- and WSW-dipp<strong>in</strong>g and NNE-SSW and NNW-SSW strik<strong>in</strong>g<br />
discont<strong>in</strong>uity sets (Fig. 10 a, b). In Dürnste<strong>in</strong>, NE-SW to NNE-SSW strik<strong>in</strong>g and very steeply NW/WNW or ESE<br />
dipp<strong>in</strong>g sets of jo<strong>in</strong>ts and fault planes (slickensides) and on <strong>the</strong> o<strong>the</strong>r hand NW-SE strik<strong>in</strong>g, very steeply NE or<br />
SW dipp<strong>in</strong>g jo<strong>in</strong>ts and fault planes (slickensides) represent <strong>the</strong> most dist<strong>in</strong>ct discont<strong>in</strong>uity sets. There are two<br />
subsets allocated to <strong>the</strong> two ma<strong>in</strong> sets, both strik<strong>in</strong>g 30° rotated counter clockwise. Both ma<strong>in</strong> sets represent a<br />
conjugate system, reflect<strong>in</strong>g a large scale tectonic pattern, <strong>in</strong> which <strong>the</strong> NE-SW strik<strong>in</strong>g discont<strong>in</strong>uities are<br />
oriented parallel to <strong>the</strong> Diendorf fault, whereas <strong>the</strong> o<strong>the</strong>r set forms an obtuse-angled strike. For this reason <strong>the</strong><br />
system of discont<strong>in</strong>uities <strong>in</strong> Dürnste<strong>in</strong> (Fig. 11 a, b) can be related to <strong>the</strong> tectonics of <strong>the</strong> Diendorf fault stress<br />
field (Scheidegger 1976).<br />
Discont<strong>in</strong>uity characteristics<br />
The ma<strong>in</strong> discont<strong>in</strong>uity characteristics <strong>in</strong> Spitz as well as <strong>in</strong> Dürnste<strong>in</strong> are similar. The persistency of <strong>the</strong><br />
discont<strong>in</strong>uities is very high. Bedd<strong>in</strong>g planes (Spitz) and foliation planes (Dürnste<strong>in</strong>) can be tracked over several<br />
metres, tens of metres, and even up to 100 m. The surfaces of <strong>the</strong> discont<strong>in</strong>uities are mostly wavy, with<br />
amplitudes <strong>in</strong> a range of decimetres (bedd<strong>in</strong>g planes), partly planar (fault planes) and range from smooth to<br />
polished. The discont<strong>in</strong>uities are mostly wide open <strong>in</strong> a range of centimetres to decimetres. The walls are<br />
partly coated with limonite but dry. No <strong>in</strong>filligs were observed. Bedd<strong>in</strong>g planes <strong>in</strong> Spitz are mostly closed and<br />
<strong>the</strong> beds well-bonded. However, <strong>the</strong>re are also bedd<strong>in</strong>g planes show<strong>in</strong>g an aperture <strong>in</strong> <strong>the</strong> range of<br />
centimetres disconnect<strong>in</strong>g beds. The aperture of steeply dipp<strong>in</strong>g jo<strong>in</strong>ts and fault planes is also <strong>in</strong> a range of<br />
centimetres, rarely of decimetres <strong>in</strong> Spitz, whereas <strong>in</strong> Dürnste<strong>in</strong> jo<strong>in</strong>ts and fault planes are generally wide open<br />
<strong>in</strong> a magnitude of several centimetres and up to more than 10 cm. In this case, <strong>the</strong> wide open<strong>in</strong>g of jo<strong>in</strong>ts is <strong>the</strong><br />
result of a major blast carried out <strong>in</strong> 1909, which caused an additional loosen<strong>in</strong>g of <strong>the</strong> rock <strong>mass</strong> (cf. follow<strong>in</strong>g<br />
chapter).<br />
The spac<strong>in</strong>g of <strong>the</strong> discont<strong>in</strong>uities (shortest distance between two discont<strong>in</strong>uities of <strong>the</strong> same set) is <strong>in</strong> a range<br />
between decimetres to metres, which applies to all sets of discont<strong>in</strong>uities <strong>in</strong> both locations. The blocks<br />
result<strong>in</strong>g from <strong>the</strong> fractur<strong>in</strong>g of <strong>the</strong> rock <strong>mass</strong> ow<strong>in</strong>g to <strong>the</strong> discont<strong>in</strong>uities are, for <strong>the</strong> most part, of cubical or<br />
rhombic shape with edge lengths of 0.6 to > two metres.<br />
10
Fig. 9 (a) Orientation of bedd<strong>in</strong>g planes <strong>in</strong> Spitz. (b) Orientation of foliation planes <strong>in</strong> Dürnste<strong>in</strong>. (stereographic<br />
projection of poles and major planes, equal angle overlay, lower hemisphere)<br />
11
Fig. 10 (a) Orientation of jo<strong>in</strong>ts and slickensides <strong>in</strong> Spitz. (stereographic projection of poles and major planes,<br />
equal angle overlay, lower hemisphere) (b) Kluftrose.<br />
12
Fig. 11 (a) Orientation of jo<strong>in</strong>ts and slickensides <strong>in</strong> Dürnste<strong>in</strong>. (stereographic projection of poles and major<br />
planes, equal angle overlay, lower hemisphere) (b) Kluftrose.<br />
13
Results<br />
Rock mechanical failure analysis<br />
Rock mechanical failure analysis performed on <strong>the</strong> rockslides/rock-<strong>mass</strong> <strong>falls</strong> of Spitz and Dürnste<strong>in</strong> clearly<br />
show, that block slid<strong>in</strong>g along failure planes, dipp<strong>in</strong>g out of <strong>the</strong> slope is k<strong>in</strong>ematically possible and very likely.<br />
Figure 12a shows <strong>the</strong> results of <strong>the</strong> Markland test for <strong>the</strong> bedd<strong>in</strong>g planes <strong>in</strong> Spitz and Fig. 12b for <strong>the</strong> foliation<br />
planes <strong>in</strong> Dürnste<strong>in</strong>. In <strong>the</strong> diagram, poles of s<strong>in</strong>gle discont<strong>in</strong>uities are represented by white squares, <strong>the</strong> major<br />
planes are marked with a black spot. The <strong>in</strong>ner circle represents a friction angle along discont<strong>in</strong>uities of 35°.<br />
S<strong>in</strong>ce <strong>the</strong> major bedd<strong>in</strong>g planes <strong>in</strong> Spitz and <strong>the</strong> foliation planes <strong>in</strong> Dürnste<strong>in</strong> lie with<strong>in</strong> <strong>the</strong> grey shaded region,<br />
s<strong>in</strong>gle plane slid<strong>in</strong>g is k<strong>in</strong>ematically possible <strong>in</strong> both cases. A higher friction angle represented by a larger <strong>in</strong>ner<br />
circle <strong>in</strong> <strong>the</strong> stereonet, thus reduc<strong>in</strong>g <strong>the</strong> grey-shaded region, would reduce <strong>the</strong> number of poles with<strong>in</strong> <strong>the</strong><br />
grey shaded region, which means, <strong>in</strong> practice, that <strong>the</strong> number of potential slid<strong>in</strong>g planes that are k<strong>in</strong>ematically<br />
free to slide would be decreased. With a friction angle of 40° for example, <strong>the</strong> major plane (black spot) borders<br />
exactly on <strong>the</strong> shaded region, <strong>in</strong>dicat<strong>in</strong>g a critical state of equilibrium. In practical terms, planes dipp<strong>in</strong>g 40° out<br />
of <strong>the</strong> rock face and undercut by man-made morphology ow<strong>in</strong>g to m<strong>in</strong><strong>in</strong>g are <strong>in</strong> a state of critical equilibrium.<br />
Planes dipp<strong>in</strong>g at a lower angle of e.g. 30° can be expected to be stable due to friction. Lower friction angles<br />
(e.g. due to wet slid<strong>in</strong>g surfaces) represented by smaller <strong>in</strong>ner circles <strong>in</strong> <strong>the</strong> stereonet, lead to an enlargement<br />
of <strong>the</strong> grey-shaded region, which signifies an <strong>in</strong>creas<strong>in</strong>g number of potential slid<strong>in</strong>g planes. Slid<strong>in</strong>g blocks are<br />
term<strong>in</strong>ated ei<strong>the</strong>r by slope morphology, or by two sets of steeply dipp<strong>in</strong>g jo<strong>in</strong>ts.<br />
Fig. 12 (a) Markland test for s<strong>in</strong>gle plane slid<strong>in</strong>g at Spitz. (b) Markland test for s<strong>in</strong>gle plane slid<strong>in</strong>g at Dürnste<strong>in</strong>.<br />
(equal angle overlay, lower hemisphere, friction angle along discont<strong>in</strong>uities 35°)<br />
Regard<strong>in</strong>g <strong>the</strong> potential of wedge failure, <strong>the</strong> majority of <strong>the</strong> analysed <strong>in</strong>tersection l<strong>in</strong>es of <strong>the</strong> present<br />
discont<strong>in</strong>uity sets at Spitz as well as at Dürnste<strong>in</strong> are ei<strong>the</strong>r too steep, or too flat, so that wedge slid<strong>in</strong>g is<br />
unlikely <strong>in</strong> both cases. Similar results were achieved concern<strong>in</strong>g toppl<strong>in</strong>g failure.<br />
The basic friction angle estimated from tilt tests at Dürnste<strong>in</strong> is <strong>in</strong> <strong>the</strong> magnitude of 35-40° for dry, flat and<br />
smooth slickensides with limonitic coat<strong>in</strong>g without any <strong>in</strong>fill<strong>in</strong>gs. Tests under wet conditions showed that<br />
friction angles decreased at a magnitude of five degrees. In <strong>the</strong> case of <strong>the</strong> former quarry at Spitz, similar<br />
conditions can be assumed.<br />
At Spitz, block slid<strong>in</strong>g is fur<strong>the</strong>r favoured by sheet silicates sandwiched between <strong>the</strong> marble layers. A 0.5 m<br />
thick layer of biotite schist formed <strong>the</strong> slid<strong>in</strong>g plane for a 15 m thick marble complex dur<strong>in</strong>g <strong>the</strong> rockslide <strong>in</strong><br />
2002. While fissured and partly karstified marbles dra<strong>in</strong> very fast, <strong>the</strong> mica-rich layers function as an aquiclude.<br />
Penetrat<strong>in</strong>g water softens <strong>the</strong> rock and <strong>the</strong> friction angle decreases with <strong>in</strong>creas<strong>in</strong>g water content.<br />
In his back analysis of <strong>the</strong> 2002 rockslide/rock-<strong>mass</strong> fall, Wagner (2006, unpubl.) <strong>in</strong>vestigated <strong>the</strong> relationship<br />
between <strong>the</strong> failure surface´s shear strength and layer thickness accord<strong>in</strong>g to Barton (1971). Assum<strong>in</strong>g a<br />
14
strength of eight MPa, and a friction angle of 25° for <strong>the</strong> biotite schists and jo<strong>in</strong>t roughness coefficients (JRC) of<br />
five to 10 respectively, <strong>the</strong> effective friction angles for a 20 m thick marble complex are <strong>in</strong> <strong>the</strong> range 30.8° -<br />
36.7°. With <strong>in</strong>creas<strong>in</strong>g layer thickness, <strong>the</strong> effective friction angle dim<strong>in</strong>ishes as does <strong>the</strong> <strong>in</strong>fluence of cohesion<br />
on <strong>the</strong> safety coefficient. As a consequence, all major rockslides <strong>in</strong> <strong>the</strong> former quarry of Spitz were limited to<br />
marble layers thicker than 10 m and underlaid by biotite schists. Future rockslide events are most likely to be<br />
tied to this failure model.<br />
Results from monitor<strong>in</strong>g system<br />
In <strong>the</strong> former quarry at Spitz no measurable movements were observed s<strong>in</strong>ce <strong>the</strong> implementation of <strong>the</strong><br />
monitor<strong>in</strong>g system.<br />
At <strong>the</strong> Dürnste<strong>in</strong> quarry, movements were observed <strong>in</strong> six out of seven fissurometers. The total deformation<br />
rates <strong>in</strong> five fissurometers were < five mm and <strong>the</strong>refore of m<strong>in</strong>or geotechnical relevance. Larger deformations,<br />
however were observed at fissurometer number three, <strong>in</strong>stalled at <strong>the</strong> foot of a 1000 m 3 wedge-shaped rock,<br />
slid<strong>in</strong>g on a s<strong>in</strong>gle plane <strong>in</strong> <strong>the</strong> central part of <strong>the</strong> rock face (Fig. 4), especially follow<strong>in</strong>g heavy ra<strong>in</strong><strong>falls</strong>. The daily<br />
volumes of ra<strong>in</strong>fall recorded aga<strong>in</strong>st movements at fissurometer number three, <strong>in</strong> <strong>the</strong> time between<br />
September 2009 and October 2010, are shown <strong>in</strong> <strong>the</strong> follow<strong>in</strong>g diagram (Fig. 13).<br />
Fig. 13 Movement measured by fissurometer number 3 versus ra<strong>in</strong>fall (daily sums), time span: September 2009-<br />
October 2010.<br />
The largest rate of deformation was measured <strong>in</strong> <strong>the</strong> period from <strong>the</strong> middle of April until <strong>the</strong> end of August<br />
2010, exactly <strong>the</strong> same period when <strong>the</strong> most ra<strong>in</strong>fall was logged. The diagram shows <strong>in</strong> great detail that s<strong>in</strong>gle<br />
heavy ra<strong>in</strong><strong>falls</strong> with daily amounts of > 10 mm (as <strong>in</strong> April 2009) led to a sudden <strong>in</strong>crease <strong>in</strong> <strong>the</strong> deformation<br />
curve with a time delay of approximately two days. In total, <strong>the</strong> period of accelerated movement lasted for<br />
approximately 14 days. With<strong>in</strong> this time, <strong>the</strong> curve flattened to a hyperbolic decl<strong>in</strong>e, after <strong>the</strong> first steep ascent.<br />
However, an obvious analogy between frequent frost alternat<strong>in</strong>g with thaw (e.g. <strong>in</strong> <strong>the</strong> period between<br />
December 2009 and March 2010) cannot be drawn from <strong>the</strong> diagram <strong>in</strong> Fig. 14, <strong>in</strong> which movement of<br />
fissurometer number three is plotted aga<strong>in</strong>st temperature.<br />
Results from DTM comparison<br />
By <strong>in</strong>tersect<strong>in</strong>g two digital terra<strong>in</strong> models before and after <strong>the</strong> rock-<strong>mass</strong> fall of 2009, <strong>the</strong> total volume of <strong>the</strong><br />
slid<strong>in</strong>g <strong>mass</strong> could be estimated at 13 000 +/- 2000 m³ (Fig. 15). The <strong>in</strong>accuracy <strong>in</strong> this volume calculation can<br />
be expla<strong>in</strong>ed by <strong>the</strong> follow<strong>in</strong>g facts: In <strong>the</strong> DTM provided by <strong>the</strong> federal government of Lower Austria,<br />
overhangs were not considered. M<strong>in</strong>or rock<strong>falls</strong> could have occurred <strong>in</strong> <strong>the</strong> time span between <strong>the</strong> acquisition<br />
of ALS data and <strong>the</strong> 2009 rock-<strong>mass</strong> fall. Fur<strong>the</strong>r on <strong>the</strong> loosen<strong>in</strong>g factor between <strong>in</strong>tact rock and debris could<br />
not be considered sufficiently.<br />
15
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
Discussion<br />
Interpretation of failure analysis<br />
While GIS-based methods (Meentemeyer and Moody 2000 or Gün<strong>the</strong>r, Carstensen and Pohl 2002) are a useful<br />
tool for mapp<strong>in</strong>g conformities between topographic and geological surfaces at a regional scale, a geotechnical<br />
approach us<strong>in</strong>g <strong>the</strong> stereonet was necessary <strong>in</strong> <strong>the</strong>se projects, where detailed analyses were required for <strong>the</strong><br />
design of local preventive measures. The used method on <strong>the</strong> one hand allows to process not only jo<strong>in</strong>t<br />
orientation data, but also considers jo<strong>in</strong>t characteristics, and on <strong>the</strong> o<strong>the</strong>r hand enables a probabilistic<br />
k<strong>in</strong>ematical failure analysis for all three major failure modes (block slid<strong>in</strong>g, wedge slid<strong>in</strong>g and toppl<strong>in</strong>g).<br />
The results of <strong>the</strong> failure analysis can be <strong>in</strong>terpreted <strong>in</strong> a way that block slid<strong>in</strong>g is <strong>the</strong> major failure mode, s<strong>in</strong>ce<br />
wedge slid<strong>in</strong>g and toppl<strong>in</strong>g are unlikely. As shown <strong>in</strong> Fig. 12, <strong>the</strong> variation <strong>in</strong> <strong>the</strong> dip angle of potential failure<br />
planes is high. Practically this means that an <strong>in</strong>crease of <strong>the</strong> basic friction angle does not guarantees stable<br />
conditions, though <strong>the</strong> number of potential failure planes is reduced (cf. subchapter results of failure analysis).<br />
Interpretation of monitor<strong>in</strong>g data and trigger of failure<br />
The monitor<strong>in</strong>g data illustrate that heavy ra<strong>in</strong><strong>falls</strong> triggered <strong>the</strong> failure of <strong>the</strong> 2009 rock-<strong>mass</strong> fall near<br />
Dürnste<strong>in</strong>. It is worth discuss<strong>in</strong>g how ra<strong>in</strong> <strong>in</strong>fluenced <strong>the</strong> slope equilibrium and what led to failure. Ei<strong>the</strong>r water<br />
reduced friction along <strong>the</strong> slid<strong>in</strong>g planes, or a jo<strong>in</strong>t water pressure was built up, or both. We suggest that, <strong>in</strong><br />
this case, jo<strong>in</strong>t water pressure was of m<strong>in</strong>or relevance for trigger<strong>in</strong>g rock-<strong>mass</strong> <strong>falls</strong>, s<strong>in</strong>ce <strong>the</strong> very widely open<br />
jo<strong>in</strong>ts dra<strong>in</strong> very fast. Two observations support this <strong>the</strong>sis: On <strong>the</strong> one hand no water-leakages along open<br />
jo<strong>in</strong>ts were observed after ra<strong>in</strong><strong>falls</strong> and on <strong>the</strong> o<strong>the</strong>r hand, <strong>in</strong>creas<strong>in</strong>g movement rates occurred with a delay of<br />
days.<br />
The fact, that no correlation between frost and deformation was observed is also worth discuss<strong>in</strong>g. Assum<strong>in</strong>g<br />
that congelifraction can only take effect <strong>in</strong> completely water-filled cracks, <strong>the</strong> high aperture of jo<strong>in</strong>ts comb<strong>in</strong>ed<br />
with large block sizes could be an explanation. In <strong>the</strong> studied areas congelifraction might be an important<br />
trigger for small-scale rock<strong>falls</strong> (cf. Krähenbühl 2004), but not for rock-<strong>mass</strong> <strong>falls</strong> and rockslides.<br />
The proximity of <strong>the</strong> study area to <strong>the</strong> Diendorf fault suggests, that seismic activities could play a role <strong>in</strong><br />
trigger<strong>in</strong>g rock-<strong>mass</strong> <strong>falls</strong>. A correlation of seismic data from <strong>the</strong> study area (ZAMG unpubl.) with known rock<strong>mass</strong><br />
<strong>falls</strong> with<strong>in</strong> <strong>the</strong> last century however did not show any l<strong>in</strong>k between earthquakes and <strong>mass</strong> movements.<br />
Accord<strong>in</strong>g to Lenhardt (2007) <strong>the</strong> probability for trigger<strong>in</strong>g landslides by ground vibrations <strong>in</strong>duced by seismic<br />
activity is generally low <strong>in</strong> <strong>the</strong> study area.<br />
Preventive Measures<br />
Preventive measures to protect <strong>the</strong> local <strong>in</strong>frastructure from fur<strong>the</strong>r possible rock-<strong>mass</strong> <strong>falls</strong> are difficult to<br />
accomplish, both <strong>in</strong> Spitz as well as <strong>in</strong> Dürnste<strong>in</strong>, as nei<strong>the</strong>r <strong>the</strong> discont<strong>in</strong>uity situation, which is a given, nor <strong>the</strong><br />
artificial modification of <strong>the</strong> slope geometry can be changed or undone.<br />
A long-term stable equilibrium can only be achieved by remov<strong>in</strong>g all potentially unstable <strong>falls</strong> and by cutt<strong>in</strong>g<br />
<strong>the</strong> rock back extensively, which, however, means fur<strong>the</strong>r, possibly harmful <strong>in</strong>terference to slope morphology<br />
and landscape by man. S<strong>in</strong>ce <strong>the</strong> implementation of <strong>the</strong>se measures is a long-term project and also dependent<br />
on <strong>the</strong> political, economic and environmental situation, passive protection measures (protection dams) <strong>in</strong><br />
comb<strong>in</strong>ation with monitor<strong>in</strong>g systems were carried out. A similar approach was already applied to <strong>the</strong><br />
150 000 m 3 Eiblschrofen rock-<strong>mass</strong> fall (Scheikl et al. 2000; Roth et al. 2002).<br />
In <strong>the</strong> former quarry at Spitz, a 130 m long earth dam, runn<strong>in</strong>g parallel to <strong>the</strong> railway l<strong>in</strong>e, was built <strong>in</strong> 2004 (Fig.<br />
9). The energy-absorb<strong>in</strong>g capacity of <strong>the</strong> dam, however, is limited. Major comb<strong>in</strong>ed rockslides/rock-<strong>mass</strong> <strong>falls</strong><br />
could easily exceed its energy-absorb<strong>in</strong>g capacity. In order to protect <strong>the</strong> <strong>in</strong>frastructure aga<strong>in</strong>st such events, a<br />
rockfall barrier with an <strong>in</strong>tegrated warn<strong>in</strong>g system was <strong>in</strong>stalled on <strong>the</strong> crest of <strong>the</strong> dam <strong>in</strong> 2006. Any<br />
deformation of <strong>the</strong> lightly supported posts triggers an alarm system, activat<strong>in</strong>g red traffic lights on <strong>the</strong> railway<br />
track as well as on <strong>the</strong> ma<strong>in</strong> road and send<strong>in</strong>g an alarm signal to <strong>the</strong> authorities of <strong>the</strong> federal government of<br />
Lower Austria via SMS. In addition, an automatic 3D survey system has been used s<strong>in</strong>ce 2008 to identify<br />
movements <strong>in</strong> potentially unstable regions at an early stage. There are 37 prisms <strong>in</strong>stalled <strong>in</strong> <strong>the</strong> rock face that<br />
are automatically measured by a permanently mounted laser <strong>the</strong>odolite at an <strong>in</strong>terval of two hours. After a test<br />
phase of one year, parameters for alarm trigger<strong>in</strong>g were def<strong>in</strong>ed and implemented <strong>in</strong> <strong>the</strong> alarm system. It has<br />
17
not yet proved possible to f<strong>in</strong>d <strong>the</strong> f<strong>in</strong>ances necessary to fund an exist<strong>in</strong>g, well-def<strong>in</strong>ed remedial concept based<br />
on <strong>the</strong> cutback of a total volume of 369 000 m³, to be realised by open m<strong>in</strong><strong>in</strong>g over a period of several years.<br />
The concept <strong>in</strong> question would provide for a total cutback of all bedd<strong>in</strong>g planes which had been undercut by<br />
quarry<strong>in</strong>g. The resultant catacl<strong>in</strong>al slopes above and laterally from <strong>the</strong> former quarry would extend <strong>the</strong><br />
longitud<strong>in</strong>al surface area of m<strong>in</strong><strong>in</strong>g considerably, divid<strong>in</strong>g <strong>the</strong> whole valley flank <strong>in</strong>to berms and quarry faces.<br />
In Dürnste<strong>in</strong> <strong>the</strong> slope geometry between <strong>the</strong> foot of <strong>the</strong> rock face and <strong>the</strong> railway track was redesigned, by<br />
construct<strong>in</strong>g an eight metre high protection dam us<strong>in</strong>g debris from <strong>the</strong> failed rock <strong>mass</strong>. It was thus possible to<br />
construct a new reservoir to accommodate fur<strong>the</strong>r rock-<strong>mass</strong> <strong>falls</strong> comb<strong>in</strong>ed with a 150 metre long and up to<br />
eight metre high protection dam without hav<strong>in</strong>g to transport any construction material to or from <strong>the</strong> site. The<br />
protective effect of <strong>the</strong> dam was improved considerably by redesign<strong>in</strong>g <strong>the</strong> slope geometry <strong>in</strong> <strong>the</strong> transport<br />
and deposit area of possible future rock-<strong>mass</strong> <strong>falls</strong> and creat<strong>in</strong>g an absorption bench at <strong>the</strong> foot of <strong>the</strong> rock<br />
face. Additionally, a rockfall protection kit was <strong>in</strong>stalled (Fig. 16).<br />
Fig. 16 Geomorphological situation <strong>in</strong> <strong>the</strong> former quarry at Dürnste<strong>in</strong>.<br />
This was not enough, however, as major failure of certa<strong>in</strong> larger parts of <strong>the</strong> rock face, especially <strong>in</strong> <strong>the</strong> upper<br />
sections, could still result <strong>in</strong> partial damage to <strong>the</strong> dam. Moreover smaller blocks could bounce over <strong>in</strong> this case.<br />
Therefore <strong>the</strong>se potentially hazardous parts of <strong>the</strong> rock face were monitored. At <strong>the</strong> same time, a long-term<br />
remedial concept has been devised for cutt<strong>in</strong>g back <strong>the</strong>se parts of <strong>the</strong> rock face by blast<strong>in</strong>g. This concept was<br />
implemented <strong>in</strong> summer 2011, remov<strong>in</strong>g a total volume of more than 5000 m³.<br />
18
Conclusions<br />
There are two causes of natural and anthropogenic orig<strong>in</strong> for <strong>the</strong> rock-<strong>mass</strong> fall and rockslide events <strong>in</strong> Spitz<br />
and Dürnste<strong>in</strong>. The unfavourable orientation of discont<strong>in</strong>uities is natural given, whereas <strong>the</strong> steep slope<br />
morphology and loosen<strong>in</strong>g of <strong>the</strong> rock <strong>mass</strong> was caused by human m<strong>in</strong><strong>in</strong>g activities. The comb<strong>in</strong>ation of both<br />
factors led to <strong>the</strong> undercutt<strong>in</strong>g of <strong>the</strong> beds, pos<strong>in</strong>g a situation, where block failure is potentially possible from a<br />
k<strong>in</strong>ematic po<strong>in</strong>t of view and <strong>the</strong> slope is <strong>in</strong> a critical state of equilibrium. This allowed heavy ra<strong>in</strong>fall to trigger<br />
rock-<strong>mass</strong> <strong>falls</strong>. The need for rapid restoration of endangered transport routes required extensive protective<br />
measures.<br />
Acknowledgements<br />
We would like to thank <strong>the</strong> Austrian Railways, who allowed us to use unpublished data, Michael Bertagnoli<br />
(Geological Service of Lower Austria) for fruitful discussions, Wolfgang A. Lenhardt (Seismological Service of<br />
Austria) for provid<strong>in</strong>g actual seismic data and Klaus Legat (AVT) for provid<strong>in</strong>g DEM <strong>in</strong> figure 15b. Fur<strong>the</strong>rmore<br />
<strong>the</strong> comments of Andreas Kellerer-Pirklbauer, Ján Vlcko and an anonymous reviewer, which helped to improve<br />
<strong>the</strong> scientific quality of <strong>the</strong> paper are gratefully acknowledged. Markus Wies<strong>in</strong>ger improved <strong>the</strong> English<br />
manuscript.<br />
Authors<br />
Hans Jörg Laimer, Austrian Federal Railways (ÖBB), Infrastruktur AG, SBM,<br />
Weiserstraße 9, A-5020 Salzburg, Austria<br />
Mart<strong>in</strong> Müllegger<br />
iC consulenten Ziviltechniker GesmbH,<br />
Zollhausweg 1, A-5101 Bergheim, Austria<br />
Email: m.muellegger@ic-group.org<br />
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