mass falls in the Wachau-Danube Valley (Bohemian Massif

Geomorphological and geotechnical causes of anthropogenic-induced rockmass
falls in the Wachau-Danube Valley (Bohemian Massif, Lower Austria)
Hans J. Laimer 1 and Martin Müllegger 2
1 Austrian Federal Railways (ÖBB), Infrastruktur AG, Salzburg, Austria
2 iC consulenten Ziviltechniker GesmbH, Bergheim, Austria
Laimer, H.J. and Müllegger, M., 2012. Geomorphological and geotechnical causes of anthropogenic-induced rock-mass falls
in the Wachau-Danube Valley (Bohemian Massif, Lower Austria). Geografiska Annaler Series A Volume 94, Issue 1, p.157-
174. doi:10.1111/j.1468-0459.2012.00451.x
ABSTRACT
The Wachau-Danube Valley represents a transverse valley, intersecting the Variscian Bohemian Massif.
Weakened rocks along fault structures led to accelerated river erosion, forming relatively steep rock slopes.
The exceptional cultural position of the region generated an increasing demand for building materials. Over the
centuries quarrying had a sizeable impact on slope morphology. Interdependences between quarrying and
construction caused unstable rock slopes and four rock-mass falls have occurred at two quarries near Spitz
(1961, 1984, 2002) and Dürnstein (2009). Rock mechanical analysis at these quarries has shown that the
combination of existing geological discontinuities and artificially modified morphology is fatal in terms of slope
stability. In Spitz the bedding planes within the marble had been undercut by the mining face. Additionally, two
conjugated, steeply dipping joint sets formed large scale blocks sliding on bedding planes. In three major
rockslides/rock-mass falls, each triggered by heavy rainfalls, a total mass of 170 000 m³ of rock failed. At the
quarry near Dürnstein the geotechnical characteristics of the gneiss are also unfavourable in relation to the
exposition of the mining face. After several rockfalls, 65 000 m³ were blasted away in 1909 to remove unstable
rock slopes. The residual rock face was destabilized and rockfall activities culminated in an event with a total
volume of approximately 15 000 m³. Remedial measures for both locations are essential to maintain transport
infrastructure. Sufficiently stable conditions can only be achieved by extensive reshaping of the mining faces,
which involves adapting slope geometries to naturally stable joint faces.
Key words: Bohemian Massif, rock-mass falls, rock mechanics, slope geometry, rock engineering, protective
measures
Introduction
In contrast to alpine regions, where rock slope failures are frequent, e.g. in the steep side walls of U-shaped
valleys formed by glacial erosion in the Central Alps (Abele 1974), present day large-scale rockfalls in the slopes
of the Bohemian Massif are rare. Known events of the last decades were man induced and took place in former
quarries as described below. In this paper the term rock-mass fall will be used to describe failures of large
bodies of material at very steep, mostly undercut slopes (cf. Selby 1993). Rock-mass falls differ from fragmental
rockfall (falls of single blocks) in frequency and volume of the moving material, while the term rockslide is used
for a different type of rock-slope failure (moving of a rock mass along a sliding plane).
The former quarries of Dürnstein and Spitz are located in the so-called Wachau Cultural Landscape, stretching
from Melk to Krems along the Danube Valley, approximately 80 km west of the City of Vienna. This UNESCO
protected region represents a typical transverse valley, intersecting the southernmost part of the Variscan
Bohemian Massif. Archaeological evidence such as the "Venus of Willendorf” shows that this region has been
inhabited by man since the Upper Palaeolithic (20 000-30 000 BP). Hence, this section of the Danube Valley can
be considered one of Austria´s oldest cultural landscapes. Until the Early Middle Ages, the region was primarily
relevant as a traffic route. The main building period started in the tenth century under Bavarian rule (Lechner
1983). The exceptional cultural position of the Wachau led to an ever-increasing demand for natural resources
to support large scale building activities (religious and secular buildings together with the characteristic wine
terraces). Gneiss and marble were mined in small quarries, which even then had minor effects on slope
morphology. Quarrying peaked between the sixteenth and nineteenth centuries as a consequence of transport
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infrastructure construction (roads, railways), which eventually had a dual effect on slope morphology: on the
one hand, existing quarries were expanded and/or reactivated and, on the other hand, traffic infrastructure
construction itself had an enormous impact on the area. With the advent of railway engineering, further
difficulties were encountered especially at sites with working conditions comparable to alpine construction
sites, as the planned routes led along steep rock slopes over long distances. Route construction often required
high slope cuts or subtle engineering structures such as tunnels, bridges and retaining walls.
Fig. 1 The extent of the Bohemian Massif in Austria. The box refers to the wider study area as shown in Fig. 4
Interdependences between quarrying and transport infrastructure construction eventually resulted in the
development of unstable slopes. Besides several documented events within the 20th century the latest rockmass
falls occurred in 2002 (Spitz, Fig. 2) and in 2009 (Dürnstein, Fig. 3).
30 m
Fig. 2 Rock-mass fall near the village of Spitz (view towards WNW, photo: Müllegger, May 2010).
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Fig. 3 Rock-mass fall near the town of Dürnstein (view towards SSE, photo: Müllegger, August 2009).
Contrary to many alpine rock-mass falls in uninhabited areas (cf. Weidinger and Vortisch 2005) these two
events in abandoned quarries caused major damage and still endanger transport infrastructure, which thus
made geo-scientific investigations and protective measures necessary. Aim of this paper is to comment on
geomorphological and geotechnical causes of these rockslides/rock-mass falls.
Study area
Geomorphic overview
40 m
Between Passau and Krems, the Danube River runs along the southern edge of the Bohemian Massif, most of
which is overlaid by Tertiary and Quaternary sediments. The exposed edge of the hilly fault residual plateau
takes the form of a 500-600 m high piedmont stairway. Where higher mountain ridges reach the plateau’s
margin, the river cuts down into the granites and gneisses of the crystalline basement, and carves out
transverse valleys, such as the Wachau-Danube Valley (Kohl 1966). The morphogenesis of this river section was
last analysed by Nagl and Verginis (1987), whose research focused on the palaeogeographic development of
the catchment area. They suppose that the valley between Melk and Spitz (cf. Figs 1 and 4)was drained by the
Enns River during the Tertiary, while the Danube River at that time run further north. In consequence of Late
Tertiary tectonic processes the Danube River was captured by the Wachau-Enns River.
The geomorphological appearance is characterised by the fact that the orientation of the river bed follows two
conjugated major tectonic lineaments, striking approximately NE-SW and NW-SE. Wide basin and terraced
landscapes are therefore non-existent in contrast to adjacent areas with Tertiary and Quaternary cover
sediments. The tectonically fragmented and weakened rock mass along fault structures caused accelerated
river erosion forming V-shaped valleys with relatively steep rock slopes and triggering large rockslides during
the Oligocene. Matura (1983 and 1989) mapped a fossil slide mass deposited in the Wachau-Danube Valley
near the village of Weißenkirchen (Fig. 4). He also interprets deposits of gneiss boulders in Weißenkirchen as
rockslide sediments of the same age.
In the present day, the rock slopes of the Bohemian Massif are usually considered to be stable. Gattinger (1980)
assumes that mass movements are of minor relevance to the Bohemian Massif, owing to its relatively small
relief ratio, and are limited to single rockfalls or local slope ruptures.
3
Dürnstein
Danube river

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.
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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.
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Fig. 5 Geomorphological situation in the former quarry at Spitz. The square shows the ruin of a service block.
Be that as it may, the railroad track alignment as well as the provincial road were again completely buried, with
the debris cone even reaching the Danube River (Fig. 6b). Despite the blast, another rockfall happened close to
the northern portal of the Dürnstein tunnel following a period of heavy rainfall in September 1909. Two people
were killed and six severely injured by this event, which also buried workers’ barracks (regional newspaper
article, Niederösterreichische Presse Nr. 28, 18-Sept-1909).
Fig. 6 (a) Rock face at the former quarry near Dürnstein before and (b) after the blast of 1909. The height of the
rock wall is about 130 m (view towards E, photo: Mayreder, May 1909).
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There is not much information on further rock-mass fall events over the following decades. However, a
comparison of photographs taken in 1909 and 100 years later, just before the rock-mass fall in 2009, clearly
indicates that further rockfalls must have occurred in the time between. There is only one report of a rockfall,
which took place in the winter of 2002/2003, after an unusually rainy autumn. The latest rock-mass fall
occurred in July 2009, yet again preceded by a period of heavy rainfall. The rail infrastructure was destroyed by
this event, and some smaller boulders even reached the main road B3 “Donauuferstraße” (Fig. 7).
Fig. 7 Damage to rail infrastructure in 2009 (view towards NNW, photo: Laimer, July 2009).
Methods
Failure model analysis
To determine the mode of failure, which caused the rock-mass falls/rock slides, a kinematic failure analysis was
performed, based upon the principals of “block theory” (Goodman and Shi 1985).
The application of this method is practicable for both sites (Spitz and Dürnstein), since the exceeding of rock
mass strength can be excluded as a cause of failure, because of the fact, that in both cases, the uniaxial
compressive strength of the rock mass is by far higher than the overburden stress (subchapter Lithology).
Therefore, the rock mass can be seen as a system of rigid blocks separated by discontinuities. Large scale
displacements within the rock mass can only occur if blocks are moved relative to each other along distinct
shear planes, an effect known as “block failure” in geotechnical engineering (Hoek and Bray 1974). For the
analysis, discontinuity orientation data acquired by detailed geotechnical mapping of the rock face were used
and put in relation to the spatial orientation of the rock face, using stereographic projection. The main failure
modes block sliding (sliding of a block on a single plane), wedge sliding (sliding of a block on two planes in a
direction along the line of intersection) and toppling (rotational failure of thin columns caused by
discontinuities striking +/- parallel and dipping steeply contrary to the rock face) were examined, using the
computer program dips ® (ROCSCIENCE).
Kinematically, a block can slide on a single plane, if the following preconditions are met: firstly, the strike of the
potential sliding plane must be approximately parallel to the rock face (maximum deviation of about 30°).
Secondly, the dip angle of the potential sliding plane must be lower than the dip of the rock face and, thirdly,
7

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

The rock face of the former quarry near Dürnstein is composed of Gföhl gneiss, a fine-grained gneiss of granitic
composition, which constitutes a typical rock type of the Moldanubian gneiss complex. The rock texture is
characterised by schlieren, bumpy folding and a knobbly structure, which gives the gneiss a rather migmatitic
appearance (Fuchs and Matura 1980). Partly slab- or lens-shaped inclusions of amphibolite with a diameter of
several decimetres appear within the gneiss. These bodies are accumulated in several horizons; their
longitudinal axes are orientated parallel to the well-developed foliation of the gneiss. Coarse grained,
pegmatitic layers (muscovite) occur rarely. In some areas, the gneiss is closely folded. The striking directions of
the fold axes are orientated NNE-SSW.
Based on field tests according to EN ISO 14689-1:2003 (ON 2004), the uniaxial compressive strength (UCS) of
the rocks in Spitz and Dürnstein, respectively, can be estimated as “very high”, which means UCS is in a range
of 100-250 MPa.
Discontinuity orientation
In both former quarries, the system of discontinuities is typified by well-developed bedding and foliation planes
as well as steeply dipping joints and slickensides. In the metamorphic rocks of Spitz, sedimentary structures
such as bedding planes are well preserved. In contrast to the W-dipping foliation planes in Dürnstein, the strike
of bedding planes in the Spitz marbles is orientated N-S to NNE-SSW (subparallel to the Danube River) and dip E
to ESE, inclined by 28-45°(mean: 35-40°) (Fig. 9a). The thickness of single marble beds is up to 10 m; the
bedding planes are not planar, but uneven and wavy with synclinals plunging towards the Danube River.
The foliation planes within the Gföhl gneiss in Dürnstein and the joints and slickensides orientated parallel are
striking N-S, and dip to the W at a mean angle of 40° (Fig. 9b). The rock face of the former quarry is also
exposed towards W (towards the Danube River), therefore these discontinuities strike parallel to the rock face
but dip at a lower angle.
Apart from bedding and foliation planes dipping out of the rock face, at least two approximately perpendicular
sets of steep or even vertical discontinuities can be found in both locations. The Spitz marbles are cut by ESE-
WNW striking subvertical fault planes, accompanied by joints perpendicular to the bedding planes.
Furthermore there are two steeply WNW- and WSW-dipping and NNE-SSW and NNW-SSW striking
discontinuity sets (Fig. 10 a, b). In Dürnstein, NE-SW to NNE-SSW striking and very steeply NW/WNW or ESE
dipping sets of joints and fault planes (slickensides) and on the other hand NW-SE striking, very steeply NE or
SW dipping joints and fault planes (slickensides) represent the most distinct discontinuity sets. There are two
subsets allocated to the two main sets, both striking 30° rotated counter clockwise. Both main sets represent a
conjugate system, reflecting a large scale tectonic pattern, in which the NE-SW striking discontinuities are
oriented parallel to the Diendorf fault, whereas the other set forms an obtuse-angled strike. For this reason the
system of discontinuities in Dürnstein (Fig. 11 a, b) can be related to the tectonics of the Diendorf fault stress
field (Scheidegger 1976).
Discontinuity characteristics
The main discontinuity characteristics in Spitz as well as in Dürnstein are similar. The persistency of the
discontinuities is very high. Bedding planes (Spitz) and foliation planes (Dürnstein) can be tracked over several
metres, tens of metres, and even up to 100 m. The surfaces of the discontinuities are mostly wavy, with
amplitudes in a range of decimetres (bedding planes), partly planar (fault planes) and range from smooth to
polished. The discontinuities are mostly wide open in a range of centimetres to decimetres. The walls are
partly coated with limonite but dry. No infilligs were observed. Bedding planes in Spitz are mostly closed and
the beds well-bonded. However, there are also bedding planes showing an aperture in the range of
centimetres disconnecting beds. The aperture of steeply dipping joints and fault planes is also in a range of
centimetres, rarely of decimetres in Spitz, whereas in Dürnstein joints and fault planes are generally wide open
in a magnitude of several centimetres and up to more than 10 cm. In this case, the wide opening of joints is the
result of a major blast carried out in 1909, which caused an additional loosening of the rock mass (cf. following
chapter).
The spacing of the discontinuities (shortest distance between two discontinuities of the same set) is in a range
between decimetres to metres, which applies to all sets of discontinuities in both locations. The blocks
resulting from the fracturing of the rock mass owing to the discontinuities are, for the most part, of cubical or
rhombic shape with edge lengths of 0.6 to > two metres.
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Fig. 9 (a) Orientation of bedding planes in Spitz. (b) Orientation of foliation planes in Dürnstein. (stereographic
projection of poles and major planes, equal angle overlay, lower hemisphere)
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Fig. 10 (a) Orientation of joints and slickensides in Spitz. (stereographic projection of poles and major planes,
equal angle overlay, lower hemisphere) (b) Kluftrose.
12

Fig. 11 (a) Orientation of joints and slickensides in Dürnstein. (stereographic projection of poles and major
planes, equal angle overlay, lower hemisphere) (b) Kluftrose.
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Results
Rock mechanical failure analysis
Rock mechanical failure analysis performed on the rockslides/rock-mass falls of Spitz and Dürnstein clearly
show, that block sliding along failure planes, dipping out of the slope is kinematically possible and very likely.
Figure 12a shows the results of the Markland test for the bedding planes in Spitz and Fig. 12b for the foliation
planes in Dürnstein. In the diagram, poles of single discontinuities are represented by white squares, the major
planes are marked with a black spot. The inner circle represents a friction angle along discontinuities of 35°.
Since the major bedding planes in Spitz and the foliation planes in Dürnstein lie within the grey shaded region,
single plane sliding is kinematically possible in both cases. A higher friction angle represented by a larger inner
circle in the stereonet, thus reducing the grey-shaded region, would reduce the number of poles within the
grey shaded region, which means, in practice, that the number of potential sliding planes that are kinematically
free to slide would be decreased. With a friction angle of 40° for example, the major plane (black spot) borders
exactly on the shaded region, indicating a critical state of equilibrium. In practical terms, planes dipping 40° out
of the rock face and undercut by man-made morphology owing to mining are in a state of critical equilibrium.
Planes dipping at a lower angle of e.g. 30° can be expected to be stable due to friction. Lower friction angles
(e.g. due to wet sliding surfaces) represented by smaller inner circles in the stereonet, lead to an enlargement
of the grey-shaded region, which signifies an increasing number of potential sliding planes. Sliding blocks are
terminated either by slope morphology, or by two sets of steeply dipping joints.
Fig. 12 (a) Markland test for single plane sliding at Spitz. (b) Markland test for single plane sliding at Dürnstein.
(equal angle overlay, lower hemisphere, friction angle along discontinuities 35°)
Regarding the potential of wedge failure, the majority of the analysed intersection lines of the present
discontinuity sets at Spitz as well as at Dürnstein are either too steep, or too flat, so that wedge sliding is
unlikely in both cases. Similar results were achieved concerning toppling failure.
The basic friction angle estimated from tilt tests at Dürnstein is in the magnitude of 35-40° for dry, flat and
smooth slickensides with limonitic coating without any infillings. Tests under wet conditions showed that
friction angles decreased at a magnitude of five degrees. In the case of the former quarry at Spitz, similar
conditions can be assumed.
At Spitz, block sliding is further favoured by sheet silicates sandwiched between the marble layers. A 0.5 m
thick layer of biotite schist formed the sliding plane for a 15 m thick marble complex during the rockslide in
2002. While fissured and partly karstified marbles drain very fast, the mica-rich layers function as an aquiclude.
Penetrating water softens the rock and the friction angle decreases with increasing water content.
In his back analysis of the 2002 rockslide/rock-mass fall, Wagner (2006, unpubl.) investigated the relationship
between the failure surface´s shear strength and layer thickness according to Barton (1971). Assuming a
14

strength of eight MPa, and a friction angle of 25° for the biotite schists and joint roughness coefficients (JRC) of
five to 10 respectively, the effective friction angles for a 20 m thick marble complex are in the range 30.8° -
36.7°. With increasing layer thickness, the effective friction angle diminishes as does the influence of cohesion
on the safety coefficient. As a consequence, all major rockslides in the former quarry of Spitz were limited to
marble layers thicker than 10 m and underlaid by biotite schists. Future rockslide events are most likely to be
tied to this failure model.
Results from monitoring system
In the former quarry at Spitz no measurable movements were observed since the implementation of the
monitoring system.
At the Dürnstein quarry, movements were observed in six out of seven fissurometers. The total deformation
rates in five fissurometers were < five mm and therefore of minor geotechnical relevance. Larger deformations,
however were observed at fissurometer number three, installed at the foot of a 1000 m 3 wedge-shaped rock,
sliding on a single plane in the central part of the rock face (Fig. 4), especially following heavy rainfalls. The daily
volumes of rainfall recorded against movements at fissurometer number three, in the time between
September 2009 and October 2010, are shown in the following diagram (Fig. 13).
Fig. 13 Movement measured by fissurometer number 3 versus rainfall (daily sums), time span: September 2009-
October 2010.
The largest rate of deformation was measured in the period from the middle of April until the end of August
2010, exactly the same period when the most rainfall was logged. The diagram shows in great detail that single
heavy rainfalls with daily amounts of > 10 mm (as in April 2009) led to a sudden increase in the deformation
curve with a time delay of approximately two days. In total, the period of accelerated movement lasted for
approximately 14 days. Within this time, the curve flattened to a hyperbolic decline, after the first steep ascent.
However, an obvious analogy between frequent frost alternating with thaw (e.g. in the period between
December 2009 and March 2010) cannot be drawn from the diagram in Fig. 14, in which movement of
fissurometer number three is plotted against temperature.
Results from DTM comparison
By intersecting two digital terrain models before and after the rock-mass fall of 2009, the total volume of the
sliding mass could be estimated at 13 000 +/- 2000 m³ (Fig. 15). The inaccuracy in this volume calculation can
be explained by the following facts: In the DTM provided by the federal government of Lower Austria,
overhangs were not considered. Minor rockfalls could have occurred in the time span between the acquisition
of ALS data and the 2009 rock-mass fall. Further on the loosening factor between intact rock and debris could
not be considered sufficiently.
15

Fig. 14 Movement measured by fissurometer number 3 versus temperature (daily 05:00 a.m.), time span:
September 2009-October 2010.
a
x = E
y = N
z = vertical axis
b
Fig. 15 Morphometric evaluation of rock-mass fall deposits by means of the intersection of digital terrain
models. (a) Perspective view before (black) and after (grey) rock-mass fall event. (b) Differential DEM (top view)
showing surface with surface loss (black) and surface gain (white) including +5 m and +10 m isolines.
Insert
16
+10 m
+5 m
+5 m

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

not yet proved possible to find the finances necessary to fund an existing, well-defined remedial concept based
on the cutback of a total volume of 369 000 m³, to be realised by open mining over a period of several years.
The concept in question would provide for a total cutback of all bedding planes which had been undercut by
quarrying. The resultant cataclinal slopes above and laterally from the former quarry would extend the
longitudinal surface area of mining considerably, dividing the whole valley flank into berms and quarry faces.
In Dürnstein the slope geometry between the foot of the rock face and the railway track was redesigned, by
constructing an eight metre high protection dam using debris from the failed rock mass. It was thus possible to
construct a new reservoir to accommodate further rock-mass falls combined with a 150 metre long and up to
eight metre high protection dam without having to transport any construction material to or from the site. The
protective effect of the dam was improved considerably by redesigning the slope geometry in the transport
and deposit area of possible future rock-mass falls and creating an absorption bench at the foot of the rock
face. Additionally, a rockfall protection kit was installed (Fig. 16).
Fig. 16 Geomorphological situation in the former quarry at Dürnstein.
This was not enough, however, as major failure of certain larger parts of the rock face, especially in the upper
sections, could still result in partial damage to the dam. Moreover smaller blocks could bounce over in this case.
Therefore these potentially hazardous parts of the rock face were monitored. At the same time, a long-term
remedial concept has been devised for cutting back these parts of the rock face by blasting. This concept was
implemented in summer 2011, removing a total volume of more than 5000 m³.
18

Conclusions
There are two causes of natural and anthropogenic origin for the rock-mass fall and rockslide events in Spitz
and Dürnstein. The unfavourable orientation of discontinuities is natural given, whereas the steep slope
morphology and loosening of the rock mass was caused by human mining activities. The combination of both
factors led to the undercutting of the beds, posing a situation, where block failure is potentially possible from a
kinematic point of view and the slope is in a critical state of equilibrium. This allowed heavy rainfall to trigger
rock-mass falls. The need for rapid restoration of endangered transport routes required extensive protective
measures.
Acknowledgements
We would like to thank the Austrian Railways, who allowed us to use unpublished data, Michael Bertagnoli
(Geological Service of Lower Austria) for fruitful discussions, Wolfgang A. Lenhardt (Seismological Service of
Austria) for providing actual seismic data and Klaus Legat (AVT) for providing DEM in figure 15b. Furthermore
the comments of Andreas Kellerer-Pirklbauer, Ján Vlcko and an anonymous reviewer, which helped to improve
the scientific quality of the paper are gratefully acknowledged. Markus Wiesinger improved the English
manuscript.
Authors
Hans Jörg Laimer, Austrian Federal Railways (ÖBB), Infrastruktur AG, SBM,
Weiserstraße 9, A-5020 Salzburg, Austria
Martin Müllegger
iC consulenten Ziviltechniker GesmbH,
Zollhausweg 1, A-5101 Bergheim, Austria
Email: m.muellegger@ic-group.org
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