1

Geomorphological and geotechnical causes of anthropogenic-induced rock-

mass falls in the Wachau-Danube Valley (Bohemian Massif, Lower Austria)

Hans J. Laimer1 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 rock-

mass falls occurred in 2002 (Spitz, Fig. 2) and in 2009 (Dürnstein, Fig. 3).

Fig. 2 Rock-mass fall near the village of Spitz (view towards WNW, photo: Müllegger, May 2010).

30 m

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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

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.

Danube river

Dürnstein

40 m

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Fig. 4 Geological-tectonic map of the Wachau-Danube Valley (Matura 1983, modified) and study areas near

Spitz (1) and Dürnstein (2).

Geological and tectonic overview

From a tectonic point of view, the crystalline basement of the Wachau region is part of the mainly NE-SW

trending gneiss massifs of the Moldanubian Complex, which is a major tectonic unit of the Bohemian Massif,

representing Austria´s share of the European Variscides (Fuchs and Matura 1980). The Moldanubian zone is

further divided into several thrusts, of which the Gföhl and the Raabs units are exposed in the study area,

according to the latest tectonic classification by Matura (2003 and 2006).

The high temperature/high pressure metamorphic Gföhl unit is generally composed of migmatitic granite type

gneisses and granulite in its south-eastern part and is belted by amphibolites. The Raabs unit contains mostly

para- and orthogneisses, as well as different alkaline meta-magmatites. The marble and the Spitz calc silicate

gneiss-formation had previously been attributed to the Drosendorf unit (Fuchs and Matura 1980 and Schnabel

et al. 2002). Recently, however, this unit has been included in the Raabs unit. The former Drosendorf unit

(more recently Drosendorf formation) has been subsumed under the Bíteš unit, which forms the uppermost

tectonic unit of the Moravo-Silesian nappe complex. Since Matura (2003) defines the border between the Spitz

marbles in the hanging wall and the granodiorite gneiss in the foot wall as the contact between the Moravian

and the Moldanubian zones, this formation is of major tectonic significance.

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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 worst-

ever 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,

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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 (tilt-

angle = 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.

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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.

13

Fig. 11 (a) Orientation of joints and slickensides in Dürnstein. (stereographic projection of poles and major

planes, equal angle overlay, lower hemisphere) (b) Kluftrose.

14

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

15

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 m3 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.

16

Fig. 14 Movement measured by fissurometer number 3 versus temperature (daily 05:00 a.m.), time span:

September 2009-October 2010.

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

x = E y = N z = vertical axis

b a

+5 m

+10 m

+5 m

17

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 rock-

mass 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 m3 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

18

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.

19

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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