mass falls in the Wachau-Danube Valley (Bohemian Massif

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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