Engineering geologists and geotechnical engineers often have to deal with problems of stability of natural or man-made slopes built up by jointed and weathered rock mass. This paper deals with the stability conditions of an important rhyolite cliff, the stability of which has endangered human life, a state road and a brewery below it. Several methods used for the investigation and for the stability assessment of the cliff are described which had to take into account the complex rock mass conditions. Methods of terrestrial photogrammetry have been used in the investigation. They enable monitoring of slope deformations and they have contributed to the selection of effective, economically acceptable mitigation methods. After assessing all investigation results effective and economically profitable remedial measures have been chosen.
The rhyolite cliff is located in the village of Vyhne, on the boundary of Štiavnické vrchy Mountains, in Central Slovakia (Figure 1). The cliff is about 75 metres long and about 23 to 29 metres high (Figure 2). It is formed by neovolcanic rocks of Central Slovakia, originating as large rock bodies, of frequently different volcanic structure. Stratovolcanic structures typically prevail. The volcanic activity was limited to Badenian and Sarmatian periods (Tertiary-Neogene), related to dissection and disintegration of the volcanic bodies into individual structural blocks. During the end of the Sarmatian to Lower Pannonian periods (Neogene), the rhyolite volcanic activities were renewed. Channels of the rhyolite lava were formed along the well developed fault systems. The volcanic body represents an extrusion of a laccolith type penetrating the older Sarmatian rocks, characterised by a high viscosity and explosive behaviour. From the view point of lithology it is a rock type of rhyolite with secondary feldspar. The rhyolite was the subject to extensive denudation in the Pannonian period.