Geomorphological and geotechnical causes of anthropogenically induced rock‐mass falls in the wachau‐danube valley (bohemian massif, lower austria)

Abstract

Laimer, H.J. and M¨llegger, M., 2012. Geomorphological and geotechnical causes of anthropogenically induced rockmass falls in theWachau‐Danube Valley (Bohemian Massif, Lower Austria). Geografiska Annaler: Series A, Physical Geography, 94, 157–174. doi:10.1111/j.1468‐0459.2012.00451.xABSTRACTThe 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 170-000m3 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, 65-000m3 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 15-000m3. 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.