Pletzachkogel (Austria): Landslides caused by “Hard on Soft” geomechanics

Pletzachkogel is a mountain North of the Tyrolean Inn valley in a prominent position above the village of Kramsach. Three major rock avalanches have occurred since the last glaciation and ongoing rockfall from the head scarp area presently affects local infrastructure. This study presents geological and geomechanical investigations of Pletzachkogel that were performed using conventional and uncrewed aerial vehicle (UAV) mapping techniques and focused on understanding the different landslide processes. The investigation indicates that the gravitational instability is related to a stratigraphic sequence of “Hard on Soft” rock units: Hauptdolomit and Plattenkalk beds (hard) – Kössen Formation (soft) – Oberrhätkalk and Lower Jurassic Breccia units (hard). “Hard on Soft” slope deformation has been described for similar stratigraphic conditions and involves the overlying hard rock mass breaking apart over a soft underlying unit. This leads to distinct geomorphological features like buttresses. In addition to the “Hard on Soft” stratigraphy, synsedimentary deformation during deposition, polyphase, heteroaxial, tectonic deformation during Alpine orogenesis, and glacial loading and unloading during the last ice age exacerbated landslide processes. This study describes the “Hard on Soft” geomechanics of Pletzachkogel and also demonstrates the value of UAV‐based structural mapping, particularly for acquiring and quantifying structural rock mass data in inaccessible areas.