Engineering-geological characterisation and activity analysis of a deep-seated rockslide near Laatsch (South Tyrol)

Deep-seated rockslides in Alpine areas are common phenomena, especially if geological and tectonic conditions enable a disintegration of the rock mass extending deep into the ground. Furthermore, the failure process usually is controlled by groundwater flow, permafrost degradation and rock weathering mostly by input of surface water along geological discontinuities as well as by temperature fluctuations. Thereby, extensive slope areas can become unstable and – in the worst case – can endanger population and infrastructure.

At the valley entrance of the Münstertal at the stream Rambach (South Tyrol, Italy), close to the national road SS41 ca. road kilometres 6.5, a deep-seated rockslide was formed at a south-facing mountain slope with a gradient of ca. 30 - 50°. The U-shaped valley was formed by glaciers, whereby the valley floor is filled with alluvial sediments. The rockslide is approx. 400 m wide, measures approx.  700 m in height at its longest extension and comprise a total rock volume of approx.  500,000 m³. The geological bedrock consists of foliated metamorphic rocks (mainly orthogneisses) which partially is covered by talus and glacial sediments. In the past and still continuing, the area was exposed to major tectonic stress due to its close range to the Vinschgau and Schlinig fault zones generating a dense fracture system in the rock mass.

Since several years, the highly active rockslide shows displacements of several metres per year. In 2014, the road SS41 was relocated over a length of ca. 800 m to the other side of the Rambach due to ongoing rock fall events. Field surveys conducted at that time already showed clear geomorphological indications for the destabilization of a large area at the mountain ridge by the presence of primary and secondary scarps, tension cracks, and up-hill facing scarps in the slope area ranging up to the mountain ridge.

Geological field studies in 2018 and 2019 were carried out to investigate the rockslide geometry and kinematics as well as deformation and failure processes. Quantification of the deformation rates was carried out by multi-temporal terrestrial laser scanning (TLS). From a kinematic point of view, the rockslide can be divided into different slabs of varying activity showing actual deformation rates between approx. 0.3 to 3.6 m per year. The individual slabs show a translational movement behaviour with minor internal deformation. However, also a rotational kinematics along polygonal slip surfaces was observed. Disintegration and formation of slabs mostly takes place along pre-existing steeply dipping joint surfaces.

In this contribution, a preliminary geological, geometrical and kinematical model of the current rockslide is presented by the detailed analyses of field mapping and deformation monitoring data.