Deformation processes and failure analysis of a deep-seated rockslide near Laatsch, South Tyrol

The investigated deep-seated rock slide is located at the valley entrance of the Münstertal in South Tyrol directing to the Swiss border. The area is attributed to the Sesvenna Crystalline of the S-charl crystalline nappe, which is mainly formed by poly-metamorphic orthogneisses with intercalations of amphibolites, phyllites, paragneisses and marbles. The rockslide extends approx. 400 m in NE-SW-direction and spans 650 m from 1450 m to the main scarp at. 2015 m a.s.l. The SE-facing slope shows a main slope inclination of approx. 35° (min. 20°, max. 80°). A total rockslide volume could be estimated at approx. 3 to 4 Million m³ by means of GIS.

The rock mass shows a flat into the slope dipping foliation (mean dip angle approx. 15°) and is highly fractured by two orthogonally orientated sets of brittle joints (set 1 and set 2). Kinematic analysis suggests direct toppling for fracture set 1 and flexural toppling for fracture set 2. Geological mapping and laboratory analysis via thin section and XRD analysis identified Muscovite-rich shearing planes and phyllonite rock types in the area of the main scarp. Weathering progresses along scarps and developed tension cracks further eroding and dissembling the rock mass.

The activation of the movement occurred in the year 2000, showing a rapid expansion since the year 2012 causing a relocation of the road underneath in 2014. Multi-temporal deformation analysis based on orthoimages, ALS and TLS were able to show high velocities of at least 9 m per month during the initial formation phase in 2014, followed by a continuous velocity reduction to mean annual values of 1 to 2.5 m per year until spring 2022. In the period spring to autumn 2022 no more movements could be detected via TLS, which raises the question of the causal reasons for the movement and the different velocities of movement. The absence of significant precipitation in spring and summer 2022 can be interpreted as a probable cause, since also previous movement velocities showed a correlation with the respective amount of precipitation.

Rock fall and rock topple events with a dimension of several thousand m³ could also be observed along outbreak recesses at the rockslide flanks, scarps and at the internal slab margins and also be detected through several TLS measurement series.

Results indicate an internal slab formation along discrete shear zones recognizable on surface as main and minor scarps. The slabs show a translational movement behaviour along a fully persistent, slightly curvilinear basal shear zone. The reason for the destabilization of the valley flank is attributed to retrogressive processes caused by long-term stress release due to topographical and hydrogeological changes by adjacent, previous rockslides situated directly below the active rockslide.