Structurally and climatically-controlled progressive destabilisation of a multi-unit carbonate rockslide with marly interlayers over two decades (Hornbergl, Tyrol/Austria).

Rockslides in intercalated carbonate rocks are among the most common and hazardous rock slide types in the European Alps and other mountains worldwide. Progressive shear plane development in marly or clayey interlayered limestones is an issue of debate, especially since the disastrous 200 million m³ carbonate rockslide in Vajont in 1963. Since interlayers are often not persistently developed, the processes controlling the progressive shear plane evolution in carbonate rockslides affecting acceleration and deceleration phases are poorly understood. This paper discusses 20 years of progressive shear plane development in a 10 million m³ multi-unit rockslide with marly interlayers (Hornbergl, Austria). We present a two-decade kinematic record, complemented with field mapping, surface geodetical observations, subsurface geophysical investigations, rock and soil mechanical laboratory testing, a snow-referenced infiltration model, as well as a resulting discontinuum mechanical model. Monthly tape extensometer measurements (3,300 individual measurements) from 1996 to 2016, mapping and geodetic investigations reveal mean absolute displacement rates of 0.2–14.5 mm/month between multiple units of the translational rockslide, often separated by >20 m deep and meter-wide open fractures. Electrical resistivity tomography (ERT) uncovers the geological and structural setting of the upper 40 m of the rockslide’s inner structure. Laboratory analysis shows that full (100%) saturation of marly interlayers decreases the cohesion from 65 to 21 kPa in comparison to 50% saturation, presumably explaining seasonal velocity variations. Correspondingly, infiltration models including snow accumulation and melt appear to control movement patterns in some years after snow melt and after heavy convective rainfalls. Discontinuum mechanical models based on mapping, geophysical reconnaissance and mechanical laboratory testing show that the multi-unit rockslide is at the fringe of stability with 50% water saturation in marly interlayers and that strain rates rapidly accelerate with the cohesion loss upon their full water-saturation matching with the described kinematic record. Over multiple years, compression and decompression waves between rockslide units become a first order control for progressive rockslide development. Here we show in a 20-year multi-unit rockslide record, that infiltration after snowmelt and extreme precipitation may control monthly and seasonal patterns of carbonate rockslide deformation, but compression and decompression waves may become key control on a multiannual time scale for progressive destabilisation. [ABSTRACT FROM AUTHOR]