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Understanding landslide behavior over medium and long timescales is crucial for predicting landslide hazard and constructing accurate landscape evolution models. The behavior of landslides in soil that undergo periodic displacements, termed earthflows or compound soil slides, is especially difficult to forecast at these timescales. This is because velocities can increase by orders of magnitude over annual to decadal timescales, due to processes such as changing recharge conditions, erosion of the landslide toe, and retrogression of the landslide head. In this paper, we provide a detailed analysis of the Schlucher landslide, an unusual earthflow that is perched above the village of Malbun, Liechtenstein. This landslide had been displacing by 10 to 20 cm/year until 2015, when displacements on the order of 2 m/year occurred from 2016 to 2018. These large displacements damaged landslide mitigation measures, caused numerous surface deformation features, and threatened the local population downstream of the earthflow. This landslide has an unusually long monitoring record, with accurate displacement and climatic data available since 1983. We analyze this nearly 40-year monitoring time series to estimate recharge from snowmelt and rainfall, and its correlation with displacement. We also analyze recently collected, high-resolution surface and subsurface data in order to understand landslide response to recharge, landslide kinematics through time, and catastrophic failure potential. We find that interannual displacements can be explained with variations in recharge; however, periodic surges with recurrence times of tens of years must be explained by other mechanisms. In particular, recharge into the landslide during the recent acceleration (2016 to 2018) was not anomalously high. Instead, we argue that loss of internal strength is responsible for this recent acceleration period, and that this mechanism should be considered when forecasting the surge potential for certain earthflows and soil slides. [ABSTRACT FROM AUTHOR]

Extremely-rapid, flowlike landslides can impact people and property far from their source, and predicting the initiation and runout of these events remains a challenging task. The ongoing risk analysis of the Schlucher Landslide, located near the town of Malbun, Liechtenstein, highlights the uncertainties associated with this type of analysis. This landslide, composed of clayey sand and clayey gravel, has accumulated 10s of meters of displacement in the last 3 years. Managing the risk posed by this unstable colluvial mass requires addressing the following questions: Can the Schlucher Landslide transition into an extremely-rapid flowlike landslide? If such a transition were to occur, what would be the consequence for the community located at the outflow of the Schlucher Catchment?Addressing these questions requires an understanding of potential mechanisms that can lead to catastrophic failure of the unstable mass. A number of mechanisms, including static and dynamic liquefaction, loss of true and/or apparent cohesion and shearing to residual strength have been well documented in literature, and can be identified by routine geotechnical analyses. The focus of this work is on macroscopic brittleness, a poorly understood mechanism that may have governed the failure of a number of well documented case histories that were composed of fine grained colluvium, similar to the Schlucher Landslide. Here we present detailed field observations and 3D numerical runout modelling of three extremely-rapid, flowlike landslides that occurred in fine grained colluvium: The 2014 Oso Flowslide, the 2012 Johnsons Landing Landslide and the 1973 Attachie Slide. These analyses reveal that the catastrophic runout of all three cases can be explained by the landslide debris undergoing a large, undrained strength loss. The potential for this mechanism to occur at these sites would have been difficult to identify by conventional geotechnical analyses. We then compare surface and subsurface data obtained for the Schlucher Landslide to these three cases to qualitatively assess the potential that this landslide could experience a similar undrained strength loss. The results of this work have significant implications for understanding catastrophic failure potential of fine grained colluvium, and highlight the need for further research on the mechanism of macroscopic brittleness. [ABSTRACT FROM AUTHOR]