The Landslide Blog is written by Dave Petley, who is widely recognized as a world leader in the study and management of landslides.
Large rock slope failures are a very substantial hazard in areas with steep topography, as was recently shown by the catastrophic 24 May 2024 landslide at Kaokalam in Enga province, Papua New Guinea. Understanding their behaviour, and in particular the processes that lead to failure, is a fascinating and very active research area. In many cases, these failures occur without an immediately obvious external trigger, implying that internal processes may play a key role.
A really significant paper (Bottelin and Baillet 2024) has just been published in the journal Geophysical Research Letters, which presents a major step forward in the understanding of these processes. I think this is one of the most important papers in this area in the last decade. The paper uses passive seismic monitoring to examine the internal processes occurring in the the La Praz landslide, a 14,000 m3 rock slope failure in the French Alps. The landslide is located at [49.19833, 6.60288], as shown in the Google Earth image below:-
Google Earth image of the La Praz landslide in 2022.
This landslide failed on 27-28 August 2023, causing extensive disruption to both the A43 Autoroute, a motorway that links eastern France and northern Italy, and the high-speed (TGV) and Italian Trenitalia rail services between Chambery and Turin. But the slope had been identified as being unstable a decade earlier, and thus was being extensively monitored. Bottelin and Baillet (2024) have examined a key parameter that can be extracted from the passive seismic data, which is the fundamental resonance frequency of the slope, a measure of the overall rock mass stiffness. Alongside this, data was collected on the displacement of the landslide and on environmental factors that might be expected to cause the slope to move, such as rainfall.
For me, the most interesting aspect of the behaviour of this slope was observed from February 2023, a few months before the final failureevent. At this time, the rock mass stiffness suddenly reduced in a way that had not been seen in the previous years. Interestingly, this change in behaviour was not associated with rainfall or with a period of low air temperature. In the following months, further abrupt falls in rock mass stiffness were seen at various points.
The final failure was linked to a period of summer rainfall, during which the rock mass stiffness was observed to decrease further. This is all summarised in a really neat plot by Bottelin and Baillet (2024) that links the change in the fundamental resonance frequency of the slope (y-axis) with the measured velocity of the slope (x-axis) over time (the colour of the dots):-
Plot of the fundamental resonance frequency of the slope (y-axis) with the measured velocity of the La Praz landslide (x-axis) over time (the colour of the dots), from Bottelin and Baillet (2024).
Bottelin and Baillet (2024) have correctly concluded that these sudden drops in the stiffness of the slope indicate that brittle failure was occurring in the rock mass, probably caused by, for example, the failure of rock bridges in the evolving shear surface. At other times, the rock mass stiffness was reducing in a continuous manner (see the grey arrows in the plot), which was probably linked to processes associated with progressive failure.
For me, this study of the La Praz landslide is important for two reasons. First, it demonstrates the power of the use of passive seismic monitoring for understanding the evolving behaviour of rock slopes, in a manner that is not possible using surface monitoring alone. And second, it demonstrates that failure in large rock slopes is controlled by internal processes that allow the evolution of a rupture surface upon which failure can occur. This is a powerful insight.
Reference
Bottelin, P., & Baillet, L. 2024. Original insights into rock slope damage processesuntil collapse from passive seismic monitoring. Geophysical Research Letters,51, e2024GL109139. https://doi.org/10.1029/2024GL109139
Text © 2023. The authors. CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
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