Three cliff collapses in two days in Occitanie: “The freeze-thaw made the rock unstable”

L’éboulement survenu à Causse-de-la-Selle a été causé par l’épisode de gel qui a rendu la roche instable.

Romain Meddour, ingénieur géotechnicien de l’entreprise Ginger CEBTP à Jacou (Hérault) décrypte les éboulements de falaises qui surviennent dans la région.

Romain Meddour, engineer and rope access technician, performs diagnostics on the region's cliffs. DR

You are specialized in natural rock risks and you intervened last week in Causse-de-la-Selle. How do these natural phenomena occur ?

The three landslides that occurred last week in a relatively small area, Causse-de-la-Selle, Millau and Anduze, have similar characteristics: limestone terrain and a fall of roughly equivalent masses, of the order of ten to twenty cubic metres. These are events that occur in valleys dug by watercourses in the rock, which has created areas of instability on the slopes over the centuries, with erosion and the penetration of vegetation. As we are in a relatively rainy year, water has infiltrated the cracks and with the recent frost phase, it has taken on more volume, which has pushed the rock blocks to the end. The landslide then occurred with the thaw.

Does this mean that the risk of landslides is greater in winter? ?

200% Deposit Bonus up to €3,000 180% First Deposit Bonus up to $20,000

Not only. Of course, freezing-thawing is one of the main factors of rock instability, but two other important parameters can cause landslides. First, in the autumn, there are the Mediterranean or Cévennes episodes: heavy rains generate hydraulic pressure in the fractures, a pressure roughly similar to that caused by frost. Drought can also cause the fall of rocks weakened by the root network of shrubs and trees that infiltrates the fractures; earth or clay penetrates them and the shrinkage-swelling phenomenon creates volume differences that cause landslides. This generally occurs during the first rains after a very dry summer, as we have seen particularly over the last ten years.

Global warming can therefore increase the risk ?

It is difficult to say. It is a natural phenomenon that will occur in any case, because we are at a latitude where there is seasonality, therefore rock elements subject to these climatic variations, in addition to the erosion of time which naturally plays a role. Only a climatologist can predict it, but it is true that the succession of very dry summers and very rainy winters, with marked episodes of frost, can generate an amplitude that the rock will have difficulty supporting. In fact, the instabilities will tend to be pushed downstream, and therefore to roll down the slopes.

Are there some types of rock that are more sensitive than others ?

Although it is no longer relevant today, the Mediterranean rim has a seismic past that has created a network of faults and fractures that allow weather agents, rain and frost, to infiltrate the rock. All are therefore generally sensitive to these phenomena. However, there are differences: limestone, which is particularly found in Hérault, will tend to fall in larger blocks, and therefore have greater impacts on roads. With the Cévennes schists, the masses are broken up into plates, so they are not very large unit volumes that are detached, except that they can cause landslides in the process that can submerge the roadway.

How can we, all the same, prevent them??

Our job is mainly to intervene in events, but we also do, most of the time on behalf of communities, prevention on slopes or rocky embankments that we know or think are problematic. In this case, we map an entire area, using aerial photographs or LiDAR technology which allows, thanks to a drone, to obtain a very precise rendering of the topography through the plant cover. This allows us to scan large areas to then choose the areas that we will investigate on foot, in the embankment. We then inspect the rock outcrops likely to present instability to determine their risk of rupture and, depending on the slope, to recommend the necessary work. Once this has been done, we check these areas regularly. But it is difficult to predict all landslides, particularly on limestone terrain.

I subscribe to read the rest