Three-ton block falls on car: roads under surveillance due to risk of landslide

A Causse-de-la-Selle, les travaux de sécurisation vont durer une dizaine de jours. Sylvie Cambon

Three rock falls last week, in Hérault, Gard and Aveyron, reminded us that nature can be unstable. The roads are regularly secured.

“Miraculous survivors”. The day after the landslide on the RD 122, between Saint-Guilhem-le-Désert and Ganges, not far from Causse-de-la-Selle where she lives, Karine did not hide her emotion on Ici Hérault. While she was driving home, her partner Leaven suddenly shouted at her to stop. “He first thought it was wild boars coming down. Then he saw a big rock and told me to steer. The stone rolled over him and sent us back to the other side of the road“. By “luck”, it was the back of the vehicle that was crushed by a 3-ton block. “By a second”, Karine and Leaven escaped “unharmed, with small bruises”. But a significant psychological backlash.

The same day, in the Gard, between Anduze and Générargues, another cliff collapse occurred, this time without causing any casualties, but forcing the departmental council to close the RD 129 for ten days, the time to carry out safety work. Two days later, it was above Millau, on the Cavalerie climb leading to Larzac, that a 30-ton block of stone crashed onto the road, hitting a camper van, fortunately there too without any human tragedy. But with the same disruptions on a very busy road.

686 monitoring points in Hérault

The concordance of the three events leads experts to believe that it was the recent frost episode that weakened the rock blocks, which then fell with the thaw. However, these cliff landslides occur regularly here and there, like the one that occurred last November in the Pyrénées-Orientales on the RD53B, which connects the Pagris valley to Amélie-les-Bains. The 5,000 tons of rock that fell into a 100-meter precipice even prompted local authorities to “abandon it definitively”this small road and to launch studies for the construction of a new road.

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This ultimate solution is obviously rare. In Hérault, the RD225, between Lodève and Soubès, was only seriously disrupted for two months this fall, the time to stop the disintegration of marl banks by nailing the rock masses fixed with shotcrete, before reinstalling a safety net. “Along our 5,000 km road network, we have 686 OPFs that we monitor regularly”, assures Philippe Vidal, vice-president in charge of roads in the Department. OPF, for “cliff protection works”, “which can be sensors, fences, enclosures or other protection systems”, deciphers the elected official.

This work represents “a budget of €500,000″per year, which can increase in the event of more significant work that nature suddenly imposes. In 2022, the road between Avène and Le Bousquet d’Orb had to be closed for several months, after an initial landslide showed that a very large mass of at least 100 m3 was threatening the RD08, a major axis for the activity of the Pierre-Fabre factory.

“A block that we knew…”

This new rockfall at Causse-de-la-Selle worries Philippe Vidal, however.“There were already, in this sector, cliffs that were monitored, even developed, but this block, which we knew, had not been considered at risk. The services tell me that it could be the impact of global warming, so it may force us to expand our protection system, we will think about it”.

In Hérault, we are aware that we should not take the subject lightly. In the summer of 2018, a camper van driver who had set up camp on the side of the road in the Vis valley was killed instantly by a 1 m3 rock. Since then, for several kilometers, the RD25, although popular with river swimmers, has been closed to parking.

Daily patrols

In the Gard too, the cliffs that border the departmental network are regularly monitored. “Our agents patrol the mountain roads daily and intervene as soon as they spot something, either by carrying out a purge operation themselves or by calling a specialized company”,describes Martin Delord, a departmental elected official in charge of roads, who himself found himself a few years ago “with a landslide in front of the bonnet of my car. Since then, nets have been installed at this location”, he recalls.

The vice-president of the Gard Department therefore speaks of a “day-to-day” monitoring, less complex according to him to follow “than the monitoring of bridges which are not in good condition and which can pose more problems for the continuity of service“. Rebuilding a structure is longer, more expensive and has a greater impact on traffic than securing a cliff. “That doesn't stop us from taking this subject seriously, even if, up until now, we haven't experienced a landslide that is very serious from a human point of view“. “A stroke of luck”,he adds. In their misfortune, Karine and Leaven can say the same thing.

A company specializing in the Cévennes

Based in Saint-Hippolyte-du-Fort, in the Cévennes, Hypogée MCV and its twenty employees, up to 60 with temporary workers, regularly work on the region's cliffs. “In the event of a one-off landslide, we will purge, with crowbars, what is unstable. This makes it possible to secure the area”, explains Benoît Martinez, site manager. “We can also reinforce the blocks by anchoring them to the rock or installing “screens”, he continues. These interventions are necessarily perilous. In Anduze, until January 31, his teams are working on a cliff of… 150 meters.

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