2023-09-09 16:15:00
It seems that fate is besetting the commune of Oreye and its inhabitants. After the heavy floods which affected the villages of the commune at the end of August, it is now their soil which is suffering damage. Indeed, strange underground caves have appeared in several places, notably in a field along the Chaussée Romaine not far from Rue d’Opheers or even on the construction site of the new changing rooms dedicated to municipal workers. Obviously, these two unfortunate chain events are linked, explains the mayor, Jean-Marc Daerden. “We received more than 92 liters per square meter during the heavy rainfall. All this water weakened the roof of these already existing and unmapped cavities. They therefore collapsed.” Fortunately, so far, no casualties have been reported, he said.
The presence of these cavities in the Hesbignon soil is not surprising, however, underlines Jean-Marc Daerden. “At the time, it was common for farmers in the region to dig holes several meters deep in order to extract the white chalk present in the regional soil. The farmers then used it as fertilizer for their fields.” We are then talking about phosphate mining, which is particularly rich in Hesbaye.
According to the information given on the website of the Walloon Geology Service, the Hesbignonne marlstones are more modest since they were used for personal purposes. They measure on average two meters wide and two to four meters deep. This is not the case in Borinage for example where they were more often used for the industrial extraction of chalk. They could therefore reach a depth of 35 meters.
Concerning the holes observed in Oreye, geologists do not prefer to move forward too quickly. An in-depth study by researchers from the Walloon Geology Service will be necessary so that they can confirm that these are indeed marl beds, as has already been the case in Wasseiges and Lens-Saint-Remy.
Water, the main culprit
Land collapses often occur when water infiltrates the ground. “This is what happens in more than 90% of cases,” explains Patricia Ruscart, agent of the Geological Service of Wallonia. This could be due to water coming from a gutter or obviously due to flooding. The landslide phenomenon will then result from the fact that the walls of these cellars are made of chalk and that the water will dissolve this chalk.
A difficult census
It is common for these cavities to collapse and appear as a surprise in gardens or during work. The Walloon Geology Service has therefore put maps online which list the marl pits already discovered. These maps are also used to warn if work plans to be undertaken in risk areas. The data was collected thanks to the work of geologists who surveyed the entire Walloon territory. After making these discoveries, they must then look into the origins of these cavities. “The origins of marl stones can go back up to 2,000 years. We are trying to find out why they were made? How were they dug? With what means?”
If it still often happens that marl pits are revealed without having been recorded, it is for the simple reason that at the time, the owners of the land exploited their soil without declaring it. There is therefore no trace of these cavities, although they are still very present. “We strongly encourage the population to notify us of collapses, but also during work so that we can complete and enrich our maps,” advises Patricia Ruscart.
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