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Val-d’Oise: part of its garden collapses in a mysterious tunnel

It is a surprising discovery that Pascal made when he left his home one morning. “It must have been around December 10 that I saw this hole when I left my house,” said this inhabitant of the Maisons d’Arcole housing estate in Montsoult. At the foot of his house, the ground collapsed about a meter in diameter. He estimates the depth at four meters. Firefighters measured him two meters. “It’s a bit of a funny situation,” he says. When I saw this, I wasn’t sure what to do. “He first warned his insurance then called the firefighters. Part of the roadway near his house also sagged.

Intervention of firefighters and gendarmes

The departmental fire and rescue service (Sdis) intervened on Monday, but also the general inspection of quarries, the gendarmes and the town hall. “The town hall immediately put the barriers in front,” adds Pascal. She also took a decree to make the places inaccessible. Pascal was evacuated on Monday and had to spend a night with relatives. An expert intervened the next day to assess the dangerousness. “He has just passed, he believes that we can return to the house,” he says.


Mystery about the origin of this fontis

If the risk of the house collapsing seems to be eliminated, the mystery as to the origin of this cavity remains. The hole reveals a stone vault which suggests the existence of a conduit. “It seems that the bottom is masonry”, emphasizes Pascal. According to the General Quarry Inspectorate, it is likely that this conduit was discovered during the construction of the house and that it was walled up where the foundations of the house begin.

However, Jean-Claude Boistard, former mayor and president of this subdivision, indicates that he never had any knowledge of this underground structure, although he was one of the first inhabitants. It does not appear on any map. On the other hand, there are many old quarries near the house in question. According to Jean-Claude Boistard, it is clay that was exploited at one time. “The housing estate was built in the 1970s. It was split in two, because in the middle there is a piece of land that cannot be built on,” says the mayor (SE), Silvio Biello. “Perhaps when they made these careers, they went a little further,” asks Jean-Claude Boistard. But this is not the most likely explanation.


Different hypotheses raised

Silvio Biello mentions the existence in this sector of an underground aqueduct. “The famous aqueduct, it is two meters high and it goes from the castle of Bonshommes in Maffliers, to that of Baillet”, assures Jean-Claude Boistard. He indicates that his children went to play in this conduit when they were younger. It could be accessed, according to him, by two entrances, located on either side of the subdivision. It would therefore pass under the houses, but not where the hole was discovered. “The pond at Clos-Mangeot is supplied by a pipe which passes through this underground”, continues Jean-Claude Boistard.

For the general inspection of quarries, it is not an underground, but probably a drainage structure. It could be an aqueduct which served to supply the gardens of the Château de Baillet. “It’s a very small book,” said the general inspectorate of careers. The latter recalls that underground works greatly stimulate the imagination and that people often tend to exaggerate their scale. “What I would like to know is what is behind this wall and what is the origin of this tunnel,” says Pascal. He may never have the answer.