Cherbourg-en-Cotentin.  This underground gallery is in danger of collapse, planned work

Cherbourg-en-Cotentin. This underground gallery is in danger of collapse, planned work

This underground gallery in Cherbourg-en-Cotentin (Manche) is in danger of collapsing. (©BRGM)

On higher ground Equeurdreville (transferred municipality Cherbourg-en-CotentinChannel), v residential area rue de la Palière-au-Renard, nothing allows us to guess the existence of eight galleries that loom in the basement. They have been there ever since. between the two warscarved into the granite at depths of 15 to 25 meters.

“When the land was being sold, they didn’t tell us anything…”

“I’ve lived here since 1983 and I didn’t know about it,” testifies one resident of the neighborhood.

“It used to be a field here. I had this house built for me. When the land was sold, they didn’t tell us anything… It wasn’t until the town hall came to inform us that work was going to be done that we knew that an tunnel passed right under the house. We have five or six affected houses on the street and in the neighborhood. »

One of the residents of the neighborhood

If the residence is surrounded by lots surrounded by signs “Military land: no entry without permission”, from there imagine this underground maze and the danger it can pose on the surface… It took initial studies carried out since 2013 with BRGM (Bureau of Geological and Mining Research), followed by a more detailed inventory to reveal risks.

In a document published online, the BRGM recalls that in the territory of Cherbourg-en-Cotentin “numerous cavities and underground tunnels were dug for passive defense, equipment storage and command of military operations”. “Some may present themselves. danger for possible surface problems. »

Supervision and prevention

The goal is to inform about the existence of these cavities and “to be able to recommend possible programs.” securedof surveillance az prevention “. The BRGM has identified 35 cavities, two of which are defined as a “major surface danger zone due to the high instability observed underground”.

and April 27, 2022based on a message sent to the community in March, the Municipal Council of Cherbourg-en-Cotentin decided to start work to secure one of the galleries Brecourt Park, “G0 Gallery”. When considering, it is emphasized that this is project management shared with state“right-of-way section” belonging to Ministry of Armed Forces.

Work from this Monday

The “relentless nature of the collapse of the G0 tunnel in the period of one to ten years, which characterizes the immediate danger and justifies urgent community intervention for security people and property’.

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“If there are several underground galleries in Cherbourg-en-Cotentin, this is the first time this type of work has been done here. It is a complex file in the process of processing, with 50-50 military support. Civil services planned to fund this work, but the ministry denied that it would shift the burden to residents, which was categorically denied. We are waiting for the latest studies from BRGM, but work should start on October 5. It will be for a week. »

Dominique HebertDeputy Mayor and Mayor of Équeurdreville-Hainneville, from 21

The private owners affected have been informed and will be contacted during the operation: “We have indeed been invited to several meetings and had visitors to the house. The bailiff also came to carry out an inventory, especially to determine the presence of cracks before the work. And there will be one after the works. »

The operation will consist of excavating various drill points to pour in flowable concrete to fill 100% of this G0 tunnel.

Mapped and buried military cavities

“The situation is related to the history of the city of Cherbourg, marked by various wars. Many tunnels and cavities were dug there for military purposes. In 2015, BRGM was commissioned by the Prefect of the Manche department and carried out an initial assessment of the cavity in the Saint-Sauveur district.
Instabilities were revealed, prompting the creation of an inventory to map and value all the volumes,” confirms Élisabeth Castellotti, sub-prefect of Cherbourg.
This process had two goals: to know where these military cavities were located and to assess the risks associated with their aging. For the nine cavities selected as priorities, BRGM writes that “precise 3D laser mapping of their influence” was carried out, followed by “stability diagnostics”.
“At the end of this work, all data related to the geometry of underground structures (contours, footprints, etc.) and their state of stability (vault instability, fracture state, etc.) were stored in a dedicated GIS (Geographic Information System) database usable by state services. »

“Inventory and assessment of risks associated with the presence of voids in Cherbourg-en-Cotentin” BRGM are online with accompanying photos at www.brgm.fr.

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