RTL Today - Ongoing risk: Nine months on, Villerupt probes if sinkhole danger is expanding

New subsurface drilling has begun in Villerupt to assess the stability around a giant sinkhole that appeared nine months ago, as officials seek answers on whether the underground threat is growing.

Nine months after a massive sinkhole appeared on a residential street in Villerupt, France, new investigative drilling has begun to determine if other hidden cavities lie beneath the surface.

The sinkhole, which emerged on 2 January 2025 on Rue François Raspail, remains open. It appeared on the pavement and a home's driveway, narrowly missing a resident whose car had been parked on the exact spot just one day earlier.

Initially measuring 2.5m deep, the hole expanded in both width and depth before stabilising. It now contains debris such as old guardrails, concrete blocks, and pipes, with grass beginning to grow inside. The two closest houses, numbers 16 and 18, remain uninhabitable.

A company will conduct drilling operatoins in the next few days to probe the surrounding area and locate any other cavities. / © RTL

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On Monday, 22 September, a geotechnical specialist company returned to the site to conduct a thorough subsurface investigation. A manager on site explained to our colleagues from RTL Infos that crews will spend two to three weeks drilling around and underneath the affected property to a depth of 25–30m, aiming to locate any further voids.

The operation aims to map the underground area and ensure no other cavities are hidden nearby. As the manager noted, the sheer volume of displaced earth indicates that "it must have gone somewhere", underscoring the need for the survey.

Mayor: Studies will determine if the fault is spreading

Villerupt Mayor Pierrick Spizak confirmed to our colleagues that the municipality has commissioned the new studies after initial surveys last spring yielded inconclusive results. The cost of these investigations, nearly €80,000 to date, falls to the local council.

"As the site has been recognised as a natural disaster area, the damage and repair work will be covered by insurance; however, the studies are the responsibility of the council", Spizak explained.

The primary goal is to assess ongoing risks. "The aim is to pinpoint if there are other risks in the surrounding area", the mayor stated. "At the end of the studies, we will know if the fault is spreading or not, if the homes are still habitable, and if we can start to fill in the cavity. We hope they will tell us what measures to take", he said.

Addressing residents' concerns, Spizak acknowledged their impatience but emphasised that safety is the priority. "It's not that we don't want to provide information, but while the studies are ongoing, we can't draw any conclusions", he said. "Our priority is the safety of the residents, and to be able to rehouse the people who lived there", the mayor stressed.

The possibility of a karstic collapse, a widespread threat in France

A report from the French Geological and Mining Research Bureau (BRGM) obtained last February concluded that "many uncertainties remain about the cavity", but pointed to a karstic collapse as the primary hypothesis.

In red, the area where the collapse occurred in January. / © BRGM

Detailed location of the area affected by the instability. / © BRGM

According to the BRGM geological map, the area concerned is located in the limestones of the Middle Bajocian. These limestones are regularly karstified (presence of karstic networks or cavities at depth) and may sometimes contain iron clusters filling fractures. They are covered by a sometimes thick layer of plateau silt. / © BRGM

Summary map of fault zones (hatched in red) and cavities identified in the BRGM cavities inventory at the time of this study. / © BRGM

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The report indicated that the area sits on a limestone plateau (Bajocian limestone), which can contain natural cavities. Given the volume of collapsed material, a karstic cavity extending beneath the two affected houses and the road surface could be the cause.

Other potential causes considered by the report include the rapid "emptying" of a clay-filled cavity after heavy rainfall; the gradual washing away, or leaching, of deep clays by groundwater, which creates a void that migrates to the surface; or the rupture of a limestone cavity's roof, causing the overlying ground to collapse.

The threat of such collapses is significant across France. "Subsidence and collapses related to karst can occur in nearly 40% of French territory", warned an expert from the public research centre Cerema.

These events result from karstification, where water dissolves limestone over time, and suffosion, where fine soils are washed away. This slow process creates underground voids that can eventually reach the surface as sinkholes, threatening infrastructure.

This is not an isolated incident in Villerupt. Mayor Spizak recalled a collapse on Rue Emile Zola in 2020, another near the Robespierre residence in the early 2010s, and in the 1970s, a house that "completely collapsed" in the same area.