Ground collapse in 1992 in the Peyrui district on the Route de Seillans above the village (Bargemon-Var, 2024)
© BRGM
The need
The aim of the work carried out by BRGM was to build an overall strategy from a multidisciplinary perspective in order to manage the risk of ground subsidence or collapses linked to the dissolution of gypsum in the municipality.
The results
The following results enabled us to confirm (or rule out) the initial hypotheses, in order to establish risk management recommendations:
- Update on the geology of the Keuper gypsum formations and their structural organisation (e.g. alignment N110° of the main gypsum masses and the known or newly discovered sink-holes);
- Major uncertainties continue to exist about the geology of the surface (due to a lack of visible outcrops) and, above all, at depth (due to a lack of deep boreholes), in terms of establishing the scope of the gypsum deposits;
- Better understanding of the hydrogeological system of the massifs;
- New insights into the geological and structural context, conducive to the occurrence of events such as the one at Peyrui in 1992: accumulation of gypsum under the effect of anticlinal folds, combined with fracturing that allows water infiltration (electrical geophysics);
- Certain hypotheses about the origin of the damage to buildings in the village of Bargemon were ruled out (contribution of seismic geophysics data);
- Interferometric analyses made it possible to identify and closely define an active subsidence zone around the Riou plain, partially recognised in geophysics;
- Status of damage to buildings taken into account as a reference base to be monitored in the event of further deterioration;
- Assessment of vulnerability of areas to collapse due to gypsum dissolution: 14.7% of the area with high vulnerability, 22.7% with medium vulnerability and 18.5% with low vulnerability (44.1% of the area not affected);
- Cross-referencing of vulnerability zones with the buildings in place (1,503 buildings of all types inventoried): 99.5% of buildings in the municipality in vulnerability zones (26.8% high vulnerability, 65.1% medium vulnerability and 7.7% low vulnerability).
Outcrop of Triassic Keuper gypsum in a former quarry used to produce plaster (Bargemon, 2024)
© BRGM
Using the results
The municipality of Bargemon has now been provided with a series of recommendations for managing the risk of ground subsidence and collapses. Some of these recommendations are general, while others are more specific, notably including the implementation of appropriate surface-water management and the creation of a system for monitoring problems that occur (in the short and medium term). However, additional longer-term investigations are still required.
A thesis on the "Tectonics of the evaporitic and carbonate Triassic formations of the Haut-Var: structural characterisation, dating and thermal analysis of deformation - Implications in terms of gravity-related hazards (collapse - dissolution)" was started in 2024 for the RGF (Référentiel Géologique de la France [French geological repository]), and will contribute to a better understanding of the processes involved.
The partners
- Municipality of Bargemon
- Var Departmental Authority for Inland and Marine Areas (DDTM83)
- Var Departmental Council
