View to the north of the Hournech spur (foreground) through which the RN125 (visible in the background), will pass, with the Saint-Béat gorge (Garonne valley in Haute-Garonne, 2023) in the distance.
© DREAL Occitanie
The need
The Occitanie environmental, spatial planning and housing directorate (DREAL Occitanie) asked BRGM to carry out a geological and structural study of the Hournech massif based on the scientific data acquired over the last 20 years as part of its geological surveys. The aim was to specify the geological model that would enable the optimal design and sizing of the structure to be built on the Hournech excavation linked to the construction of the RN 125, which can be seen below it.
The results
The land affected by the construction work has very contrasting lithological facies. These stacked units were formed from bottom to top (from south to north) by: the formation of Silurian graphite schists, the formation of slate schists (Devonian) changing at the top to silky schists, the "Pierre Blanche limestones" which form the rocky spur, transitional limestone formations and the massive dolomite formation.
The construction area is located on the southern flank of a south-facing synclinal fold, faulted at its core. This flank is made up of Lower Devonian strata that are inclined at an average of 70° to the north. The stratification (S0) is strongly affected by a south-verging schistosity (S1) (S1 is steeper than S0). The reverse fault to the south, which intersects the syncline, has been deduced from the structural reconstruction but has never been observed in the field, as it intersects the dolomite unit in a large scree-covered depression. At the scale of the massif, the dispersion of stratification measurements is due to anticlinal and synclinal folding. At the scale of the construction area, located on the southern flank of the syncline, the dispersion of S0 is due to local undulations.
In order to carry out the development work, one of the study's recommendations is to categorise sectors of the construction area according to the different lithologies defined by the geological study. Each category of rock has different characteristics in terms of resistance to stress and deformation, which will have an impact on the progress of the work.
In 2023, following the identification of a risk of a major landslide occurring in one of the construction areas, we asked BRGM to provide a structural analysis of the Hournech massif. The aim was to gain a better understanding of the mechanisms at work in a complex zone of folds crossed by a fault, in order to optimise the design assumptions for future road structures. This study, supplemented by other investigative techniques, enabled us to confirm the risk of a major landslide and the nature of the engineering required to eliminate the risk.
1:10,000 geological map and N-S structural section of the Hournech massif. The position of the spur affected by the construction is framed in purple.
© BRGM
Using the results
The geological model based on the structural and lithostratigraphic study provides a framework for understanding the organisation and nature of the terrain in the Hournech massif. It identified a number of points to watch out for and will enable the considerable amount of data acquired by drilling in the area to be reinterpreted in order to redesign and re-size the structure to be built on the Hournech excavation.
The partners
- DREAL Occitanie
