Geological model for a tunnel in Pyrenees (France): importance of the feedback loop based on re-examined investigation data

When designing an underground structure, the geologist’s job is to reveal the arrangement of the terrain of the massifs crossed and thus to highlight what is not obvious at first sight. Sometimes, the geological model produced is not completely satisfactory, especially when there are still grey areas, inconsistencies, and mismatches. In such cases, the conceptual model must be revised, and the main difficulty lies in questioning the data available at all levels (regional, local) and how these data are interpreted. This article explains how such a situation was encountered for a tunnel project in the Coniacian flysch of the Western Pyrenees. In its initial phase, the geological model conceptually considered that the terrain was interspersed with faults associated with numerous crushed zones. However, the overall coherence of the model, its connection to the regional context, and the detailed interpretation of borehole logging data eventually called into question the structural basis of the model. An adaptation of this model, by better taking into account the local geological folds, allowed the interpretation to be oriented toward a ductile model. In particular, the geological core surveys and the acoustic borehole imaging of discontinuities were at fault for the erroneous interpretation. The re-examination of the collected cores and the re-positioning of the discontinuities confirmed the new conceptual orientation of the model. The comprehensiveness of the identified folds and their crushed cores was analyzed and summarized to produce a more precise and clearer model despite the chaotic layout of the terrain.