Shale microstructure evolution due to tunnel excavation after 100 years and impact of tectonic paleo-fracturing. Case of Tournemire, France

The mechanical excavation of an underground work results in stress redistribution on the opening, which can induce fracturing. The example of the Toarcian shale formation of the Tournemire experimental site was used to evaluate the impact of tunnel excavation on the microstructure of the formation in the near field of the tunnel. The effects of the excavation were compared to the effect of tectonic fracturing. Studies have been made on core samples located at various distances from the tunnel by X-ray diffraction, X-ray transmission and water content and volumes measurement. Although the Tournemire mudstone is overconsolidated, the excavation of the tunnel resulted both in the relaxing of constraints and in hydration/ dehydration of the rock. An increase of the porosity and a better orientation parallel to the bedding of the clay particles are observed. Around fractures, damage of tectonic origin has had a significant impact on the properties of rocks, broadly the same as the tunnel excavation, and contributes to the specific heterogeneities of rock properties in the formation. (C) 2003 Elsevier Science B.V. All rights reserved.