On the state of stress in non-homogeneous rock masses: A study in a clay formation with application to waste management in France

The state of stress in a rock mass is an important piece of information for the investigation of High-Level Waste management disposal in deep geological formations. Andra has conducted a large in-situ stress measurement campaign in several boreholes located around the site chosen for the construction of their underground research laboratory. Reliable data have been collected and have highlighted the high dependency of the state of stress on the formation rheology. As a result of the campaign, the maximum horizontal stress has been estimated to be close to the lithostatic pressure in the host rock a clay formation - at the main level (—490 m depth). This result has important consequences both for the geotechnical design of the galleries of the site and for the long term safety aspect of the site. The role of formation rheology on the state of stress is confirmed by other results like the high ratio - between 1.5 and 2.0 - of the two horizontal stresses in the limestones that sandwich the host rock.