Architecture and management of a geological repository - Andra

2 – General DescriptionIn the Dogger, bore-hole tests have showed low permeability (10 -8 m/s in the greatest water-producinglevels). No water-producing level of regional extent has been identified. Water recharge takes placesmainly via the outcrops about thirty kilometres to the east of the site. There is no natural outlet to thisformation in the sector studied.2.3.4 Geo-prospective evolutionThe long-term evolution of the geological environement, like its past evolution on geological timescales, is the result of the climate at the ground surface, as well as the internal geodynamic evolutionof the plates forming the earth’s crust.2.3.4.1 Climatic evolution and consequencesSince the beginning of the Quaternary Period, oscillating climate cycles succeed each other,responding to astronomic parameters, with alternating glacial and interglacial periods. Periodically, thesurface soils are frozen to a significant depth for a long time (permafrost) at the Meuse/Haute-Marnesite (40 to 50% of the time over the last 130,000 years). The frost penetrates to a depth of around ahundred metres. The deeper Callovo-Oxfordian formation is not therefore directly affected by frost([6] – Volume 3). Notwithstanding the influence of the greenhouse effect which could slow down thisevolution, permafrost could reappear in around 100 000 years’ time.These climate cycles result in a periodical boost to surface erosion. The main erosion phenomena arethe incision of valleys and the removal of limestone plateaux which modify the surface flows throughthe evolution in the karst networks and possible river captures.These phenomena have left marks on the landscape (alluvial terraces, for example) which make itpossible to estimate their rate. The laboratory site is situated on a plateau zone away from the majorvalleys and located at the head of a secondary hydrographic network, where erosion is slower. Thegradual disappearance of the Calcaire du Barrois (Tithonian) is possible beyond a period of 500 000years.2.3.4.2 Long-term geodynamic stabilityThe only tectonic movements anticipated are limited to the region’s faults (Marne faults to the westand the Gondrecourt-le-Château rift to the south-east). Outside of these zones, no deformation of thegeological layers is expected ([6] - Volume 3). The high geodynamic stability of the region explainsthe practically aseismic character of the sector on a historical time scale.2.3.4.3 Seismic uncertaintyAccording to historical accounts or more recent records, no earthquake exceeding 3.5 in magnitudehas been related or identified in the zone close to the Meuse/Haute-Marne sector (the Calcaire duBarrois (Tithonian) plateau between the Marne and Meuse valleys). The nearest known historicalearthquakes were located over 60 kilometres from the site (essentially in the Vosges) and presented amagnitude ranging from 3 to 6.In order to dimension the structures in accordance with regulations, a safety margin computedearthquake may be defined for the repository operating period. The magnitude of this earthquake(magnitude of 6 at a distance of 25 km from the site) is conservatively determined by increasing themagnitude of the strongest known historical earthquake in the zone and moving the focus from theplace of occurrence to the closest point to the site. This earthquake is characterised by maximumacceleration of approximately 0.15 g (1.5 m/s 2 ) at the depth of the repository and approximately 0.2 g(2 m/s 2 ) at the surface. The difference is linked to the amplification of the movement on propagationof the seismic waves at the surface ( [6] - Volume 3, [6]).DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM52/495