ABSTRACT

An experimental programme for damage assessment was performed in the vicinity of the main shaft of Andra's Underground Research Laboratory, currently under construction in the East of France, when the shaft is crossing the Kimmeridgian marls between 80 and 100 m depth. The mechanical behaviour was analysed by mean of convergence and displacement measurements. The disturbance into the wall was assessed using numerous methods such as measurement of sonic velocities, permeability measurement and VSP measurement in radial boreholes. The intensity of the damaged zone is low, and its extension does not exceed 0.5 time the radius of the shaft. It does not seem to depend on blasting patterns.

ZUSAMMENFASSUNG

Ein Forschungs- und Versuchsprogramm wurde aufgestellt, um die beschadigte Zone im Bereich der Kimmeridgemergel zwischen 80 und 100 Meter Tiefe beim Zugangsschacht des im Bau befindlichen Andralabors im Osten Frankreichs zu berechnen. Das mechanische Verhalten der Formationen wird auf der Grundlage von Konvergenz- und Verschiebungsmessungen ermittelt. Verschiedene Messmethoden, wie z.B. mikroseismische Geschwindigkeite Messungen, Permeabilitas- und Beschleunigungsbestimmungen in strahlenförmigen Bohranordnungen, erlaubten die Ermittlung des Beschadigungscharakters der Schachtwand. Die Intensitat der beschadigten Zone ist niedrig, ihre Ausdehnung uebersteigt nicht die Halfte des Radius des Zugangsstollens und scheint unabhangig vom Schema der Sprengstoffanordnungen zu sein.

RESUME

Un programme experimental visant à evaluer la zone endommagee autour du puits d'accès du Laboratoire de Recherche Souterrain de l'Andra actuellement en construction dans l'Est de la France a ete mis en oeuvre lors de la traversee des marnes du Kimmeridgien entre 80 et 100 m de profondeur. Le comportement mecanique des formations est evalue à partir de mesures de convergence et de deplacements. La mise en oeuvre de nombreuses methodes, telles que des mesures microsismiques, des mesures de permeabilite et d'accelerometrie en forages radiaux, a permis de caracteriser l'endommagement en paroi du puits. L'intensite de la zone endommagee est faible et son extension ne depasse pas 0,5 fois le rayon du puits. L'endommagement semble independant des schemas de tirs testes.

Introduction and objectives

Andra (Agence Nationale pour la gestion des Dechets RAdioactifs) is managing an experimental programme in France to study the feasability of radioactive waste disposal in deep argillitic rock formation (claystone). An Underground Research Laboratory (URL) is currently being constructed at Bure (Meuse/Haute-Marne) in the East part of Paris Basin to investigate the Callovo-Oxfordian argillitic formation located at a depth of about 420–510 m (Piguet, 2001).

Before reaching the target argillitic formation, the two vertical shafts, currently under construction, cross a sequence of Kimmeridgian marls interlayed with calcareous beds (up to 120 m depth) and then the Oxfordian calcareous formations.

The performance assessment of a disposal has to take into consideration the disturbance due to excavation and particularly the extension of the microcracked zone behind the excavation's wall, as well as its intensity. This so called EDZ (Excavation Damaged Zone) could be associated with a locally increased permeability and represents a risk that radioelements could leak from an eventual disposal to the biosphere. This point has to be carefully controlled and the influence of the excavation method has to be appreciated.

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