Architecture and management of a geological repository - Andra

2 – General DescriptionFigure 2.3.5View of the Meuse/Haute-Marne underground research laboratory drift at-445 mSubjected to deviator stress (i.e. non-isotropic stress), the argillites deform gradually towards a moreisotropic stress state. This deferred behaviour mechanism (creep) is slow; a creep deformation rate of10 -3 to 10 -4 per year has been measured on water-saturated samples after two to three years underconstant deviator stresses of 5 to 15 MPa ( [6] - Volume 2) 14 . The deferred behaviour of the argilliteswill result in the gradual build-up of pressure on the underground structures. The liners of theunderground excavations, and in the longer term all of the materials filling the excavations, will beable to absorb the resulting stresses and to control damage to the rock.The ventilation of underground structures causes gradual desaturation of the surrounding argillites. Asa result, their water content is reduced and a gaseous phase is formed in the rock pores. Desaturationincreases rock stiffness and strength and slows down deferred deformations considerably, or evensuspends them, for as long as the argillite remains unsaturated [6] - Volume 2).The upper levels of the Callovo-Oxfordian formation have greater strength than the median levels ofthe formation (30 MPa on average). Damage here is naturally slight, as shown by the observationsmade in the shaft of the underground research laboratory. This element represents an advantage for thesealing of shafts at a possible repository installation.14By way of comparison, Boom clay, studied in Belgium for a possible repository (Hadès underground laboratory, Mol site), has aconsiderably higher creep rate: it varies from 0.06 to 0.3 per year under deviator stress of 2.25 to 2.75 MPa [34].DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM50/495