Architecture and management of a geological repository - Andra

2 – General Description2.3.2.2 Geochemistry of the geological mediumThe Callovo-Oxfordian is a naturally reducing medium; this property is particularly favourable inlimiting the corrosion of metallic materials and the release of radionuclides (§ 2.2.1.2).During the operation of underground structures, ventilation will cause an oxidation phenomenon in thevicinity of the wall. This phenomenon will also concern the argillite excavated rock extracted to thesurface. However, argillites contain minerals (pyrites in particular) and organic matter which reactwith oxygen, which will limit this phenomenon to a very slight thickness.After closing underground structures, the interstitial water of the argillites which will percolatetowards these structures may transport chloride ions; the interaction of these ions with the materialsemplaced in the structures (steel and concrete) is considered in the assessment of their long-termdegradation.Argillites can also act as effective buffers to alkaline disturbance, such as that which would be inducedby the gradual degradation of concrete structures over time.2.3.2.3 Mechanical properties and excavatabilityThe mineralogical composition of the Callovo-Oxfordian argillites confers them relatively highstrength strong for an argillaceous rock and limited deformability, whereas; damage occurs above acertain load threshold and whereas they are prone to fearing (brittle). Their high clay mineral contentlimits the reversible deformation threshold, attenuates the brittle behaviour (plasticity), tends to reducethe breaking strength of the rock and confers a significant viscous behaviour (long-term creep).As far as short-term mechanical behaviour is concerned, the simple compressive strength of argillitesaverages 21 megapascals (MPa) in the middle of the formation studied ([6]- Volume 2) 13 .It will be noted that this strength is comparable to that of rocks where major underground work hasbeen carried out, under similar conditions. A particular example is the Chamoise motorway tunnel inthe French Alps (see Figure 2.3.4) which, at a depth of approximately 400 metres, passes through anargillaceous rock of the same age, with simple compressive strength of 20 to 35 MPa [32], [33]).The modulus of elastic deformation of Callovo-Oxfordien argillites ranges from 3 000 to 5 000 MPa –within the same range of values as the argillaceous rock traversed by the Chamoise tunnel (3 000 to6 000 MPa).13It will be noted that this strength is only slightly lower than that of common construction-grade concrete (30 MPa).DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM48/495