Tome Architecture and management of a geological repository - Andra

6 – C waste repository zoneKnowledge of a swelling clay engineered barrier behaviour leads to limiting the temperature at thehottest point of the engineered barrier during the thermal phase to 90°C. This allowance is built intorepository module dimensioning by adjusting the spacing between disposal cells to the thermalcharacteristics of French granites (cf. § 6.5).In addition, various possibilities of clay engineered barrier formulation are another way of managingthe variability of the chemical compositions of French granite water.6.2.3 Limiting disturbances to the granite by the repositoryThe hydro-geological and hydro-geochemical disturbances of a granite rock, arising from theexcavation of the underground installations are managed as part of the general disposal process (cf. §6.3).The other mechanical, thermal or chemical disturbances likely to affect the properties of a C wasterepository are limited either by structure dimensioning or by disposal process management.6.2.3.1 Limiting mechanical damage to the rock during excavation work ("EDZ")Excavating the underground structures is likely to incur damage to the granite rock at the wall bycreating fissures. This damage creates a potential path for water circulation. It is technically possibleto limit its extent.Experiments in foreign underground laboratories ([14] [46]) have demonstrated the possibility ofcontrolling this damage. Drilling techniques in particular result in very slight damage and may be usedto excavate the small disposal boreholes. At this stage the choice of drift excavation techniques isopen, if the possibilities of adapting blasting methods to the context are exploited. Limiting thedamage incurred by blasting methods involves using familiar techniques such as smooth blasting, precuttingand excavating in divided sections. These low-damage techniques are based on reducing theinstantaneous quantity of energy liberated in the rock by the explosives (spreading the firing sequence)and distributing blasting pattern explosives in line with their impact (shattering) and gas energy.Added to this, sawing methods for cutting out the damaged zone such as those used in ornamentalstone quarries can be used at the sites of the seals.6.2.3.2 Limiting thermo-mechanical deformations in the graniteThe granite that hosts C waste is likely to warm up and thus be subjected to thermo-mechanicalstresses, primarily during the early phases of the repository when temperatures and temperaturegradients are at their highest [47]. These stresses may result in granite medium deformations primarilywhere it is weakest, namely at the fractures or faults. Thus these deformations may trigger offmodifications to fracture hydraulic and transfer properties. The intensity of these deformations musttherefore be controlled by suitable design measuresDossier 2005 granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY169/228