Architecture and management of a geological repository - Andra

5 – Repository ModulesGeotechnical hypotheses adoptedThe instantaneous behaviour of argillites can be modelled using an elastoplastic law with theappearance of irreversible plastic deformation before rupture. Dimensioning calculations do notrequire taking this complex behaviour into account; simplifications according to standard engineeringpractice are accepted for these. Argillites in particular are treated as being elastic until peak resistance,then as behaving in a perfectly plastic way after rupture. The adopted characteristics, deduced fromlaboratory tests by adopting the lower limits of the results obtained (20% quartile) are as follows:- Simple compressive strength of the rock mass (Rc): 12 MPa- Cohesion: 4.5 MPa- Internal angle of friction: 25°- Young’s modulus: 3,800 MPaThe choice of this behaviour model (elastic - perfect plastic) with minimum properties, leads to amaximizing evaluation of the stresses on the ground support and lining.The deferred behaviour of the argillites subject to an anisotropic stress is of the viscoplastic type,which means that the rock is deformed slowly when subjected to this stress. In order to beconservative, no minimum stress value that might halt this deferred deformation has been introduced:the rock is therefore assumed to deform until it reaches an isotropic stress state. The viscoplastic lawadopted is of the Lemaître type. The characteristics assigned to this law, resulting from theconservative extrapolation of laboratory tests, lead to predicting relatively high deferred deformationsfor this kind of rock (of the order of 1.2% deferred convergence for an unlined tunnel over 100 years).The favourable effect of the desaturation of the rock in the vicinity of the cell, which slows down thedeferred deformations and improves the mechanical characteristics of the rock, was not taken intoaccount, whereas this phenomenon could emerge not only before the closure of the cell, but also overseveral centuries after 71 .The initial stresses are assumed to be isotropic in the plane perpendicular to the cells, with a naturalstress in situ of 12 MPa at 500 m depth.Calculation principle and dimensioning criteriaIn order to take into account the uncertainties in estimating the stresses to which the structure issubject and regarding the characteristics of the materials employed, the method used is thatrecommended by regulations such as the Eurocode [45].. This method uses safety factors for takinginto account the uncertainties in estimating the stresses exerted on the structure to be dimensioned andregarding the characteristics of the materials used.Two kinds of numerical calculations were performed: finite element calculations, which can be used totake into account the detailed geometry of the structure, and, as a check and for sensitivity studies,calculations by the conventional convergence-confinement method 72 , which is a simplified approachapplicable to a structure of circular cross-section.The concrete strengths taken into account were chosen cautiously, taking the ageing of the concreteinto consideration, as is normally done, through the use of safety factors defined by standardengineering practice. In fact, the concrete of a cell is subject to a physical and chemical environmentmore favourable than the concretes considered in establishing standard practice, which are subject tobad weather conditions and seasonal variations of temperature.7172The rock is desaturated for several tens of centimetres under the effect of ventilation. Resaturation occurs after ventilation stops. Thisresaturation is very slow due to the very low permeability of the argillite.The convergence-confinement method uses the force-deformation curves of the lining (or of the ground support) and the terrain. Theequilibrium of the lining-terrain system is found at the intersection of these two curves.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM178/495