DEEP REPOSITORY R&D FOR RADIOACTIVE WASTE | ROCK TUNNELS


In this type of sealing configuration, equilibrium


basically relies on shear strengths at the following interfaces: between the plugs and the lining (i.e., concrete–concrete interface); between the backfill and the lining (i.e., clay–concrete interface); and, on the strength and stiffness of the backfill itself reacting to compression. Here, the performance of the sealing structure as


a whole is evaluated by considering the individual performance associate with capability of the sealing core to reach full saturation at all points; develop the target swelling pressure as uniformly as possible; and, recompress the EDZ. Also, the stabilising components (plugs and backfill) must provide long-term stability to the swelling core and maintain the swelling pressure.


NUMERICAL SIMULATION Basic Theoretical Formulation Simulations were carried out with the software CODE_BRIGHT, a finite-element code designed to model coupled thermo-hydro-mechanical processes in porous media (Olivella et al. 1994). The basic theoretical formulation consists of governing equations based on mass and momentum conservation principles, completed by constitutive laws and equilibrium constraints. For the sake of simplicity, a particular case of


the general formulation is considered. The porous medium is assumed to be composed of a solid (s) phase (composed only of minerals), a liquid (l) phase (composed only of water), and a gas (g) phase (composed only of dry air). Moreover, gas phase is assumed perfectly mobile, and therefore, gas pressure (Pg


) remains constant during all the analysis. Under these considerations, the relevant balance


equations are the balance of solid mass and water mass, respectively, and total stress equilibrium. Analysis involves the simultaneous solution of stress equilibrium and water mass balance equations. The main unknowns, usually called independent or state variables, are the displacements and the liquid pressure linked to the dependent variables (solid and liquid densities, saturation degree, and total stress tensor) through a set of constitutive laws, some being generic to all materials. Details about the hydro-mechanical constitutive


model considered for each material are given in the following sections.


Geometry and Main Features of the Numerical Model Model dimensions and the finite-element mesh are illustrated in Figure. 5. Details of the model around the sealing core can be seen in Figure. 6, where, for ease of viewing, the upper half of the model is hidden. This figure also shows the position of six points within the core used as results’ output points. The mesh is constituted by 8-node hexahedral linear elements (total amount of almost 150,000 elements and 160,000 nodes). Each node has four degrees of freedom or unknowns


(displacements in the three spatial directions and liquid pressure). Advantage was taken of two planes of symmetry


(“x–y” and “y–z”, labelled “sym. boundary” in Figure. 5) to analyse only a quarter of the full problem domain. Null displacement and null flux of water were prescribed on these planes during the entire simulation. Since gravity effects are considered, it is not possible to take advantage of a third plane of symmetry. For the initial stress state of the COx claystone, the


anisotropic in-situ state estimated by Wileveau et al. (2007) was considered. At the main disposal level, the major principal stress is horizontal and aligned with the drift (σH


≈ 15.6 MPa). On the other hand, the vertical (σv ≈ 12 MPa). To account for gravity, a linear )


and the minor horizontal (σh) stresses are close to each other (σv ≈ σh


variation with depth of the three stress components was prescribed as initial values in the model, setting the previously mentioned values at the depth of gallery centreline. For the initial water pressure, a hydrostatic distribution was considered with a value of 4.7 MPa, set at the gallery centreline (Armand et al. 2013).


Below, figure 5: Model geometry and mesh: general overview


Backfill Plug Core


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100m


10m


70m


boundary sym.


50m


10m


10m


50m


boundary


sym.


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