ROCK TUNNELS | DEEP REPOSITORY R&D FOR RADIOACTIVE WASTE


Upper shear fractures


σh


σH → Major horizontal


stress


σh → Minor horizontal


stress σH


Lower shear fractures


Extensional fractures


Above, figure 10: Conceptual model of the induced fractures network around a drift excavated parallel to the horizontal major stress (Armand et al. 2014)


The compressible material, installed between


the concrete lining and the host rock, exhibits a high deformability. It is intended to allow for the convergence of the excavation wall due to the time-dependent deformation of the host rock. This is an innovative feature that aims to reduce the transmission of forces to the concrete lining, thus reducing its thickness. Each layer (concrete and compressible material) has been


simulated separately considering a specific constitutive model for each. According to concrete lining specifications,


Equivalent plastic strain [-]


compressive stresses should remain below 50% of the maximum compressive strength (60 MPa). Consequently, it is expected that the concrete lining remains elastic during the full time of simulation. However, the concrete lining around the sealing core could reach the maximum tensile strength of the concrete (5 MPa) under the pressure exerted by the expansive material during swelling. For this reason, an elastoplastic model is employed. When the maximum compressive strength is reached, the response is assumed perfectly elastoplastic, while when the maximum tensile strength is reached, a brittle failure occurs and the tensile strength is assumed to be zero. The behaviour of the compressible lining material


is characterised by three regimes (Souley et al. 2017; Zghondi et al. 2017): a stiff response for a radial strain


RP1


up to ~ 1.5%; a high deformability limiting the radial stress within a range of ~ 2 MPa –3 MPa for radial strain up to ~ 50%); and, a stiff response for radial strain higher than 50%. For simplicity, at this stage,


this response is simulated through a three-step linear elastic law (see Figure 9). The values of Young’s modulus adopted for the three branches are in accordance with the specifications given by Andra for the material.


time = 100 years 0 0.007 0.013 0.020 0.027 Above, figure 11: Equivalent plastic strain contour fill 28 | February 2025


RESULTS The discussion of the simulation results is in four parts: first, the EDZ creation during the excavation procedure; second, natural hydration and performance of the sealing core; third, equilibrium of the whole system; fourth, recompression of the EDZ.


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