New development using the "convergence-confinement" method in an anisotropic stress field

The French project CIGEO for an underground nuclear wastes repository in the Callovo-oxfordian claystone of the Eastern France entered a new phase of design, which should be the last one before the works launching. The 1st design stage, named as "esquisse" requires some rough but fast analysis of the structural lining of the galleries. The service life of it, is assumed to be of approximately 100 years. In the tectonic context of the Parisian basin, the ratio of anisotropic stresses is quite high at 500m depth and the horizontal high stress leads some squeezing, moderate but associated to the creep behaviour. That is why a new development using an approximate solution for the elasto-plastic stress field, according to the "convergence-confinement" method, is coupled with an experimental procedure for the fitting of the long term ground behaviour, in order to estimate the equivalent modulus and residual cohesion to the long term. The short term rendering is roughly based on the semi-analytical solution from Detournay & Fairhurst in an elasto-plastic medium, ruled by a Mohr-Coulomb criterion with softening. The empirical creep law is of the p-power type, according to the available measurements in the underground laboratory in Bure. Then, the method is applied as load concept of the structure, depending on the setting delay, according to the tunnelling means, with or without TBM.