At the Meuse/Haute-Marne Underground Research Laboratory several research programs have been dedicated to: (1) knowledge improvement of the Callovo-Oxfordian claystone response to the excavation; (2) development of the different disposal systems. In order to increase the understanding of the rock mass formation and support interaction, a drift composed of different support systems has been monitored for several years. As this one is oriented along the minor horizontal principal stress, the measurements exhibit an anisotropic closure of the cross section with the vertical convergence being higher than the horizontal one. It is thus of interest to evaluate the effect of flexible support composed of compressible wedges and of the final concrete lining on the closure evolution. Also, extensometers measurements and the stresses on the temporary support are analyzed in different zones of the drift. Finally, the results are compared to those obtained from other drifts excavated in similar conditions.
The French National Radioactive Waste Management Agency (Andra) began the construction of the Meuse/Haute-Marne Underground Research Laboratory (MHM URL) in 2000 with the first objective to study the feasibility of radioactive waste deep geological repository in the Callovo-Oxfordian (COx) claystone. Experimental drifts are designed as a mean to test and optimize the construction methods using different excavations and support techniques.
At MHM URL, the principal vertical stress, σv, is similar to the principal horizontal minor stress, σh, (about 12 MPa) whereas the principal horizontal major one, σH, is 1.3 times greater (about 16 MPa). During excavation, an induced fractured zone around the drift was observed (Armand et al., 2014). The fracture patterns depend on the direction of the drift relative to the principal stress directions and lead to an anisotropic closure of the drift. Most of the drifts at the main level of the Laboratory (-490 m) are constructed following the principal horizontal stresses, because it is the direction in which the fractured zones are minimized. Thus, it is the preferred direction for the safe storage of radioactive waste.
