The French National Radioactive Waste Management Agency (Andra) is in charge of the study on the possibility of disposal for radioactive wastes in a deep claystone formation. A deep geological disposal facility for high-level radioactive waste (HLW) and intermediate level long life radioactive waste (IL-LLW) called Cigéo is planned to be constructed in a deep Callovo-Oxfordian claystone formation (COx), if licensed. In 2000, Andra began to build the Underground Research Laboratory (URL) at Meuse\Haute-Marne to (1) characterize the confining properties of the COx; (2) demonstrate the construability and working of the repository components and (3) optimize concept/design of the different components.
Actually, several types of retaining structure have been constructed and instrumented in the URL to follow stress/strain evolutions, as well as better understanding of the interaction between the concrete structure and the surrounding rock. The lining stress measurement is a complicated measurement and often carried out indirectly from the strain measurement. One of the rare direct stress measurement is the flat jack technique, which provides a relatively simple and almost non-destructive punctual measurement of stress. Typically, the flat jack is withdrawn from the structure after the test and the structure is restored to the original state before the test. This is not the case in URL where the flat jack is kept in place, in a way to follow the stress time evolution by conducting regularly the test.
This article proposes an interpretation method of flat jack measurement based on semi analytical methods and numerical modelling in a way to follow the time dependence evolution of orthoradial stress evolution in a tunnel lining. Those analysis have been conducted on measurements performed at URL. Comparisons with other measurements in URL were made to validate the proposed interpretation method.
In order to demonstrate the feasibility of a radioactive waste repository in claystone formation, Andra started in 2000 to build the URL at Bure (nearly 300 km east of Paris). The host formation consists of COx claystone, which exhibits very favorable conditions for repository of radioactive waste, as they generally have a very low permeability, small molecular diffusion and significant retention capacity for radionuclide. The objectives of the research program is to (1) characterize the confining properties of the clay; (2) test and optimize concepts at full scale on site (Armand et al. 2016). Among them, understanding the impact of excavation methods and the hydro-mechanical behavior of claystone on the support system of the drift at short and long term is a key issue for optimizing the support designs. Measurements/estimations of stresses evolution as function of time in the URL concrete structure is important to characterise the creep behaviour of surrounding rock and to design Cigéo lining structures.