France – Structural engineering consultancy services – Installation par méthodes innovantes des matériaux compressibles dans les carrefours du Laboratoire souterrain

Law No. 91-1381 of 30 December 1991 (known as the "Bataille" law) set out the main research directions to be pursued on the management of HA and MA-VL waste. It identified three research axes: separation and transmutation, long-term storage (both entrusted to the CEA) and deep geological disposal (entrusted to the Andra). In 2005, the Andra and the CEA submitted to the State the results of the fifteen years of research conducted on these three axes. Law No. 2006-739 of 28 June 2006 on the sustainable management of radioactive materials and waste is based on the 2005 public debate and on the evaluations of the research. This 2006 law adopted the implementation of a deep geological repository as the only solution capable of ensuring the long-term safety of radioactive waste and set a requirement for reversibility of at least one hundred years. The 2006 law also entrusted the Andra with the mission of designing and implementing a reversible storage facility in a deep geological layer. Law No. 2016-1015 of 25 July 2016 specifies the modalities for the creation of a reversible storage facility in a deep geological layer for high-level and intermediate-level long-lived radioactive waste (Basic Nuclear Installation (INB). The project corresponding to this INB is called Cigéo for "Industrial Centre for Geological Storage". On 3 August 2020, the Andra submitted, to the ministry in charge of the environment and energy, a file for a public inquiry prior to the declaration of public utility (DUP) of the Cigéo storage centre. The DUP decree as well as the decree qualifying this "National Interest Operation" (OIN) were obtained on 8 July 2022. These files constitute the first milestones prior to a series of authorisations that will be granted as the project progresses. A request for authorisation and creation of the Cigéo INB (DAC) was submitted on 16 January 2023. The examination of this request is scheduled to last 5 years. In parallel with the examination, the Andra continues the research work necessary for the detailed design of the underground structures and for demonstrating the expected performance of the Cigéo project. This work is based in particular on more than twenty years of research conducted at the Meuse/Haute-Marne Underground Laboratory, located within the Callovo-Oxfordian clay formation, at a depth of approximately 490 metres. This experimental laboratory was specifically designed to study, under representative conditions, the behaviour of the host rock, the methods of excavation of underground structures, the support and lining systems, the associated hydro-mechanical phenomena, as well as the long-term safety requirements specific to deep geological disposal. The experiments conducted at the Laboratory have made it possible: • to acquire a fine understanding of the mechanical and hydraulic properties of the rock; • to characterise the evolution of the damaged zone (EDZ) around the excavated structures; • to develop and evaluate different excavation methods, including the conventional method and the tunnel boring machine; • to test several types of support and lining; • and to develop the numerical models used in the design and safety studies of Cigéo. This work has highlighted the importance of controlling the convergence and the pressures exerted on the concrete linings of the galleries. The experimental campaigns carried out since 2005 have shown that the use of compressible materials, inserted between the rock and the lining, constitutes a relevant solution to reduce the stresses transmitted to the structures and improve their durability. By acting as absorption elements, these materials make it possible to accommodate the slow deformations of a clayey mass whose mechanical behaviour is viscoplastic. The results obtained in the CTM, TBM, OMA, VMC or GRD4 programmes have confirmed the interest of this approach, while revealing the complexity of its implementation, particularly in areas of complex geometry. In particular, the intersections of underground galleries have large dimensions (≈ 8.60 m of intrados), multiple interfaces, and localised concentrations of stresses, which make them a particularly sensitive point in the design of the storage architecture. The engineering of Cigéo provides for: • galleries of different diameters; • large intersections; • linings whose thickness must be controlled; • strict mechanical performance taking into account safety requirements; • and construction methods compatible with a confined, deep and high HSE criticality environment. At this stage of development, the Andra must demonstrate the operational feasibility, reproducibility and mechanical performance of the compressible solution envisaged for the intersections, with a view to its integration into the future underground intersections. However, despite the encouraging results obtained on certain straight sections of the gallery, no existing solution currently allows: • to achieve the required mechanical performance; • to ensure a homogeneous application of 20 cm of material in an intersection; • to guarantee implementation under real conditions at the Laboratory (access via shaft, coactivity, logistical and safety constraints); • nor to achieve the level of technological maturity required for industrial exploitation. It is in this context, combining legislative framework, progress of the Cigéo project, geotechnical requirements, experimental feedback and engineering needs, that the Andra has decided to enter into an innovation partnership specifically dedicated to the development, testing and demonstration of a solution of compressible materials adapted to the intersections of underground structures.