a Department of Civil, Environmental and Geomatic Engineering, ETH Zurich, Zurich, 8093, Switzerland
b Department of Construction and Environment, National Cooperative for the Disposal of Radioactive Waste (Nagra), Wettingen, 5430, Switzerland
2024, 16(12): 5162-5178. doi:10.1016/j.jrmge.2024.10.002
Received: 2023-08-04 / Revised: 2024-10-18 / Accepted: 2024-10-24 / Available online: 2024-10-26
2024, 16(12): 5162-5178.
doi:10.1016/j.jrmge.2024.10.002
Received: 2023-08-04
Revised: 2024-10-18
Accepted: 2024-10-24
Available online: 2024-10-26
The deep geological repository for radioactive waste in Switzerland will be embedded in an approximately 100 m thick layer of Opalinus Clay. The emplacement drifts for high-level waste (approximately 3.5 m diameter) are planned to be excavated with a shielded tunnel boring machine (TBM) and supported by a segmental lining. At the repository depth of 900 m in the designated siting region Nördlich Lägern, squeezing conditions may be encountered due to the rock strength and the high hydrostatic pressure (90 bar). This paper presents a detailed assessment of the shield jamming and lining overstressing hazards, considering a stiff lining (resistance principle) and a deformable lining (yielding principle), and proposes conceptual design solutions. The assessment is based on three-dimensional transient hydromechanical simulations, which additionally consider the effects of ground anisotropy and the desaturation that may occur under negative pore pressures generated during the drift excavation. By addressing these design issues, the paper takes the opportunity to analyse some more fundamental aspects related to the influences of anisotropy and desaturation on the development of rock convergences and pressures over time, and their markedly different effects on the two lining systems. The results demonstrate that, regardless of these effects, shield jamming can be avoided with a moderate TBM overcut, however overstressing of a stiff lining may be critical depending on whether the ground desaturates. This uncertainty is eliminated using a deformable system with reasonable dimensions of yielding elements, which can also accommodate thermal strains generated due to the high temperature of the disposal canisters.
Keywords: Radioactive waste disposal, Squeezing, Opalinus clay, Anisotropy, Ground desaturation, Transient consolidation, Tunnel boring machine
Alexandros N. Nordas
Dr. Alexandros N. Nordas is a Senior Scientific Associate and Lecturer in the Chair of Underground Construction at ETH Zurich. He is involved as expert technical consultant in industrial underground infrastructure projects, the principal being the deep geological repository for the disposal of radioactive waste in Switzerland overseen by Nagra. His research focuses on the life-cycle simulation and hazard assessment of underground systems, squeezing ground, time-dependent ground processes (creep, consolidation), TBM tunnelling, and the development of novel design aids for engineering practice. Alexandros holds a 5-year Diploma in Civil Engineering from the National Technical University of Athens (2013; Honours), along with a M.Sc. in Earthquake Engineering (2014; Distinction) and a Ph.D. in Computational Structural Mechanics (2019) from Imperial College London. He has authored and co-authored numerous expert technical consulting reports and scientific publications in international, peer-reviewed journals and conference proceedings. Throughout the years, Alexandros has received several honours and awards, including the Telford Premium Prize by the Institution of Civil Engineers (ICE), as well as the Letitia Chitty Centenary Memorial Prize and the Patrick J Dowling Prize in Advanced Structural Engineering by Imperial College London.