Feasibility analysis for a radioactive waste repository tunnel

This paper addresses the feasibility of constructing deep tunnels in the Opalinuston rock (Opalinus Clay Shale) formation in Switzerland. Laboratory and field tests show that the characteristics of Opalinuston vary over a wide range, influenced by different geologic conditions, by inherent spatial variability even under similar geologic conditions, and by the testing conditions. Tunnelling feasibility is examined through an initial elastic analysis followed by an elasto-plastic analysis. The objective of the elastic analysis is the investigation of the effect of a wide variety of parameters, particularly regarding the ground but also the liner material and a range of liner thicknesses. Since these analyses assume elastic behavior and, most importantly, the simultaneous excavation of the opening and installation, of the liner which rarely corresponds to reality, high to very high liner stresses are produced. What becomes quite clear from these analyses is the importance of modeling the actual ground behavior, which the elastic analysis can do to quite a limited extent only, and to consider the construction procedure with support installation following excavation with a delay. This is done in the elasto-plastic analysis in which a specific material model for Opalinuston with somewhat conservative ground parameters is used together with the realistic condition of delayed support installation. The results of the elasto-plastic analysis show that in most cases liner stresses are in a range that can be handled with normal to high strength concrete. In addition, we also investigate the effect of drained versus undrained conditions behind the liner where the latter as expected lead to higher liner stresses. Finally, the effect of ground stiffness and of permeability on the results is studied. As expected a greater ground stiffness and higher permeabilities produces lower liner stresses. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.