Feasibility analysis for a radioactive waste repository tunnel

This paper addresses the feasibility of constructing deep tunnels in the Op alinuston rock (Opalinus Clay Shale) formation in Switzerland. Laboratory a nd field tests show that the characteristics of Opalinuston vary over a wid e range, influenced by different geologic conditions, by inherent spatial v ariability even under similar geologic conditions, and by the testing condi tions. Tunnelling feasibility is examined through an initial elastic analys is followed by an elasto-plastic analysis. The objective of the elastic ana lysis is the investigation of the effect of a wide variety of parameters, p articularly 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, o f the liner which rarely corresponds to reality, high to very high liner st resses are produced. What becomes quite clear from these analyses is the im portance of modeling the actual ground behavior, which the elastic analysis can do to quite a limited extent only, and to consider the construction pr ocedure with support installation following excavation with a delay. This i s done in the elasto-plastic analysis in which a specific material model fo r 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 i n a range that can be handled with normal to high strength concrete. In add ition, we also investigate the effect of drained versus undrained condition s behind the liner where the latter as expected lead to higher liner stress es. Finally, the effect of ground stiffness and of permeability on the resu lts is studied. As expected a greater ground stiffness and higher permeabil ities produces lower liner stresses. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.