Technischer Bericht NTB 91-05
StollenversiegelungKonzept und Machbarkeitsnachweis
This report discusses first the aim and purpose of tunnel seals as well as the requirements which should be satisfied. The basic seal concept is a zoned plug consisting of key zones and intermediate zones. The key zones act as barrier elements and will be placed into sections of competent and sound rock. The main function of the intermediate zones is that of a support and the requirements for sealing efficiency may be somewhat relaxed.
Three sealing concepts have been devised for both the key zones and the intermediate zones. They differ in the materials used for the seal and in the placement method. For the key zones highly compacted bentonite is recommended, but also cement-based materials, such as standard concrete or prepact concrete are considered suitable. For the intermediate zones, the use of pumped concrete with subsequent grouting of the roof zone is favourable, but also a combination of concrete with a sand/gravel mixture or with properly compacted excavation material is feasible. The concepts introduced can all be realized by conventional tunnelling methods. Excavation by tunnel boring machine is most advantageous as it minimizes disturbance of the rock caused by the cavity-forming process.
By employing simple material models, it can be shown that the depth of the excavation disturbed zone can be minimized if support of the tunnel is provided as early as possible after excavation. The cutting of a groove in the tunnel wall along the key zone can further contribute to reduce the depth of the excavation disturbed zone.
In order to ensure the quality of a seal, the quantities of the materials used can be checked and the work procedures to place the seal can be supervised. For the latter the experiences obtained from a large-scale test should be available.
Finally, it is also shown that when considering safety analytical aspects, the proposed sealing concepts represent adequate solutions in spite of the probably increased permeability in the excavation disturbed zone. The sealed tunnel is not a preferential pathway when considering the large-scale characteristics of the rock mass surrounding the repository system.