Tunnel geometries and the relation with the in-situ stresses in the Stockholm region. Case study of the special tunnel geometries in the Stockholm metro - Blue line extension.

The new Blue line extension is one of the key parts of Stockholm's metro extension project. The existing blue line will be extended southwards from Kungsträdgården to Nacka and the southern suburbs. The new tunnels are going to be excavated through igneous and metamorphic rock masses using the Drill and Blast method and SCL (Sprayed concrete lining). It is well known that the Stockholm basin can carry high residual tectonic stresses as a consequence of its Glacial past. The influence of the in-situ 3D stresses tensor in the tunnel design is explicitly stated in the design requirements for tunnels in rock and the challenge is how to transfer the general assessment into a specific localization: the size and the geometry of the tunnel in relation to the stress field presented in the rock mass will govern this stress redistribution, having a key importance in the stability of the excavation. Along the alignment of the new Stockholm metro project, two areas of special interest in terms of nonconventional tunnel geometries and in-situ rock stresses can be identified: the required geometry modifications of the tunnels in the connection with the existing Kungsträdgården station to be adapted to the new alignment, where narrow tunnels with wall heights up to 14m are required, and the large caverns for station platforms and operational switch gear systems, with spans up to 31m in several rock conditions. A summary of these and the design approaches followed in the detailed design is included in this paper. [ABSTRACT FROM AUTHOR]