Tunnel Support. Use of Lattice Girders in Sedimentary Rock

Abstract

Icelandic geology is highly influenced by its location on the Mid Atlantic ridge. Changes in geological conditions are therefore frequent in Icelandic tunnels and flexible rock support methods are required to deal with constant variation of rock mass properties. Main focus of this thesis is tunnel excavation and rock support in sedimentary rock mass. Instability problems in thick sedimentary rock layers in the Óshlíð tunnel are discussed. Installed support is evaluated using finite element modelling of the rock mass and bearing capacity calculations of the tunnel lining. Special attention is given to the usage of lattice girders and how it was used as both temporary support to secure safer working environment at the tunnel face and as a part of the final rock support for the tunnel. Good estimation of rock mass properties to construct a reliable finite element model is important. Limited laboratory test data of the sedimentary rock mass turned out to be a problematic factor. A number of uni-axial compression tests were made on core samples from sedimentary layers in the tunnel but no tri-axial tests were executed. That led to very conservative approach in the estimation of the rock mass parameters. Result from the modelling indicates that the tunnel lining used in the Óshlíð tunnel is sufficient if average material parameters are used for the sedimentary rock mass. The rock support although seems to be slightly insufficient if lower limit (90% of test data above) is used for sediment strength. These results emphasise the importance of estimation of rock mass properties in preparation of tunnel constructions.