Big diameter tunnelling beneath low rock cover / Bau eines Großtunnels unter geringer Felsüberdeckung

The Clem Jones Tunnel (former North-South Bypass Tunnel) is the first project to get underway as part of Brisbane's transport plan which aims to improve the urban road network. The total length to be excavated is 6.8 km which includes 4.8 km of driven tunnel and associated road connections. Construction commenced in August 2006 with project completion targeted well before the contractual completion date of October 2010. Due to the local geology, in particular the hard Brisbane tuff and Neranleigh-Fernvale formation a combination of tunnel excavation methods are used. The majority of the tunnel is excavated by two tunnel boring machines in rock having a compressive strength of between 80 and 150 MPa. Ten roadheaders are excavating the remaining tunnels such as ramps, access tunnels, cross passages and merges.
A major challenge arose during the planning of the initial mainline excavation underneath the Royal National Association Showground in Bowen Hills. This section contains low rock cover and historical maps indicate that the Showground is located within a former topographic depression (alluvial valley) where a creek once flowed through. Due to traffic merge design requirements, both TBM and roadheader excavation methods were required to be used in this challenging geological profile. The answer was to stabilise the alluvium above both TBM section tunnels from the surface prior to excavation and to operate the TBM in single shield mode with immediate grouting of annular void from the tail shield. In addition, the roadheader section of tunnel required additional support by spiles and canopy tubes installed from the tunnel face.