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Big Diameter Tunnelling Beneath Low Rock Cover


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Author D Meyer, F Brodbeck and N McKenzie


The 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
which have a rate of progress of 16 – 20 m per day in rock having a compressive
strength of between 80 and 150 MPa. Ten roadheaders operate at a rate of up to 2
m per day excavating the remaining tunnels, such as ramps, access tunnels, cross
passages and merges.

The tunnel
team are managing the challenges and complexities associated with tunnelling
through the hard Brisbane rock. 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), which a creek once flowed through.
Due to traffic merge design requirements, both tunnel
boring machine (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