The tunnel had to be constructed in soft saturated
river delta material called Coode Island Silt. This
silt extends to approximately 20 meters below the
surface. This meant that special design and
construction techniques had to be adopted. The final
design solution was based on the use of sheet piles
driven into the soft ground with the use of reinforced
concrete to form the roof and floor while the internal
walls were constructed using structural shotcrete.
Most of the sheet piles were driven to a depth of 15m.
Every sixth sheet was driven to refusal in underlying
dense granular material, called Moray Street Gravel,
at depths of 20-25m to support the tunnel. A patented
beam called a Bonacci Beam® consisting of steel shells
placed at 3.6m centres and a metal tray deck spanning
between these steel shells was adopted to support the
roof loads unpropped over 11m.
This steelwork also acted as formwork for the concrete
placed on top of it. Bulk excavation to the final
depth within the tunnel was carried out in 3.6m
'drives' progressively installing structural steel
props to control the lateral movement of the sheet
piles. After a drainage layer was laid down over the
props, the permanent concrete floors were constructed,
followed by the spraying of concrete walls to line the
sheet piles. In some areas the active sides of the
excavations adjacent to the cantilevered sheet piles
were "laid back" to reduce the lateral load on the
sheet piles. In other areas "dead men" anchors were
installed to restrain the high cantilevered sections
of sheet pile wall.
Geotech completed the works well ahead of time and
completed all the civil construction activities
without the need to engage sub-contactors.
