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Tugun Bypass Tunnel Using Top-Down Cut-and-Cover Method


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Author J Hsi, S Lambert and M Thomas


The Tugun Bypass Tunnel in Gold Coast, Australia was
constructed using diaphragm walls with the top-down cut-and-cover method to
allow simultaneous construction of an airport runway extension above the tunnel,
whilst excavation of the tunnel continued underneath. The tunnel was built in an
environment of high groundwater table and deep deposits of alluvial and
estuarine soils with the toes of the walls founded in soil deposits. There was a
potential risk for differential settlements to occur between the diaphragm wall
panels, caused by the runway fill placed over the tunnel roof during excavation.
Dewatering within the diaphragm walls was required to facilitate the
construction of the tunnel. The tunnel was also built in an area where
environmental considerations were of great importance. Three-dimensional
numerical modelling was undertaken to predict the differential settlements of
the tunnel with considerations of varying subsurface profile, staged excavation
and dewatering, non- uniform surface loading and complex soil-structure
interaction. Field instrumentation and monitoring was implemented to confirm
numerical predictions.