Civil Engineering Reference
In-Depth Information
three days after the initial placing of the concrete. As an alternative to
cooling, it is possible to pre-heat concrete at construction joints to avoid
the temperature differential across the joint. However, this technique is less
common and infrequently used for immersed tunnels.
Full section casting
A technique that has developed significantly as a result of the Øresund
Tunnel is the full section casting technique. This is also described in
Chapter 14, and entails pouring the concrete for a single tunnel segment in
one continuous operation, starting at the base slab and continuing up the
walls and completing the roof slab without any construction joints. This
technique has the great merit of avoiding any restraint in the tunnel struc-
ture due to construction joints, which are the primary cause of through-
section cracking. Cast-in cooling pipes are therefore not required. The
method was first developed for small-section utility tunnels, notably in
the Netherlands, and later for power station outfall tunnels, such as the
ones constructed for the Sizewell power station in the United Kingdom
and Pulau Seraya and Tuas Bay cable tunnels in Singapore. For these
projects, the small tunnel sections were cast on end, and once the concrete
had gained sufficient strength, they were lifted, rotated, and assembled
together to form a tunnel element. The short segments were permanently
prestressed together.
The Øresund Tunnel was the first large transport tunnel to use this tech-
nique. This was not just a scaled-up version of the method used for util-
ity tunnels. It used the same principle for the casting of concrete in one
continuous concrete pour, but had to use a different process for assembling
tunnel segments into a tunnel element. As the tunnel was some 40 m wide,
the weight was such that it was not possible to lift and rotate the tunnel
segments. Therefore, they could not be cast on end and had to be cast in the
correct orientation and slid together to form a tunnel element. Even with
the full section casting method, there needed to be strict control on the
ambient temperature conditions, particularly in the Scandinavian winter,
when very low air temperatures would cause a problem with thermal gra-
dients through the structural sections. A casting factory was constructed
in this instance, with curing tents to protect the tunnel segments from cold
temperatures as they were slid out of the factory after casting.
The technique was 100% successful in eliminating early age cracks due
to thermal effects and set a new benchmark in the quality of concrete con-
struction that is possible to achieve for immersed tunnels. The technique
was subsequently used for the Busan Tunnel in South Korea, although it
was adapted there such that the tunnel segments did not need to be slid,
as the warmer climate permitted construction in situ with a lighter curing
tent. Again, the technique was successful in providing high-quality concrete
