Civil Engineering Reference
In-Depth Information
often the wrong shape for tunnel elements as they narrow towards the
bottom. In assessing whether a dry dock has sufficient depth, allowance
has to be made for a foundation layer beneath the elements to create the
required profile to the underside of the tunnel, to protect the dock floor,
and to permit water percolation under the element for float up. Docks
that are available are often disused and in need of refurbishment. This is
particularly true of the dock gate. The water depth over the cill must be
checked to ensure it is sufficient to float out the tunnel element as well
as the water depth in the channel outside the dock. Substantial reha-
bilitation and modification may therefore be required. In some cases,
planning consent may be needed. Nevertheless, large flat-bottomed dry
docks do exist, and if one can be found, it is likely to have access to
services and be surrounded by working areas for plant and materials.
There are a number of recent examples where docks have successfully
been used, including the New Tyne Crossing in the United Kingdom,
the Marmaray Tunnel in Istanbul, and the Second Midtown Tunnel in
Virginia.
Large concrete tunnel elements have not so far been built on slipways.
The elements are very heavy and it would be difficult to control the move-
ment as they are launched. Launching of an element would need to be car-
ried out in a very controlled manner to avoid overstressing the structure
when it is part in and part out of the water and subject to dynamic loads.
The tolerances of the sliding surface for launching are also likely to be
very tight, to avoid imposing any unacceptable distortions into the concrete
structure. Consideration has been given to using water skids or multiwheel
self-leveling platforms to place the element in the water, but this has never
been done on large tunnel elements although such techniques have been
used on smaller utility tunnel elements.
Construction of concrete tunnel elements
Construction of a monolithic concrete tunnel element is typically carried
out in 12 m bays. This length stemmed originally from the 12 m length of
reinforcement bars that were available. This constraint no longer applies
as longer bars are available, but 12 m length pours are still widely used.
The section is built up from a number of individual pours—first the base,
then the external and internal walls, and finally, the roof. Typically, the
concreting proceeds sequentially from one end of the element to the other,
although in some circumstances, other sequences may be used to reduce
the effects of induced deformations. On many tunnels, the outer walls and
roof slab are combined into a single concrete pour to reduce the number of
construction joints in the outer perimeter of the structure. The practical-
ity of this must always be considered according to the size of the concrete
pour that this creates. The joints between the pours are normal reinforced
