Civil Engineering Reference
In-Depth Information
Transporting steel tunnels generally involves a two-stage tow.
First, the steel shell is taken from the fabrication yard to the concret-
ing facility. Then, the completed element is taken to the tunnel site. For
the first stage, the element is relatively light. It will normally have some
ballast concrete cast inside to act as a keel, but even so, it will only
have a draught of about 3 m. In this condition, it can be carried on
a semi-submersible barge as an alternative to towing, as illustrated in
Figure 14.24. Loading the element onto the barge is a critical opera-
tion to ensure that the element is not overstressed and that the combina-
tion of barge and element remains stable at all times. Steel elements can
also be towed large distances in their launched condition. Here, wind-
age must also be taken into consideration when determining the tow-
ing forces—something that does not really apply to the low freeboard
concrete elements.
Once the steel element has been concreted, its behavior is then similar to
that of a concrete element and the towed journey to the tunnel site is simi-
lar. If the tow is a short distance in sheltered water, it may well be sufficient
to use just one tug at the head of the element with a small multicat-type
craft in attendance to assist with control of the element. Pushboats are also
used to move elements in inland waterways.
Figure 14.24 Steel shell element being transported.
(Photo courtesy of W. Grantz.)
