Civil Engineering Reference
In-Depth Information
the steel and the concrete. The single steel shell arrangement shown in
Figure 3.6 was developed as a design option for the Conwy Tunnel in the
United Kingdom, a dual two-lane road tunnel. The rectangular shell is
25 m wide and 9 m high, and the length of the tunnel elements was chosen
as 110 m.
DOUBLE STEEL SHELL
The cross section of a double steel shell has two steel skins. There is an
inner steel shell, which is thinner than the steel shell of a single shell tunnel
and typically 8 mm thick. This provides strength and watertightness and
is stiffened and constructed in a similar way to the single steel shell form.
This inner steel shell is reinforced by an internal concrete lining that is
designed to act compositely with the steel shell. Around this inner shell is
an outer steel box, referred to as a form plate that forms a void for the bal-
last concrete. This outer form plate is slightly thinner than the inner shell
at typically 6 mm. The inner shell and form plate are connected by a series
of transverse diaphragm plates that compartmentalize the void between the
shells and add stiffness to the structure. The space between the two shells
is filled with concrete, which acts as ballast and also protects the inner steel
shell against corrosion. A typical double steel shell is shown in Figure 3.7
and can be seen under construction in Figure 3.8.
The method of construction is similar to that of the single steel section.
The steel shell is fabricated from a series of modules, then assembled on a
slipway, and launched. It is then towed to an outfitting site and the concrete
placed while the element is afloat. This tunnel form is the direct descendant
of the original Detroit River Tunnel where the outer “shell” was formed
out of timber.
Cap concrete
Form plate
Tremie
concrete
Inner steel
shell
Inner
reinforced
concrete lining
Diaphragm
Stiffeners
Keel concrete
Figure 3.7
Double steel shell.
