TUNNEL ENGINEERING

stressed together longitudinally for ease of transportation
and placing. After placing and release of
the segments, each may act as a mini-element free
to move at the segment joints. The ability to use
discrete segments can depend upon subsurface
conditions, acceptable displacements, and sufficient
capacity to resist seismic effects.
Most tunnel elements are cast in bays, similar to
segments, but continuous across the joints. Typically
the floor slab is cast first. The walls and roof
may be cast in either one or two operations. Special
efforts must be made to reduce or preferably
eliminate cracking in the concrete during fabrication.
Testing and repair of leaks should be
completed before submerging elements.
(L.C.F. Ingerslev, “Concrete Immersed Tunnels:
The Design Process,” Immersed Tunnel Techniques,
The Institution of Civil Engineers, Telford, UK, 1989.)
Steel Single-Shell Tunnels n Of the eight
single-shell tunnels with some external curvature,
TUNNEL ENGINEERING
Fig. 20.24 Western Harbour Crossing, Hong kong.
Tunnel Engineering n 20.47
three are for rail in Tokyo, Japan, and three are for
rail in the US including the unique two over two
configuration of the 63 rd Street Tunnel in Manhattan.
The Baytown tunnel in Texas was circular with
two lanes for highway, and the Cross Harbour
Tunnel in Hong Kong is similar but binocular to
give two lanes in each direction. The Detroit River
tunnel (1910) and the Harlem River tunnel (1914)
may not quite fit into this category, being the first
two immersed transportation tunnels ever built,
but do have similarities to single-shell tunnels and
carry rail.
Figure 20.25 shows the cross-section of the San
Francisco Bay Area Rapid Transit (BART) trans-bay
tube with one track in each direction, separated by
an exhaust air duct and a service passage. The 57
elements have a total length of about 19,000 ft. The
steel shell is welded to make it continuous across
the joints, as is the reinforced concrete lining, to
provide security against major earthquake loading.
At the landfall junctions with the ventilation
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