TUNNEL ENGINEERING

deep, as long as risks from ship and submarine
collision can be handled.
(“International Tunneling Association
Immersed and Floating Tunnels Working Group
State-of-the-Art Report,” Second Edition, Pergamon,
April 1997.)
Internal Dimensions n Highway design or
client requirements should determine the required
number of traffic lanes, tracks, or internal spaces. It
is usual to avoid climbing lanes within immersed
tunnels elements themselves, and nominal widths
of emergency lanes or shoulders have almost
always been used to minimize costs. If tunnels are
particularly long, extra width may have to be
provided at intervals to permit emergency stopping.
Curbs, or more usually barriers, are provided
to protect the walls from traffic impact. Barriers
over 2 ft high may make the lane width seem
narrower and slow motorists. Emergency access
into an adjacent tunnel should be available, say at
300 ft intervals, which would require an emergency
“walkway” at least 2 ft wide on top of the adjacent
barrier. Such emergency “cross-passages” may
need to be provided at intervals of say 100 meters.
Extra space may be needed for tunnel and other
utilities, construction and misalignment tolerances,
lighting, lane signs, and highway signs, while
keeping the clearance height as low as possible.
Escape ducts, when provided, should be slightly
over-pressurized relative to adjacent ducts to
prevent entry of noxious fumes. With the minimum
spaces determined, space allowances for any
necessary ventilation system (such as for jet fans
or additional ducts) can then be evaluated. The
critical design case may be for moving or stalled
TUNNEL ENGINEERING
Tunnel Engineering n 20.45
Fig. 20.22 Immersed tunnels are set in a trench, which is then backfilled.
traffic, but a fuel fire usually governs. Permitted
classes of vehicles may be restricted by legislation
or owner requirements to limit it the potential size
of the fire. Finally, additional air space may be
needed for the tunnel elements to be able to just
float when completed with bulkheads in place, and
perhaps space for additional ballast, either inside
or out, to stay submerged when completed and
bulkheads removed. Floating tunnels that rely on
buoyancy must have sufficient compartmentalized
buoyancy to stay afloat in case of accidental
damage to two adjacent compartments.
[L.C.F. Ingerslev et al, Chapter 15B Tunnel
Structures, ‘‘Structural Engineering Handbook,’’
2000 Update for ENGnetBASE, Edited by Wai-Fah
Chen and Lian Duan, CRC Press, 2000 (www.
crcpress.com).]
Construction n The technique of immersed
tunneling is often less risky than bored tunneling,
since tunnel element manufacture can be better
controlled due to the construction of the elements
in a controlled environment in the dry. As a result,
immersed tunnels are nearly always much more
watertight and therefore drier than bored tunnels.
Two main types of tunnel have emerged, known
as steel and concrete. Steel tunnels use structural
steel, usually in the form of stiffened plate, working
compositely with the interior concrete, whereas
concrete tunnels do not, relying on steel reinforcing
bars or prestressing cables. The number of concrete
tunnels is a almost twice that of steel tunnels. Steel
tunnels can have a draft of as little as about 8 ft,
whereas concrete tunnels have a draft of almost the
full depth. Tunnel cross-sections may have flat
sides or curved sides. Historically, concrete tunnels
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