TUNNEL ENGINEERING

Fig. 20.3 Clearance diagram for San Francisco Bay Area Rapid Transit System.
tunnels to provide overhead lane signals to show
which lanes are open to traffic in the direction of
travel, so extra overhead allowance is required for
these, and when appropriate also for lighting,
Fig. 20.4 Clearance diagram for interstate highway
tunnels.
TUNNEL ENGINEERING
Tunnel Engineering n 20.5
overhead signs, jet fans for ventilation, and any
other ceiling-mounted items. Minimum overhead
traffic clearances depend upon which alternative
routes are available for over-height vehicles and
the classification of the highway, but accepted
values usually lie between 14 ft and 5.1 m.
Additional height may be required on vertical
curves to allow for long trucks. Additional space
may be required for ventilation, ventilation equipment,
and ventilation ducts.
20.3 Alignment and Grades
for Tunnels
Alignment of a tunnel, both horizontal and vertical,
generally consists of straight lines connected by
curves. Minimum grades are established to ensure
adequate drainage. Maximum grades depend on
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