TUNNEL ENGINEERING

The ventilation system also must be capable of
controlling smoke and hot gases in case of fire (see
Ventilation Systems for Road Tunnels following).
The Federal government or health authorities of
states place restrictions on permissible carbon
monoxide (CO) content. With new standards
limiting contaminants in vehicle exhaust gases,
however, it may eventually be possible to meet the
CO limitations without extensive increase in
ventilation. Engineers should check current rules
at time of design.
Haze from vehicle exhaust gases, particularly
from Diesel engined vehicles, does reduce visibility
in the tunnel. In practice, when the CO level within
the tunnel is maintained at levels as proposed in
Table 20.1, adequate dilution of the irritating parts
of exhaust gases and adequate visibility is assured.
New road tunnels built in the United States
must comply with the time-weighted limits for
concentration of CO established by the U.S.
Environmental Protection Agency and the Federal
Highway Administration. These limits are listed in
Table 20.1.
Other countries may set other standards for
carbon monoxide (CO) concentrations within their
tunnels. The World Road Association (PIARC)
publications provide documentation on the
subject.
For tunnels in which traffic may incorporate a
high percentage (10% or more) of diesel vehicles,
the ventilation requirements for dilution of NO x
particles of nitrogen and particulates (smoke)
become significant. The NOx emitted by vehicles
consists mainly of nitric oxide (NO), which
oxidizes in the atmosphere to form nitrogen
dioxide (NO 2). Based on exposure limits recommended
by the American Conference of
Governmental Industrial Hygienists and a typical
4-to-1 ratio for NO to NO2, the maximum
permissible concentration of NO x is about 10 ppm.
Table 20.1 Limits on CO in Road Tunnels
Exposure time,
min
Maximum CO
concentration, ppm
0–15 120
16–30 65
31–41 45
46–60 35
TUNNEL ENGINEERING
Tunnel Engineering n 20.9
Carbon Monoxide (CO) has been proven to be
the umbrella pollutant in most road tunnels. That
is, when the CO level in any given road tunnel is
maintained at or below the levels shown in Table
20.1, all of the other vehicle pollutants will be
within appropriate levels. The only exception to
this is the case of particulate matter emitted by
Diesel engined vehicles when the tunnel traffic
stream contains on an average more than 15%
Diesel engined vehicles.
The current method of determining the vehicle
emissions to be considered for a road tunnel
ventilation system design is to apply the United
Stated Environmental Protection Agency’s MOBIL
series of computer programs. Mobil5B is the
current version in use today.
20.6.4 Ventilation Systems for
Road Tunnels
In straight tunnels up to about 1000 ft in length,
natural air flow is usually sufficient, particularly
with traffic in one direction. If a tunnel is exposed
to heavy traffic congestion at times, installation of
exhaust fans in a shaft or adit near the center for
emergency ventilation is advisable if the length
exceeds 500 ft.
Natural Ventilation n Naturally ventilated
tunnels rely primarily on atmospheric conditions
to maintain airflow and a satisfactory environment
in the tunnel. The piston effect of traffic provides
additional airflow when the traffic is moving.
Naturally ventilated tunnels over 1,000 feet (305
meters) long require emergency mechanical ventilation
to extract smoke and hot gases generated
during a fire as defined by NFPA 502 “Standard for
Road Tunnels, Bridges, and Other Limited Access
Highways”. Tunnels with lengths between 800 and
1,000 feet (240 and 305 meters) will require the
performance of an engineering analysis to determine
the need for emergency ventilation. Because
of the uncertainties of natural ventilation,
especially the effect of adverse meteorological and
operating conditions, reliance on natural ventilation,
to maintain carbon monoxide (CO) levels,
for tunnels over 800 ft (240 m) long should be
thoroughly evaluated. If the natural ventilation is
demonstrated to be inadequate, the installation of a
mechanical system with fans should be considered
for normal operations.
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