TUNNEL ENGINEERING

20.10 n Section Twenty
Smoke from a fire in a tunnel with only natural
ventilation moves up the grade driven primarily by
the buoyant effect of the hot smoke and gases. The
steeper the grade the faster the smoke will move
thus restricting the ability of motorists trapped
between the incident and the portal at the higher
elevation to evacuate the tunnel safely.
Mechanical Ventilation n A tunnel that is
sufficiently long, has heavy traffic flow, or
experiences adverse atmospheric conditions
requires mechanical ventilation with fans. Mechanical
ventilation layouts in road tunnels are either
of the longitudinal or transverse type.
Longitudinal Ventilation n This type of
ventilation introduces or removes air from the
tunnel at a limited number of points, thus creating
a longitudinal flow of air along the roadway.
Ventilation is either by injection, or by jet fans.
Injection Longitudinal Ventilation is frequently
used in rail tunnels and is also found in
road tunnels. Air injected at one end of the tunnel
mixes with air brought in by the piston effect of the
incoming traffic. This type of ventilation is most
effective where traffic is unidirectional. The air
speed remains uniform throughout the tunnel, and
the concentration of contaminants increases from
zero at the entrance to a maximum at the exit.
Injection longitudinal ventilation with the supply
at a limited number of locations in the tunnel
is economical because it requires the least number
of fans, places the least operating burden
on these fans, and requires no distribution air
ducts.
Jet Fan Longitudinal Ventilation has been
installed in a significant number of tunnels worldwide.
Longitudinal ventilation is achieved with
specially designed axial fans (jet fans) mounted at
the tunnel ceiling. Such a system eliminates the
spaces needed to house ventilation fans in a
separate structure or ventilation building; however,
it may require a tunnel of greater height or width to
accommodate jet fans so that they are out of the
tunnel’s dynamic clearance envelope. This envelope,
formed by the vertical and horizontal planes
surrounding the roadway pavement in a tunnel,
define the maximum limits of predicted vertical
and lateral movement of vehicles traveling on the
road at design speed. As the length of the tunnel
increases, however, the disadvantages of longi-
TUNNEL ENGINEERING
tudinal systems, such as excessive air speed in the
roadway and smoke being drawn the entire length
of the roadway during an emergency, become
apparent.
The longitudinal form of ventilation is the most
effective method of smoke control in a road tunnel
with unidirectional traffic as was determined in the
Memorial Tunnel Fire Ventilation Test Program. A
longitudinal ventilation system must generate
sufficient longitudinal air velocity to prevent the
backlayering of smoke. Backlayering is the movement
of smoke and hot gases contrary to the
direction of the ventilation airflow in the tunnel
roadway. The air velocity necessary to prevent
backlayering of smoke over the stalled motor
vehicles is the minimum velocity needed for smoke
control in a longitudinal ventilation system and is
known as the critical velocity.
Transverse Ventilation n Transverse ventilation
includes systems that distribute supply air
and collect exhaust air uniformly along the length
of the tunnel. There are several such systems
including the full transverse system which
includes both supply and exhaust air uniformly
distributed and collected. The semi- or partial
transverse systems incorporate only one, either
supply or exhaust air.
Semi transverse ventilation can be configured
as either a supply system or an exhaust system.
Semi transverse ventilation is normally used in
tunnels up to about 7,000 feet (2,000 meters);
beyond that length the tunnel air velocity speed
near the portals may become excessive.
Supply semi transverse ventilation applied to
a tunnel with bi-directional traffic produces a
uniform level of contaminants throughout the
tunnel because the air and the vehicle exhaust
gases enter the roadway area at the same uniform
rate. In a tunnel with unidirectional traffic,
additional airflow is generated in the roadway by
the movement of the vehicles, thus reducing the
contaminant level in portions of the tunnel.
Because the tunnel airflow is fan-generated, this
type of ventilation is not adversely affected by
atmospheric conditions. The supply air travels the
length of the tunnel in a tunnel duct fitted with
supply outlets spaced at predetermined distances.
If a fire occurs in the tunnel, the supply air initially
dilutes the smoke, which was shown in the
Memorial Tunnel Fire Ventilation Test Program to
be an ineffective method for controlling smoke
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