TUNNEL ENGINEERING

from larger fires. Supply semi transverse ventilation
should be operated in a reversed mode for
the emergency so that fresh air enters the tunnel
through the portals to create a longitudinal flow
of air equivalent to the critical velocity. It also
provides a tenable environment for fire-fighting
efforts and emergency egress.
Exhaust semi transverse ventilation installed in
a unidirectional tunnel produces a maximum
contaminant concentration at the exit portal. In a
bi-directional tunnel, the maximum level of contaminants
is located near the center of the tunnel.
In a fire emergency both the exhaust semi
transverse ventilation system and the reversed
semi transverse supply system create a longitudinal
air velocity in the tunnel roadway thus
extracting smoke and hot gases uniformly along
the tunnel length.
Full Transverse Ventilation n Full transverse
ventilation has been used in extremely long
tunnels and in tunnels with heavy traffic volume.
Full transverse ventilation includes both a supply
duct and an exhaust duct to achieve uniform
distribution of supply air and uniform collection of
vitiated air throughout the tunnel length. During a
fire emergency the exhaust system in the incident
zone should be operated at the highest available
capacity while the supply system in the adjacent
incident zone is operated. This mode of operation
creates a longitudinal airflow (achieving the critical
velocity) towards the incident zone and allows the
smoke and heated gases to be extracted as close as
possible to the fire and keep the upstream stopped
traffic clear of smoke.
Other Ventilation Systems n There are
many variations and combinations of the systems
described previously. Most of the hybrid systems
are configured to solve a particular problem faced
in the development and planning of the specific
tunnel, such as excessive air contaminants exiting
at the portal(s).
Ventilation System Enhancements n A
few enhancements are available for the systems
described previously. The two major enhancements
are single point extraction and oversized exhaust
ports.
Single point extraction is an enhancement to a
transverse system that adds large openings to the
TUNNEL ENGINEERING
Tunnel Engineering n 20.11
exhaust duct. These openings include devices that
can be operated during a fire emergency to extract
a large volume of smoke as close to the fire source
as possible. Tests conducted as a part of the
Memorial Tunnel Fire Ventilation Test Program
concluded that this concept is extremely effective
in reducing the temperature and smoke in the
tunnel. The size of openings tested ranged from 100
to 300 ft 2 (9.3 to 28 m 2 ).
Oversized exhaust ports are simply an expansion
of the standard exhaust port installed in the
exhaust duct of a transverse or semi-transverse
ventilation system. Two methods are used to create
such a configuration. One is to install on each port
expansion a damper with a fusible link; the other
uses a material that when heated to a specific
temperature melts and opens the airway. Several
tests of such meltable material were conducted as
part of the Memorial Tunnel Fire Ventilation Test
Program but with limited success.
20.6.5 Elements of Road Tunnel
Ventilation Systems
Major components of ventilation systems commonly
used for road tunnels are described in the
following.
Ventilation Buildings (Figs. 20.5 and
20.6) n Fans, electrical transformers and switchgear,
control board, and auxiliary equipment are
housed in ventilation buildings. In short- and
medium-length tunnels, one building at either
portal is sufficient. Longer tunnels should have a
building at each portal. A few of the longest have
three or four buildings. For underwater tunnels,
ventilation buildings may be at the water’s edge,
each building controlling a land and a river section
of the tunnel.
Fresh air is taken in through large louver areas
in the walls of the building. The louvers should be
protected by bird screens. Louvers are usually
aluminum and arranged for shedding water.
Adequate drains should be provided in the fan
room to remove rainwater, which may blow in
through the louvers. Vitiated air is discharged
through vertical stacks, which also should be
covered by screens.
Tunnel Ducts are usually of constant area
throughout their length. Concrete surfaces should
be smooth for minimum friction. Obstructions,
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