TUNNEL ENGINEERING

A safety screen placed in the upper part of the
tunnel near the heading will trap air above this
safety walk in case of flooding and permit workers
to escape. Some safety laws require installation of
two worker locks.
A special decompression chamber capable of
accommodating an entire shift of workers should
be available when decompression time required is
more than 75 min. A passageway should be
provided to give workers in a man lock access to
the special chamber.
Safety and Health n For all compressed-air
work, a well-equipped first-aid station and
decompression chamber are required, staffed by a
trained attendant at all times. A physician must be
available at all times for emergency calls while
work is in progress.
Most states and countries have laws regulating
the working hours and locking rates for compressed-air
work. Regulations of the U.S. Occupational
Safety and Health Administration for
work in compressed air as well as for construction
in general and all underground operations should
be observed. (See also Arts. 20.6, 20.8, and 20.15.)
OSHA requires that a record be kept outside
worker locks of the time in each shift that workers
spend in compression and decompression. A copy
should be given to the supervising physician.
During the first minute of compression in a lock,
pressure may be increased up to 3 psig and should
be held at that level, and again at 7 psig, long
enough to determine if anyone in the lock is being
adversely affected. After the first minute, pressure
may be raised gradually at a rate up to 10 psi/min.
If personnel experience discomfort, pressure
should be reduced to atmospheric and distressed
personnel should be evacuated from the lock.
Except in emergencies, pressure in a lock should
not exceed 50 psi. Temperature in a lock should be
at least 70 8F but not more than 90 8F., whereas
temperature in compressed-air working areas
should not exceed 85 8F.
Unless air pressure in the working chamber is
less than 12 psig, decompression in a worker lock
should be automatic. Manual controls, however,
should be provided inside and outside the lock to
override the automatic mechanism in emergencies.
The lock should have a window at least 4 in in
diameter to permit observation of the occupants
TUNNEL ENGINEERING
Tunnel Engineering n 20.35
from the working chamber and free-air side of
the lock.
OSHA requires also that at least 30 ft 3 /min of
ventilation air be supplied per worker in the
working chamber. In addition, OSHA specifies that
at least 10 ft-c be provided by electric lights on
walkways, ladders, stairways, or working levels.
Two independent supply sources should be used,
including an emergency source that becomes
operative if the regular source should fail. External
parts of electrical equipment, including lighting
fixtures, when installed within 8 ft of the floor,
should be made of grounded metal or noncombustible,
nonabsorptive, insulating material.
OSHA also requires that sanitary, comfortable
dressing and drying rooms be provided for
workers employed in compressed air. Facilities
should include at least one shower for every 10
workers and at least one toilet for every 15 workers.
Fire-fighting equipment should be available at all
times in working chambers and worker locks.
OSHA requirements for total decompression
time, which depends on the air pressure in the
working chamber and the time of worker exposure
to that pressure, are listed in Table 20.3. Decompression
should take place in two or more stages,
but not more than four. (Four stages are required
for pressures of 40 psig or more.) In Stage 1,
pressure may be reduced at a rate up to 5 psi/min
from 10 to 16 psi, but not to less than 4 psig. In later
stages, the rate of pressure reduction may not
exceed 1 psi/min. Local rules, however, also
should be checked. Limits of union contracts,
though, are sometimes more stringent than legal
requirements.
The amount of air required for compressed-air
tunneling depends on so many variables that exact
rules cannot be given. To determine the size of the
compressor plant for a given job requires a great
deal of judgment by the engineer, based on past
experience. Low-pressure machines are installed
for the tunnel air and high-pressure units for air
tools. Adequate standby capacity must be provided
by using a number of compressors. High-pressure
air may be used as an emergency tunnel supply by
interconnecting the compressors through reducing
valves.
Shieldless Tunneling n Some tunnels have
been built in water-bearing ground by using
compressed air in conjunction with liner plates,
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