TUNNEL ENGINEERING

tunneled with tunnel boring machines (TBMs). Size
varies from about 7 to 15 ft. Drainage tunnels for
storm water are usually less extensive since they
can discharge into nearest open waters.
The cross section of sewer and drainage tunnels
is usually horseshoe or circular, with concrete
lining. Quality of concrete is of special importance
to resist the detrimental effect of wastewater.
Generally, they are grade tunnels, except for
siphons under rivers, which are under pressure.
A circular or egg-shaped section maintains velocity
at low flow to prevent excessive settling of solids.
Alignment is dictated by location of treatment
plants, soil conditions, and the street plan of the
city. Continuous grades should be maintained
except for siphons. A minimum grade should be
maintained for gravity flow.
20.12 Cut-and-Cover Tunnels
Shallow-depth tunnels, such as rapid-transit lines
under city streets, underpasses, land sections of
underwater tunnels, and end sections of tunnels
through hills, are built by cut-and-cover methods.
A trench is excavated from the surface, within
which a concrete tunnel is constructed. With
bottom-up construction, the completed tunnel is
covered up, and the surface reinstated. With topdown
construction, the walls are constructed first,
perhaps using bentonite slurry in narrow trenches.
The roof is constructed next, backfilled and the
surface reinstated. Excavation and construction of
the floors below roof level then follow, using access
from the ends or from glory holes. Both bottom-up
and top-down construction almost always use castin
place concrete. Depth of invert on subways and
underpasses usually does not exceed 35 to 40 ft. For
connections to subaqueous tunnels, cuts up to
100 ft have been used under special circumstances,
and depths to 60 ft are not uncommon.
The Occupational Safety and Health Administration
(OSHA) sets standards, regulations, and
procedures for protection of personnel during
excavation. OSHA requires that all surface encumbrances
and underground utility installations, such
as sewers, electrical and telephone conduits, and
water pipes, be protected, supported, or removed
as necessary to safeguard the workers. Also,
structural ramps used for access or egress should
be designed by a structural engineer and constructed
in accordance with the design. For trench
TUNNEL ENGINEERING
Tunnel Engineering n 20.19
Fig. 20.7 Taipei Metro Cut-and-Cover.
excavations that are 4 ft or more deep, a stairway,
ladder, or ramp should be provided for egress so as
to require no more than 25 ft of lateral travel for
workers.
Among the measures that OSHA specifies for
safeguarding personnel in excavations are the
following: Precautions should be taken to prevent
exposure of personnel to harmful levels of
atmospheric contaminants (Art. 20.6). If natural
lighting is inadequate for safe working conditions,
illumination to meet OSHA requirements for
excavation should be provided (Art. 20.8). Personnel
should not be allowed to work in excavations in
which water accumulates unless the workers are
protected by safety harnesses and lifelines, water is
being removed to control the water level within
safe limits, and special supports or shields are used
to protect against cave-ins.
To avoid exposure to falling objects, personnel
should not be permitted below loads carried by
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