TUNNEL ENGINEERING

interior. The length of each of these zones should
be approximately one safe-stopping sight-distance
(SSSD) at design speed. Reduction between zones
should not exceed 3:1.
At night, a pavement luminance of 2–5 cd/m 2
minimum is recommended for the entire length of
the tunnel. The approach and exit roadways should
have a luminance level of no less than one third the
tunnel interior level for a distance of a SSSD.
There are four viable types of light sources used
in tunnels, fluorescent, low-pressure sodium (LPS),
high-pressure sodium (HPS), and metal halide
(MH). The advantages and disadvantages of each
are discussed in greater detail in ANSI/IESNA RP-
22-96 8.1. These include restrike time in the event of
momentary power interruption, linearity of source
to reduce flicker, cost, color rendering, lamp size,
lamp efficacy, control of light distribution, effects of
air temperature, lumen depreciation with time,
glare, the risk of lamp rupture, and keeping
enclosures dust-tight and water tight. Florescent
lamps frequently provide the lower illumination
levels, combined with LPS at threshold and
transition zones. Lower wattage LPS sources are
also used in interior zones. HPS and MH lamps
come in a wide selection of sizes, better lamp life,
compact size and are easily optically controlled.
20.9 Tunnel Drainage
Most tunnels through hills and mountains have
water problems. Surface water penetrates through
fissures and percolates through permeable soils.
Attempts to seal off the rock by grouting, with
either cement or chemicals, usually are not
completely successful since very high pressures
may build up even if flows are low. Cast-in-place
concrete linings may not be completely watertight.
Water may find its way through shrinkage cracks in
the linings into the interior of tunnels. There, it may
freeze in cold weather and produce an unsightly
appearance, objectionable in highway tunnels.
Consequently, provision must be made to drain
water from tunnels.
Fire fighting, washing of tunnel interiors, and
flushing of pavements also introduce water that
must be drained.
Although cut-and-cover tunnels can be waterproofed,
this is difficult with bored tunnels. If the
water problem is not serious, the most economical
solution is to seal cracks in the lining that leak. With
TUNNEL ENGINEERING
Tunnel Engineering n 20.17
good concrete control, the number of these should
be small. It is good practice to design tunnels
assuming that they will leak and therefore provide
appropriate drainage paths.
If water appears in considerable quantity during
rock tunneling operations, tight steel lagging over
the tunnel supports and grouting may prevent
leakage. In serious cases, it may be necessary to
dry-pack between the rock and the tunnel lagging
to drain water. This is a slow, costly method
requiring much manual labor. Dry pack behind the
side walls can easily be placed and is effective in
preventing the buildup of a hydrostatic head
behind the lining. Longitudinal drain pipes should
be installed behind the base of the side walls, with
the laterals at regular intervals leading to the main
drain (this is a large drain installed under the
roadway, for roadway drainage). Water will flow
through the dry packing and into the base drains.
In a rock tunnel, heavy flow of water coming
through a drill hole indicates a water-bearing fault
or seam. The flow may be stopped by drilling
additional holes and injecting cement grout. Some
holes should be slanted to reach beyond the
periphery. If dense sand or rock flour in the fault
prevents proper penetration of cement grout,
chemical grouting may give satisfactory results.
In special cases, it may be necessary to drill a pilot
hole well ahead of the face to detect severe water
conditions, especially substantial quantities under
heavy pressure. This must be done for rock
tunneling under deep bodies of water.
In highway tunnels, drainage inlets should be
installed at regular intervals along the curbs, with
cross connections to the main drain. The latter
should be of generous size, in longer tunnels
preferably large enough to provide crawl space
to remove silt accumulations, particularly when
grades are near horizontal. Traps at drainage inlets
are undesirable, because of the danger in the event
of a fuel spill.
Leakage in well-constructed underwater tunnels,
either shield-driven or immersed, is usually
minor. It can be controlled by calking joints in
segmental liners or by injecting cracks where leaks
appear. Main sources of water are washing of
tunnel interior, fire fighting, drippings from
vehicles, and rain collected in open approaches.
Pumps are usually sized to handle the full flow
from one fire hydrant.
Continuous open gutters recessed into the curbs
have been used in many subaqueous tunnels. The
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