Page 451 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.51

There is such constant improvement in tunnel-driving techniques, and increase in confidence
to undertake bigger projects, that some or all of these records may have been surpassed by the
time this book is in print.
Plant. The plant at a tunnel may include the tower, hoist, and hopper; compressors, low-pressure
ventilation system, water pumps, electric transformers or generators, change rooms with showers
and lockers, provision for emergency treatment of injuries, a blacksmith, forging, and bit dressing
shop; welding and repair equipment; and telephone or radio communication systems.
Compressors are usually at the surface. They are usually of the two-stage type, and have an after-
cooler as well as an intercooler, to avoid transporting any heat of compression into the heading,
which is often too hot already.
Alternating current is used. When possible, it is purchased from a utility. It is usually stepped down
to 220 or 110 volts at the entrance, but on some jobs is taken in at several thousand volts, in armored park-
way three-wire cable. Dry transformers (oil-filled ones are a fire hazard underground) are set about 1,000
feet back from the faces, and advanced in long jumps as progress warrants. This system avoids the power
loss and voltage drop associated with long-distance transmission of low-voltage current.
There may be three electric circuits in the tunnel, a 220- or 440-volt for power, a 110-volt for
light, and a high-voltage line for firing explosives. Some operators standardize on 220 for both
lighting and power. Sometimes 220-watt bulbs are a nuisance to get in the United States, but they
have the advantage of being useless in an ordinary lighting circuit, so they are seldom pilfered.
No drill dust can be tolerated. It may be suppressed by detergent or foam, or drowned in wet
drills with water supplied through pipes from outside the tunnel.
Surveying. Tunnel sections meet each other far from their portals or shafts, sometimes after curves,
with uncanny precision. Differences usually vary from a small fraction of an inch up to several inches.
These are too small to be noticed on the walls, but are measured at the surveyed centerline (axis).
An underground direction is obtained by establishing a baseline at the surface, running close to
the line of the tunnel. This is very carefully done, and it is marked at frequent intervals by permanent
monuments, with exact points pricked into metallic bolts embedded in concrete.
Two plumb bobs weighing 20 to 30 pounds each are suspended close to the bottom of the shaft by
piano wire from the surface. They are as far apart as shaft width permits. Vibration and tendency
to swing may be dampened by hanging them in pails of water. Very careful observations are taken
of the wires at the surface, relative to the proposed tunnel centerline. Direction is identical with
that of the same wires at the bottom.
Careful observations are taken of the bottom part of wires, using a very accurate instrument and
special sighting devices. Readings are taken over and over, and the results averaged. The tunnel
line is then established in the correct direction, by reference to surface readings.
This work must be done at a time when workers and equipment are not working, as ventilating
currents and vibration can disturb the wires.
The line is extended through the tunnel by laser beam and/or transit, and marked on spads
(markers) driven into holes drilled in the roof.

Exploration. Tunnels are seldom driven blind. Preliminary drilling is done along the route to
determine the type of rock, the amount of water to be expected, and the danger of mud slides. Test holes
are drilled from the surface, usually with diamond drills that can bring up cores for inspection.
Diamond drilling may also be done from the heading, where dangerous conditions are expected.
This precaution has often revealed the presence of such quantities of water or unstable soil ahead,
that disaster might have resulted had it been broken into by a full-face blast.
Figure 9.42 shows extensive core drilling that was done for a sewer tunnel under the East
River, New York City, in order to find a way to avoid a dangerous seam of decayed rock.
Dangers. Underground work naturally is very dangerous, and it is greatly to the credit of tun-
nel workers and labor departments that there are so few accidents.
The most evident danger is that of collapse. Most soils and many rock formations will slump
rather quickly into any hole cut under them. In any given material, this tendency increases markedly