Page 460 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.60 THE WORK

Drilling. The standard tool for small tunnel drilling has been the drifter, a medium-weight hand
drill with a hand or automatic feed, mounted on a vertical column or a horizontal bar of such
length that it can be secured between the floor and roof, or between the sides, by screw-jack ends.
Because of the weight of the columns, they become impractical for full-face work in tunnels of
greater cross section than 10 10 feet.
The drifter permits the drill crew to resume work on the top of the face as soon as blast fumes
have cleared away, with the drill operators standing on the pile of muck until it is dug away. They can
drill the bottom after it has been cleared.
Larger tunnels may be done by the heading-and-bench method shown in Fig. 9.49. This permits
the use of drifters on short columns for the advance, and approximately vertical jackhammer or
wagon drilling for the bench. Sometimes the heading is extended far ahead of the bench, and has its
own hauling equipment that dumps over the bench face into other cars, or into a pile to be dug away.
For larger tunnels to be drilled and blasted, the standard method is to use a drill carriage (jumbo)
on which power feed drills can be mounted so as to reach all parts of the face at correct angle and to
correct depth. Each drill usually does several holes. It can be positioned by hand, or by mechanical,
air, or hydraulic controls. Such jumbos may be so constructed as to straddle hauling equipment, so
that it need not interfere with removal of muck. They may also carry a cherry picker crane to pick
up empty cars to switch loaded ones through. They are backed away from the face before each blast.
On very large tunnels jumbos may be used on both levels of heading-and-bench work.
Usual drilling depth is 10 to 12 feet, but in any case is seldom deeper than two-thirds the smallest
dimension of the tunnel.
Figure 9.50 shows the typical full-face drilling patterns.
Bits. Recently tunnel drilling has been partly standardized to use steels threaded to carry detachable
bits. These may be multiuse types that can be sharpened by grinding, or sharpened and reshaped by hot
milling; one-use or throwaway bits that are discarded when dull; and carbide insert bits. The carbide insert
bit has caused a spectacular advance in speed and ease of hardrock tunneling. Carbide outwears steel at
an average of about 100 to 1, and gives much more rapid hard rock penetration. The time of handling,
transporting, and processing bits is reduced from a major to a minor problem.
Loading. Water-resistant explosives with good fume characteristics are desirable in under-
ground work. These qualities are found in gelatin dynamites.
When all holes in a face have been drilled, each is blown out with a high-pressure air jet to
remove loose cuttings and water. Cartridges are slit (unless the explosive has been damaged by
water and the hole is wet) and tamped firmly with a wooden pole. It is common practice to place
the primer after the first cartridge, with the cap pointed toward the collar of the hole.
Stemming may be taken from the drill cuttings. It is most convenient to use if wrapped in paper
bags of the same size as the cartridges. If this material is very high in silica, its use as stemming
might increase the silica in the air enough that prewrapped blanks supplied by powder manufac-
turers might be preferred. There are also wood and rubber plugs that are very satisfactory.
It is good practice to place a wad of paper between the explosive and the stemming, so that the
powder can be easily and safely located in case of a misfire.
There is danger of premature explosion from stray currents. A common precaution is to take
down or “kill” all electric wiring within 500 feet of the face before starting to load. Safety flash-
lights, of hand or cap models, or headlights from a battery locomotive can be used. It is sometimes
a question whether the poor lighting obtained offers as much of a hazard as the electricity would.
Even the complete absence of electricity on the job would not guarantee a tunnel face against
currents, as underground water is often highly mineralized and will conduct a charge for long dis-
tances. Metallic ores may be excellent conductors.
The precautions described earlier for blasting in the presence of electrical hazards should be
followed.

Firing. Any wiring hookup can be used—series, parallel, or parallel series, depending on the
preference of the blaster. If 440-volt electricity is available, it is preferred for firing, although 220
or even 110 will do. Regular blasting machines are also used, but they should not be kept in the
tunnel when not in use, because of possible damage from dampness.