Page 442 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.42 THE WORK

FIGURE 9.36 Charges for boulder blasting.

deliver such a rapid blow that they will crush objects under them, even when not confined.
However, a study of the table in Fig. 9.36, showing quantities of dynamite used for blockholing
and mudcapping, will show the waste involved in open explosions.
The mud pack over the charge is usually 2 to 6 inches thick. It serves to confine the explosion
slightly, increasing the force exerted on the rock and reducing noise and airborne concussion. Mud
is preferred to any other substance. It is much more effective at confining the explosion than dry
or damp dirt or sand, as it packs and sticks together better. It should be free of stone or pebbles
that would create a hazard by flying long distances.
Charges can be fired on bare rock, but they are less efficient and even noisier.
Mudcapping is wasteful of powder, excessively noisy, and less certain in effect than drill hole
blasting. However, it causes less rock scatter than other methods of shallow blasting, and does not
require the presence of a compressor.
Snakeholing. Boulders are most readily broken if they are lying on the surface of the ground.
If partly buried, the earth or other rock around them provides a support and a vibration-absorbing
cushion that may prevent or reduce breakage.
Embedded boulders may resist machinery that can handle them readily once they have been
loosened up.
Snakeholing consists of making a hole beside or under a boulder, and firing a charge sufficient
to roll it out of the ground, and preferably to break it also. Any further breakage required can then
be accomplished by mudcapping.
Snakeholing is more laborious than mudcapping, but is more economical of powder and is
much less noisy.

MECHANICAL BREAKAGE

Drop Ball. Balls of hard steel, familiarly known as drop balls or skullcrackers, may be carried
by a crane, and used for breaking loose rock. Weights range from 1,000 to 8,000 pounds new.
They wear down in use, and are replaced whenever they become too light for the work they are
expected to do.
The ball is carried on the hoist line of a revolving crane. It is positioned high over the rock to
be broken, the brake is released, and the ball falls almost freely to strike the rock. The brake must
be put on as it hits, to avoid spinning out of cable. Long booms held high increase striking power,
but may reduce accuracy, particularly on windy days.