Page 446 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.46 THE WORK

Cracks caused by vibration are usually fairly uniform in width with little or no displacement of
the sides relative to each other. Vibration cracks tend to fan out diagonally from corners of windows
and door frames.
Either type of stress frequently causes walls to crack along their junctions with a chimney or
column. Settlement or tilting of the chimney only, or the building only, usually results in an opened
or spread crack on one side, and a closed or even pressure-squeezed crack on the other. Vibration
more often produces equal-width cracks on both sides.
A distinction can sometimes be made on an age basis. New cracks show clean surfaces of material;
older ones become progeressively grimier, the rate depending on conditions. Dust, spot deposits,
and light-bleaching of color pigments are indicators. Recent cracks should be clean and unfaded.
Concussion. Airborne concussion is responsible for a large share of the damage in bombing,
and in accidental detonation of explosives, but is rarely a factor of importance in blast damage. It
consists of one or more waves of highly compressed air moving outward from the explosion.
Sufficient explosive to cause concussion more than a few feet away should not be used in mud-
capping. Even very heavy blasts in solid rock cause little or no concussion if they are laid out and
loaded properly.
Any damage caused by concussion is usually obvious. Glass breakage in closed windows at right
angles to the path of the waves is the most common result. In the absence of extensive glass breakage,
it is very doubtful that any other parts of a structure could be damaged.
If a building is to be endangered by blasting, windows should be opened or removed, particularly
those on the facing and far sides of the house. Store windows may be braced as a routine precaution.
Careful check should be made of the condition of plaster and masonry, so that claims need not be
paid on preexisting defects.

Rock Throw. Unexpected damage may be done by rock or other material thrown through the air by
blasts. In general, shallow blasts, overloaded holes, shots in rock with irregular resistance, and block-
holed boulders give the most trouble in proportion to the amount of powder used.
Thrown objects may cause injury or death, and their control is therefore of first importance.
Property damage may or may not be severe, but at least claims filed on this ground are usually
sincere.
Danger of damage from rock throw may be reduced by increasing the number of holes so that
smaller charges may be used, by sloping holes to throw rock away from danger points; by reducing
the quantity or strength of powder, and handling any resulting oversize fragments by blockholing
under mats or by the use of larger machinery.

Covered Blasts. Throw can be closely controlled by working downward, using small blasts and
covering them with mats or chained logs. If the cover is large and heavy in proportion to the strength
of the explosion, it will prevent any scattering of fragments. If the charge is heavy enough to lift
the cover, it will move somewhat less than the average distance of throw to be expected from an
uncovered blast, and fragments with higher-than-usual velocity will be held in.
It is important that the cover extend several feet beyond the area being shot, particularly if the
charge is heavy enough to lift the mat, as fragments might escape under its edges.
When a power shovel is used to remove the shot rock, it is advantageous to use a woven steel
mat as it is easily handled with chains, and provides a quicker and more secure cover than logs.
The mat is lowered over the holes, or dragged in such a manner that it will not damage the wiring
and cause misfires.
Logs are used when no mat is available, or when there is no machinery on the job which can han-
dle one. They should be long enough to overlap the blast at both sides, and light enough to allow
the crew to carry them by hand. Two chains should be laid on the ground first, the logs piled, and the
chains fastened over them, preferably by wired square knots.
Chaining is important, as unfastened logs may be thrown farther than rocks.
Neither mats nor logs should be laid directly over mudcaps as they are liable to be thrown long
distances and be severely damaged as well.