Page 445 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.45

An explosive, if properly used, will expend most of its energy in shattering the rock immedi-
ately around it. The remaining energy will set up waves or vibrations in the ground, and sound and
concussion in the air.
Noise. The noise of an explosion may cause most or all of the neighborhood difficulties. Mudcaps,
shallow blasts, overloaded holes, fractured rock, and other conditions that allow the explosion to
break out into open air before expending its energies, are the cause of complaints all out of pro-
portion to the amount of explosive used.
In the first place, the noise attracts attention to the fact that blasting is going on. It causes the
householder to concentrate on trying to feel the jar or shake of the blast, to look for cracks in plaster,
and to speculate about other damages that might be done. In many cases, the sound of the blasting
will annoy sensitive people so that they will invent or exaggerate physical effects. The contractor
or quarry operator’s first rule is therefore to blast as quietly as possible in any area where there is
a possibility of complaint.
This means a first rule of no mudcapping. This technique is not only wasteful of explosive, but
a sure way to lose the goodwill of the neighborhood and of the insurance adjuster.
Boulders and oversize blast fragments should be drilled before blasting. The noise is tremendously
reduced, and it will usually be found that the saving on explosive and the better fragmentation
obtained will more than outweigh the cost of the drilling. When there are only a few pieces, the
nuisance of clearing the pit for blasting may be avoided by plug-and-feather splitting. In brittle
rock, a crane with a skull-cracker steel ball may be the most economical solution.
To minimize noise, do blasting when weather conditions are more favorable, that is, clear to
partly cloudy skies and relatively warm daytime temperatures. Unfavorable weather conditions are
those when the air is relatively still. These occur when the days are foggy, hazy, or smoky and the
temperature is constant from the ground up to a high altitude. About the only general rule is that
the blaster should consider avoidance of noise one of the important objectives.
Sound travels rather slowly. Its distribution is affected by winds, as shown in Fig. 9.39; by
reflection from hills, clouds, or atmospheric layers; and by temperature and humidity.

Characteristics of Damage. In most cases, there is an observable difference between building
damage caused solely by vibration, and that resulting only from structural defects and/or settle-
ment that change the shape of the building. Most of the distinction is in the pattern of wall cracks.

FIGURE 9.39 Sound travel from blast.