Page 448 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.48 THE WORK

Keeping Out of Trouble. Under all ordinary circumstances, blasting should be kept light enough
not to damage buildings. The job should be figured on a basis of conservative blasting, and the
work done the same way.
Short-period delays are a real friend to the person who wants heavy blasts, but is surrounded
by structures. Up to 70 percent of the charge of an instantaneous blast can be used with each of
10 or 12 short delay periods, without increasing the vibration. Or to look at it another way, the
same loading can be used as for an instantaneous shot, and 10 periods used to cut the damage
potential by four-fifths.
Even with ultraconservative procedures, inspections should be made of nearby buildings
before blasting. If property is valuable, vibration-testing devices will be supplied by the insurance
company or by a blasting consultant to measure the disturbance caused. Such instruments should
be used to check the next blast in any building or area from which complaints are received.
As detailed before, noise should be kept to a minimum. If there are few people in the area, it
should be possible to notify them before blasts, so that they will not feel it necessary to be tense
all day waiting for an explosion. Another method, applicable to heavily populated areas also, is
to set a definite time or times each day for shooting, and stick to it.
If a claim is made and is justified, it should of course be paid. But if it is clearly unjustified, it
probably should not be paid even if apparently too small to be worth arguing about. One paid claim
is likely to bring in a dozen or a hundred others, and the contractor might be forced to replaster
and decorate a whole town before he or she knew it. Payment of any claim makes any other much
harder to defend in front of a jury.
Of course, contractors should protect themselves with insurance, and usually do so. But in the
long run the premiums they pay are based on what the company pays out for damages, so their
interests are identical.

DIGGING UNDERGROUND

TUNNEL WORK

Tunnels are underground passageways of any size, and may be natural (as in limestone caverns)
or made by animals or humans. Those discussed in this section are made by people. They serve a
variety of purposes, including mining, water supply and drainage, laying sewer and other pipes,
railroad and vehicular shortcuts or water crossings, and air raid protection.
Rock tunnels are driven through solid material that usually requires blasting and may support
itself permanently, or at least long enough to allow setting up of bracing after digging out a short
section. Soft ground tunnels involve digging or pushing aside soil, and the roof (called the crown)
and the walls may require support before removing the soil. Mixed-face tunnels go through both
types of ground, either together or in different sections.
People have driven tunnels since prehistoric times. They usually worked in rock, because the
difficulty of digging it was more than compensated for by its ability to hold itself up. Cutting was
done with hand tools, or by heating the face with wood fires, then throwing cold water or cold
water and vinegar on it to cause sections to crack off.
The vinegar technique, with little or no ventilation, must have been really rough on the work-
ers. A rough approximation of the atmosphere might be obtained by building a good blaze in a
fireplace, shutting off the chimney damper, then putting out the fire with vinegar.
Layout and Problems. The methods used to drive a tunnel vary tremendously with the nature
and water content of the material to be penetrated, depth and size required, surface conditions
along the route, time allowed, and background of the workers doing the job. There is space in this
section to indicate only a few of the problems most often encountered.