Page 209 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.11

FIGURE 5.7 Rock drilling patterns.

Standard delays permit firing a succession of small blasts with one jolt, but there is danger that
they will cause a periodic vibration in some building or object which will damage it more than a
heavy single explosion.
Deep shots, or those well buried, produce less damage than shallow ones. If the explosion is
so confined that it acts about equally in all directions, concussion is at a minimum. If any part of
the force can escape rather readily, its blowout will produce a reaction or back-kick against the
solid rock, which will shake the surrounding area severely.
Air waves, which are serious offenders in breaking windows, can be prevented by thorough
muffling.
In any blasting near buildings, roads, or people, elaborate safety precautions must be taken. An
absolute minimum is to confine the explosion so that no fragments can fly, give ample advance
notice of the blast, and block all roads and paths into the danger area. Woven steel mats are the
only safe cover for close blasting.
The contractor’s liability insurance company usually makes an inspection of the job before any
blasting is done. This may include the condition of nearby buildings.
A discussion of blasting damage is given in Chap. 9.