Page 410 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.10 THE WORK

FIGURE 9.8 Open and tight holes.

Snake Holing. Sometimes a face can be most economically blasted from the bottom only. This
may be when a cliff (face) is being cut into a steep hill with poor access to the crest, the rock has
a vertical cleavage so that it will break away without dangerous overhang when the bottom is
blasted out, and is brittle enough to shatter when it falls. Such an operation may be very econom-
ical of powder, as the breaking out of the crest and the fragmentation upon hitting the floor are
accomplished partly by gravity.

Coyote Holes. Coyote holes, illustrated in Fig. 9.9, are used for heavy undermining blasts. They
may be used alone to topple a cliff, or to break out heavy toe burdens in conjunction with well
drill holes loaded from the top. Faces should be at least 60 to 80 feet high to justify their use.
Coyote holes are used when material is difficult to drill, a large yardage is required at one time,
and the rock fractures readily.
They consist of small tunnels, 3 or 4 feet in diameter, driven horizontally into the face at floor
level, and one or two cross tunnels parallel to the face. Explosives are stacked in the cross tunnels,
and the entrance is securely blocked with stemming. Firing is best done by Primacord.

CONTROLLED BLASTING

Overbreak. Rock usually tends to overbreak at the top of the bank, and special drilling or loading
may be required to avoid leaving a hard bottom rib at each blast junction.
It may be necessary to set back the first row of holes for the next blast for more than the normal
burden, as their burden may be partly shattered, so that the shot is not confined as well as in the
other holes. Drills may not be able to work close to the edge, or may not be able to penetrate shat-
tered material.