Page 409 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

BLASTING AND TUNNELING 9.9

This is placed on the top edge of the face, as shown, and the bottom carefully leveled. A sight
is taken along line A″AB and the spot B marked on the quarry floor by an assistant. The distance
AB is then measured with a steel tape. Multiplying its length by .71 (the sine of 45°) gives the
height of the face and the distance BC. The distance BD is then measured, and when subtracted
from BC, gives CD, the projection of the toe.
This measurement may be repeated at various points along the face.
If the face is high, its top irregular, or considerable accuracy is necessary, it may be preferable
to make a transit survey of the site, establish benchmarks and location points on all levels, and
calculate from direct measurements from these points.
Each drill hole may be marked according to the cut to bottom grade, or by the drilling depth
desired. For convenience in loading, the projection of the toe may also be noted on the marker.
Bottom Grade. The bottom grade should have a slope for drainage that may be away from the
face or toward the sides, but not toward the face. If natural drainage is not possible, adequate
pumps should be provided.
If blasting and excavating are accidentally carried below grade, hollows can be readily filled
with fine shot rock. If the floor is too high, and the rock is soft, it may be possible to take it down
with rippers and dozers. If it is too hard for machinery, a tedious job of shallow drilling and blast-
ing, or of noisy and inefficient mudcappings, is required.
Spacing. In general, large drill holes are increasingly prevalent in faces over 30 feet high, with
proportionate increase in spacing and burden.
With most rock types, a point will be reached where enlarging and spreading the holes will
result in poor fragmentation in the center of the blocks.
Faces 50 feet or higher are usually shot with a single row of holes at a time. Low faces use
additional rows, fired simultaneously or in short-interval succession. In any height, holes in the
same row are now fired in sequence.
Each blast should supply enough rock to keep the shovel busy for at least half a day; therefore,
the lower faces must be shot back more deeply than the high ones, particularly if the shovel is
large and its working area narrow.
The entire group of holes may be drilled on a rectangular pattern, as in Fig. 9.8, or they may
be staggered to improve fragmentation.
Best results from multiple rows are obtained if there is a free cleavage plane at the new grade.
If the bottom is very hard to pull, heavier loading may be required than when firing single rows;
or burdens may have to be reduced progressively toward the back, resulting in higher costs.

Tight Holes. When blasted rock must be sheared away on two or more planes, the shots are called
“tight.” In Fig. 9.8 the holes marked a are open, those marked b are tighter, having to shear off the
back or side as well as the bottom, and the c holes must shear along back, side, and bottom. In gen-
eral, the tighter the hole, the greater the likelihood that it will fail to pull. It is usually cheaper to take
special precautions with tight holes in the original blast, than to do secondary drilling and blasting.
The tightest blasting found in open work is the start of a cut down from the surface: The first
rock blasted can move upward only. If the whole set of drill holes are shot together, each of them
will be very hard to pull. However, if the center holes are made oversize, loaded more heavily,
and fired first in a delay sequence arrangement that progresses toward the outer limits of the area,
the adjoining holes can throw into the space left, then the next holes throw into their space.

Buffers. Blasted rock may be entirely dug away before the next blast, or varying quantities may
be left against the face. Complete cleanup is required if the toe is to be accurately measured or
drilled horizontally, and is considered good practice with high faces.
If the working space is narrow, and the face low, it may be desirable to leave some shot rock
as a buffer or “blanket.” This confines the force of the explosion, and may prevent blocking of the
work area and aid fragmentation. However, the buffer should be small enough that some scatter-
ing occurs, as this makes it easier to find and blast oversize rock before the shovel gets in the heap.
Heavier loading is required.