Page 404 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.4 THE WORK

FIGURE 9.3 Through and sidehill cuts.

Rock excavation may follow highway techniques in exploiting comparatively narrow or irreg-
ular veins; or large-scale stripping work may be necessary to make pay rock accessible to surface
digging units.
Pits are often distinguished by the use of high and wide faces; or holes sunk below surround-
ing grades, with access by ramps or inclined or vertical hoists.
It is advantageous to have the face wide enough that several operations can be carried on in
different sections with minimum interference.
The fineness of fragmentation which must be obtained by blasting is generally determined by
the size of the hoppers or grizzlies on the crushers or processing machines.

BOREHOLE PATTERNS

The simplest type of drilling pattern is a straight line of vertical holes parallel to a vertical face. The
distance from each hole to the face is called its burden, and the distance between holes their spacing.
The holes are drilled somewhat more deeply than the face so that any ridges left between them
will not project above the new grade.
Blasts tend to overbreak at the top and not shatter completely at the base. As a result, faces tend
to slope back. The projection of the bottom beyond the vertical line from the top is called the toe.
The extra burden at the toe may be handled by bottom drilling, or heavier loading (more power-
ful explosive or tighter packing) in the bottoms of vertical holes.
Holes may be drilled at an angle so that they are parallel to the slope of the face. This angle
may be from 5 to 30 degrees.
Angle or slope drilling keeps the burden at the toe from being greater than at the top, so that no
especially heavy charge is required at the bottom. Improved bottom breakage reduces the need for
drilling below the floor, so that the total length of hole is usually not greater than in vertical drilling.
Cleaning. Small and medium air drills are not suited for working through soil overburden. It
tends to choke up their air passages and to fall in on top of the bit. If this falling occurs after pen-
etration into rock, it may make pulling the steel difficult or even impossible. Soil drills readily
only when frozen.