Page 401 - Moving the Earth_ The Workbook of Excavation

Source: MOVING THE EARTH

CHAPTER 9

BLASTING AND TUNNELING

For the purposes of this chapter, rock is defined as material which requires loosening by explo-
sives or rippers in order to be dug economically by machinery.

PURPOSES OF ROCK EXCAVATION

Surface excavation of rock is done chiefly for the following purposes:
1. Stripping—the removal and wasting of any type of rock or dirt in order to uncover valuable
layers.
2. Cutting—removal primarily to lower the surface. In road and airport construction the spoil
is generally used for fill elsewhere on the project. In ditching, it is often used for backfill after
installation of pipes.
3. Quarrying or mining—excavation of rock which has value in itself, either before or after
processing. A rough distinction can be made between these two, in that quarries are ordinarily
concerned with the physical characteristics of the stone, and mines with its chemical composition.
However, the terms will be used interchangeably in this discussion.
One excavation can involve all three classifications, as in a heavy road cut where some material
is wasted, other is used for road fill, and the best rock is crushed and used for aggregate.
Blasting may be divided into a primary operation in which rock is loosened from its original
position in bulk, and secondary work which consists of reducing oversize fragments and break-
ing back ridges and spurs. The latter is done in the same manner as other light blasting, such as
breaking boulders and chipping out ledges.
Rock work may also be classified as to the type and fineness of breakage required. Quarrying
of building or dimension stone involves loosening large solid pieces from the parent rock, while
blasting for fill or crushed rock requires pieces small enough to fit in the shovel bucket, the fill
layer, or the crusher.

Stripping. In most stripping work the spoil has no value, so that the cheapest way of handling
it is the best way. It is often possible to dump it in excavated areas from which pay material has
been removed.
It is common practice to shoot and dig overburdens over 100 feet deep in a single layer, and
the use of the largest shovels and draglines is required for such work.
Drilling may be done horizontally from a face, as in Fig. 9.1(A), vertically from the top, as in
(B), or in combination, as in (C).
Horizontal drilling has its best use when the mineral deposit is immediately under soft rock.
Auger-type drills with extensions 6 to 10 feet long, and diameters of about 5 inches, are used.
These have a tendency to drift downward, and since distances of 30 to 75 feet are commonly
drilled, it is necessary to start them several feet above the deposit, or to start them at a slight
upward slant. Spacing may be 10 to 30 feet.

9.1