Page 520 - Moving the Earth_ The Workbook of Excavation

PIT OPERATION

10.26 THE WORK

Occurrence. Sand and gravel deposits occur in all parts of the world, and with special frequency
on or near past or present shores, glaciers, and mountains. They may be thin, irregular deposits,
or in heavy masses. In general, gravel is more variable than sand in size and type of particles, and
thickness and shape of beds.
Running water needs higher velocity to carry large pieces than small, and in general, gravel is
deposited nearer the source than sand, or at times of heavier stream flow. However, a stream
which is building up a deposit alternates, bringing in materials and cutting parts of it away.
Channels wander over the whole area. Oversize material beyond the capacity of the water to carry
may be rolled long distances along the bottom. Clay and silt may be deposited in temporary pools
and cut-off and stagnant channels.
The result of these factors is that gravel, sand, and clay deposits are often extremely variable
and uncertain. When this is the case, mining them requires constant good judgment in deciding
which horizons should be combined and which separated; and what can be used and what must be
wasted.
Processing. Sand and gravel may be processed to clean out dirt; to separate into different sizes;
to combine different sizes and materials; to remove or crush oversize stones; and for combinations
of these purposes.
In variable formations, the primary processing is selection at the bank as discussed under
selective digging.
The processing plant proper may consist of a washer, a screen, a crusher, or multiples or com-
binations of these units, together with feed hopper, and transfer and discharge conveyors. These
plants, available in both mobile and portable types, are described in Chap. 21.
By the use of units of proper size, any desired reduction, combination, or separation can be
secured. It should be remembered, however, that no plant can produce a coarse product from fine
particles. Deficiencies in gravel content must be made up by mixing in stone of proper size, or
oversize up to the crusher capacity, in addition to the run-of-pit material.
Clay. Clay, like sand and gravel, may be found in massive deposits or in irregular layers and
lenses. It is often interbedded or mixed with other materials in very complex ways.
Underwater clay may be soft enough to be dug with a small dragline, or quite hard. Dry clay
grades from hard shovel digging to shales requiring heavy blasting.
Pit operators usually find it economical to loosen up dry clay with at least light blasting, to
facilitate digging. Electric or gasoline-driven augers are extensively used for drilling, and slow to
standard velocity explosives for blasting.
When valuable clay is in narrow and confused beds, it is often blasted, then separated by hand
into piles which are loaded by machine.

LOADING OUT OF THE BANK

Most primary pit excavation is in formations deep enough to be loaded directly from the bank.
The material may be in its natural state or loosened by blasting.

Bank Height. In free-flowing material, such as loose dry sand, the only limit to bank height is
that imposed by safety. This will be discussed below.
If a formation will stand in vertical or overhanging walls, and is dug from the bottom, the face
should not be higher than the machine can reach, as it may be necessary to dislodge overhanging
pieces with the bucket to avoid danger from falls. Half this height is usually more convenient and
may allow greater production if the top of the bank does not keep falling as the lower face is cut.
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For example, excavators of 2 ⁄ 2 -yard capacity may do their fastest loading in banks in which
the dipper teeth do not have to be lifted above 12 to 15 feet. However, they can trim banks up to
about 25 feet.