Page 495 - Moving the Earth_ The Workbook of Excavation

Source: MOVING THE EARTH

CHAPTER 10

PIT OPERATION

This chapter contains an outline of some of the principles of pit layout and operation. The term
pit is intended to cover any open excavation made to obtain material of value, whether it be coal,
mineral ore, quarry rock, gravel, or fill.
Because of the complexity of the subject, treatment will have to be brief, and many operations
omitted entirely.
Techniques of drainage, road building, and blasting have been discussed in previous chapters.
Most pit operations are started with the removal of soil or rock lying over the deposit to be
mined. The problems involved in stripping will therefore be considered first.

STRIPPING OVERBURDEN

Overburden may include topsoil, subsoil, sand, gravel, clay, shale, limestone, sandstone, and other
sedimentary deposits.
The depth of overburden that may be removed depends on its character and accessibility, the
value of the underlying formation, the comparative cost of underground mining, and the extent to
which the spoil can be sold or utilized.
Need for Stripping. Stripping overburden may be a very large part of the cost of mining, and a
number of factors should be considered before undertaking it.
First, is it necessary to strip it? It may be possible to mix it with the product, or to separate it
at lower cost during processing.
If the pit has gravel or stone screening, separating, or washing equipment of adequate capacity,
soil may be dug or blasted down with the pay dirt, and separated as part of the regular processing.
In this case, thorough clearing is necessary as sod and brush clog screens and crushers.
Utilization of Spoil. If the land must be stripped, the next question concerns possible profit or
use to be obtained from the material. Near large cities, topsoil can often be sold at a higher price
than the regular pit product. This may also be true of peat deposits.
Any substantial layer of clay, or fine earth, should be sampled and analyzed. Clays which are
superficially similar in appearance are used in widely different products, such as fire, paving, and
common brick; tile, pottery, portland cement, flux, mud for rotary drills, and specialized functions
in chemical processing.
Limestone is often found in overburden, and it is extensively quarried for crusher rock, building
stone, and cement manufacture.
If it is not possible to get good prices for any part of the spoil, it may still be possible to get enough
for it as fill to repay some of the stripping cost. The stripper is in a good competitive position
because he or she has to pay for digging, and often for dumping as well, and is better off selling
for a fraction of the excavation cost than not selling at all.

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