Page 537 - Moving the Earth_ The Workbook of Excavation

PIT OPERATION

PIT OPERATION 10.43

Conveyor belts and cable excavators and, to a smaller extent, scrapers are largely confined to
work inside pits. Trucks are equally adapted to inside hauling and outside delivery. On very long
hauls, heavy materials are more economically moved by standard gauge rail.
Conveyor belts may be considered either hauling units or part of the plant itself. They move
and elevate material with minimum effort, but are usually difficult to set up and locate. They may
be used instead of haul roads and trucks for delivery of a heavy volume of material to a single
point many miles away.
Trucks are excellent flexible, general-purpose units. They are available in a wide range of stan-
dard sizes and can be adapted to different-size loaders or production schedules by varying the
number on the run.
Scrapers, to operate as such, need ground they can dig and hoppers which they can drive
across, or storage areas giving them room to maneuver. Banks which they cannot dig can be
loaded into them. However, scrapers are more costly and are usually slower than dump trucks of
the same size, so it is not good practice to use them steadily under shovels.
A scraper can dump beside a sunken hopper which is kept filled by a dozer.
Truck hauls may be kept short by adding conveyor belts to the plant. The new belt will dump
on the receiving end of the previous belt. Such installations may be quite long and are justified
whenever considerable yardage will be handled.
Hoppers which are built so that the truck can drive straight across, instead of backing to dump,
are more expensive to construct but will allow a faster truck cycle. Such hoppers can also be used
for scrapers.

SELECTIVE DIGGING

Selective digging may be done to separate, at the face, two or more materials of value and to
remove them; to remove one or more formations, leaving unwanted material; or to dig two or
more materials so as to combine them.
Any or all of the spoil from these operations may be hauled away or sidecast.
Layers. If the different formations are in vertical sheets, as in Fig. 10.28(A), any machine which
is accurate enough to work the narrowest vein can be used. If they lie horizontally, as in (B), any
excavator can move them if they can be cut as separate banks. If they are horizontal, and two or
more must be removed at once, the excavator should be able to work from the top down. If divi-
sions run in several directions, and separation must be exact, a Gradall excavator, with assistance
from hand labor, could be used.
When horizontal layers are separated by a dragline, as in (C), it should have a boom at least
twice as long as the bank is high. The boom angle should be low and the dump cable short to make
possible picking up the bucket at a distance.
A clamshell can do the same work with a shorter boom as no allowance need be made for space
to drag the load.
Inclined strata fall into any of the above classes. In general, it is bad practice to remove enough
of any layer to leave the one above it overhanging.
Selective digging is quite commonly required in stripping overburden, and in gravel and clay
pits. The operator may have the responsibility of choosing the section of bank most suitable to
plant or customer requirements, and supplying deficiencies by mixing different sections or layers.

Mixing. A good way to mix at the bank is to build a stockpile by dumping several materials on
one spot. A conical pile will be built, with each bucket load separating and sliding down the sides.
A succession of very thin layers will be made which, upon redigging to load, should mix together
quite smoothly.
Such a pile will tend to concentrate round or coarse pieces at the bottom, but these will be
remixed in handling by a machine working from the bottom.