Page 535 - Moving the Earth_ The Workbook of Excavation

PIT OPERATION

PIT OPERATION 10.41

FIGURE 10.27 Pit patterns.

Excavating Patterns. Hill pits may be opened by a straight cut-in or by benching. After reduc-
tion to the level of the surrounding land, they are dug as sunken pits.
Subsurface workings, called sunken or dig-down pits, may be opened with front shovels and
ramps, or by dragline or backhoe work from the top, in much the same manner as a haul-away
basement excavation. The circular pattern shown in Fig. 10.27(A) and (B) is also widely used, for
both subsurface and slopes with gentle gradients.
Backhoes can take gentle slopes in a series of benches, as in (C). It is necessary to level a strip
for walking, as accurate loading is difficult on a slant.

PROCESSING PLANTS

This heading includes screens, crushers, and washers with their feeding and discharge mecha-
nisms. These units will be described in Chap. 21, and are discussed here only in relation to pit lay-
out and other operations.

Portable Plants. The simplest screening equipment is that described earlier in connection with
topsoil. These pickup or skid grizzlies can be used wherever a shovel can work, but require ramps
if they are to be used with tractor loaders. Their product is not well graded, as narrow oversize
pieces pass through readily.
Mobile plants having screening, usually crushing, and occasionally washing, equipment
mounted on one or more wheeled trailers, require from a few minutes to several hours to move up
to a bank and start work. Short moves in the pit require less downtime than highway transporta-
tion as conveyors and other projecting parts need not be removed or folded in.
One of these units is usually able to eliminate primary hauling or to reduce it to a single truck
shuttle, or a short conveyor. For direct loading from a low bank, particularly by a short-range
excavator, it may be desirable to keep a tractor constantly on hand so as to move the unit up.