Page 508 - Moving the Earth_ The Workbook of Excavation

PIT OPERATION

10.14 THE WORK

Once the edge is cleared, as in (B), the burden can be moved back farther by recasting with the
dragline, pushing with a dozer, or carrying in trucks or scrapers.
It is vital to pit efficiency to keep overburden stripped far enough ahead to be out of the way
in rush periods. Failure to do this often results in spoiling or losing valuable material, and in
inability to fill important orders.

Dump Location. A dump for waste from stripping should be as near the pit as possible to reduce
haul costs, but it should not be within an area that might be dug away because of any conceivable
extension of the stripping. These two considerations may be opposed, and deciding between them
may be difficult. It is a matter of regret that initial economies have often resulted in disproportionate
later expense in redigging and moving a dump pile.
For example, loading overburden might cost 90¢ per yard, hauling it 1 mile about 25¢, and addi-
tional miles 15¢ each. A single move of 2 miles would cost $1.30, while moving it 1 mile, redig-
ging, and moving another mile would cost $2.55 per yard. Double handling is always expensive.
If it can be managed without substantial extra expense, different types of spoil should be placed
in separate dumps in such a manner that they will be accessible for redigging. Changed conditions
may make previously worthless material valuable. Examples are the reclaiming of mine tailings
and slag heaps.
Haul Grades. Grade of haul roads is important to economical hauling. A level run from cut to
dump is desirable for speed and economy. An adverse grade (upgrade in the direction of haul) will
cut both the speed and the load-carrying capacity. The extent of this loss depends on many vari-
ables. A rule of thumb is that production will be reduced about 5 percent for each percent of
adverse grade.
The adverse grade will increase fuel consumption, tire wear, and maintenance costs. Wear on
the truck engine and drive train is increased disproportionately on grades over 6 percent.
A downgrade in the direction of haul (favorable grade) is helpful up to about 2 percent, but
steeper grades may reduce production about as much as an adverse grade. Downhill speed must
be limited for safety reasons, and even empty trucks are slowed by upgrades.
Favorable grades over 2 percent and adverse grades over 5 percent call for special retarding
devices in torque-converter-equipped trucks.
Grades may change considerably during a stripping operation. The floor of the cut moves
downward, but its edges move outward and often upward. Dumps may stay at the same level, but
if space is restricted, they usually build upward.
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Haul Routes. Two-way roads for heavy hauling should be from 4 to 4 ⁄ 2 times as wide as the
vehicles using them. That is, highway trucks should have 32 to 36 feet between gutters or banks,
and 11-foot, off-the-road haulers from 44 to 50 feet. Hauling can be done on much narrower roads
when necessary, but liberal width pays whenever large volumes must be moved. Even wider roads
are made for some mines.
A haul route that crosses a public road is subject to serious traffic delay. For example, an auto-
matic traffic light that is set against pit traffic, but trips within 10 seconds when a truck reaches
it, will delay the hauler as much as an extra 1,000 feet at 20 miles per hour. A full stop sign will
cause the same or greater delays, depending on the density and speed of highway traffic.
A signal worker at the intersection reduces delays to a minimum if the signaler is allowed to
favor the pit traffic.
Hillside Dump. The easiest way to dispose of stripping waste in trucks is to dump it off a bank,
that is high enough so that it grows outward quite slowly. Height may be anything from 10 feet to
several hundred.
Such a dump may be started by flattening off a hilltop enough to give trucks space to turn, or
by cutting a pioneer road along a slope and dumping from it.
Capacity can be figured in two ways. Annual or daily capacity depends chiefly on the length
of the dumping face, and to a smaller extent on its height. Total capacity is the volume that can