Figure 5-9.-Layout and development of a pit, using a dragline.
PIT OPERATIONS
Sand, gravel, and other construction materials are
extracted from pits with scrapers that can move huge
volumes of material in a relatively short period of time.
The material is removed from the floor of the pit in
successive thin layers over its entire width.
During excavation, scrapers should be carefully
spotted to maintain an even downgrade and prevent
cutting holes below the general level of the pit floor.
When the pit is longer than 100 feet in the direction of
loading, the scraper spots should be staggered along the
length of the cut as well as across the width of the zone.
Figure 5-8 depicts the layout and development of a
scraper pit with lines A-A and B-B showing the limits
of the pit. Lines A-A and B-B divide the area into three
zones for current excavating, stripping, and clearing.
Zone 1 is being excavated at the same time Zone 2 is
being stripped and Zone 3 is being cleared. Three
scrapers in staggered formation load downhill in Zone
1, and a dozer strips downhill in Zone 2.
In consolidated gravel or soft rock, when scrapers
assisted by pusher tractors (Push Cat) will not pickup a
heaping load within 150 feet, a ripper should be used to
loosen the material, thereby increasing the loading
efficiency of the scrapers. Rippers should be operated
downhill, and an entire zone should be ripped at one time
while the scrapers are hauling from another zone.
The dragline is the most practical piece of
construction equipment for underwater digging and is
particularly adapted to submerged gravel pit operations.
Draglines can efficiently recover sand, gravel, laterite,
or coral from beaches, the beds of streams, and the
bottoms of lakes and lagoons. Figure 5-9 depicts the
layout and development of a pit being excavated with a
dragline.
Clamshells are capable of excavating loose sand,
gravel, and crushed stone at, above, or below ground
level. The clamshell can be raised and loads dumped at
heights equal to the distance from the tip of the boom to
the ground, minus the length of the clamshell bucket, to
allow adequate clearance for the bucket when it is
opening.
Material removed from pits can seldom be used in
its inplace state. In most cases, pit material must be
processed (crushed and screened) to meet job
specifications. But, before the material can be
processed, it must be loaded and delivered to the
processing equipment. Loading and delivery may
require additional handling equipment, such as
front-end loaders equiped with either a rollback bucket
or 4-in-1 bucket and conveyers that may be used singly
or in series to load vehicles, construction equipment, or
hoppers from stockpiled material. Bucket loaders may
also be used. They consist of a power-driven endless
chain to which buckets are attached so material is loaded
on the downward travel. Handling equipment is used to
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