Page 132 - Moving the Earth_ The Workbook of Excavation

ROCK, SOIL, AND MUD

3.26 THE WORK

Shovel Rigs. Techniques of working from platforms vary with the type of shovel attachment being
used. The most efficient swamp worker is the dragline. Its long reach enables it to keep well away from
the hole it is digging, and to pile spoil far enough away to reduce slumping back into the pit or against
the shovel. The sliding action of the bucket during digging and hoisting reduces trouble with suction.
A dragline or a backhoe stands at the back of its line of platforms and digs at the rear or sides.
When its work from one position is finished, it walks ahead one or more platform widths, then
picks up the platforms behind it and swings and places them in the front; repeating this process as
often as a move is required.
Hydraulic full-revolving backhoes are competitive with draglines. They can dig much harder
material and to greater depths, and have more precise control in both digging and dumping. They
are much better able to get themselves out of trouble, and to work without platforms. But their
reach is much shorter in proportion to weight and bucket size. In swamp work, long reach is highly
desirable, and weight increases problems, cost, and risks.
The tractor-mounted hoe cannot work on platforms unless another machine places them.
Digging reach and dumping reach are poor. But if the tractor also carries a front-end loader, with
skillful operation it can work in (and get out of) very soft areas without supports.
A clamshell has almost as much reach as a dragline, and can work without dragging debris,
such as roots, stumps, and boulders, up against the platforms and itself. However, it has the severe
handicap of pulling the bucket straight up, which in sticky mud requires overcoming suction more
resistant than the weight of the bucket and load. Digging is best done off the rear or sides of the
platforms, but the ground ahead can be graded before placing the platforms.
The spaces between platforms will vary with the nature of the footing. For very soft conditions,
they should be placed in contact with chains fastening them together, or even laid in two layers,
as in Fig. 3.17(A).
For ordinarily soft or suspicious ground, the widest spacing allowed should be that which will
enable the shovel to reach the next platform before its center of gravity reaches the edge of the one
it is on, so that the shovel will not tip forward, as in (B), so much that the track will push the next
platform instead of climbing on it.

Blocking. Normally, a shovel can be worked on platforms without blocking. There is always a risk,
however, that a dragline, or particularly a pull shovel, i.e., backhoe, will unexpectedly hook into
something solid and drag itself off the platform before the operator realizes what has happened.
If the platforms are slippery with mud or ice, this danger is greatly increased, and the machine is
also likely to slide sideways in reaction to swinging.
On a solid platform, wood wedges chained to the platform, as shown in Fig. 3.18(A), are effec-
tive. For side protection, plank rails, permanently bolted to the side edges, or held there with pins,
as shown in (B), usually are adequate.
Chaining to Platforms. For extreme conditions, the shovel may be chained to the platforms.
There are many ways to do this, of which one sample is shown in Fig. 3.19. The platforms have a
cable loop at each corner, by some of which the two platforms are chained together. To the outer
corners of the truck frames are welded brackets holding rings or hooks. Chains are fastened to
these and to the corresponding outer corner loops of the platforms, and are drawn up taut with
chain tighteners in the rear. The tightness is important, as the momentum of a sliding shovel will
break chains or tear out loops which would hold against a direct pull.
All accessories, such as blocks, rails, chains, and tighteners that are to be used in mud, should be
kept painted some brilliant color (not green). They are very easily lost and dug under, particularly
if some emergency interrupts the routine of their use, and the bright color will greatly increase
chances of salvage.
Platforms can be used efficiently only in places where the shovel has room to swing. If trees
prevent swinging, they must be cut, or other methods used to support the shovel.

Suction. Lifting platforms or other objects from mud is greatly impeded by atmospheric pressure
(14.7 pounds per square inch), which is felt as suction.