Page 372 - Moving the Earth_ The Workbook of Excavation

ROADWAYS

8.34 THE WORK

is so much to the good. On the other hand, the operator may take ripping so seriously that she or
he will let scrapers wait while finishing a strip. This can result in considerable loss of production.
Soils too hard for backrippers, and many rock formations, can be ripped for scraper loading by
a rear-mounted ripper with hydraulic down pressure.
Methods of use and rate of production of these tools are discussed in Chap. 21. In most soils a
big tractor should be able to keep three teeth in the ground, and break soil much faster than an
ordinary fleet of scrapers could remove it. In frost or rock one tooth would be used, and output
might be so reduced that one scraper could handle it.
A pusher can do heavy ripping only if it has enough time between scrapers to prepare enough
soil for them. If it does not have this much time, the ripping will be incomplete or the scrapers
will have to wait, or both. Each case must be judged on its own merits, but under such conditions
it is usually more efficient to use one tractor for ripping and another for pushing.
If this arrangement leaves the ripper with idle time, it can be used as a tandem pusher when it
is available, with the regular pusher carrying the whole load during ripping periods.

Economics of Ripping. Many soils become so loose when they are broken up that they are harder
to load, or provide smaller loads, than when in solid state. Unless loading results are clearly and
definitely favorable, ripping should not be done.
Ripping rock for scrapers pays only when a fairly fine and uniform breakage is obtained.
Scrapers load badly in coarse rock, and unless specially reinforced, may sustain severe damage
from repeated contacts with big pieces and unbroken ledge.

WORK PATTERNS

Scraper work patterns should be arranged to allow for as many of the following as possible:
1. Digging downhill
2. Digging in the direction where load will be dumped
3. Utilization of pushed soil
4. Efficient turns with minimum deadheading
5. Cuts that start at high points, and fills at low ones

Direction of Digging. A favorable grade increases the speed and the effectiveness of loading
and reduces wear. The advantage becomes more marked as the downgrades get steeper.
Figure 8.25 shows three ways to make a deep scraper cut. Figure 8.25(A) is inefficient because
the downgrade is used in transporting where little power is required and does not assist the dig-
ging. Figure 8.25(B) takes full advantage of the downgrade but may create an inconveniently
sharp angle at the beginning of the cut.
In (C) the digging is started on the upgrade, just before the crest. The power requirement for the first
few yards is small as resistance increases with load. The machine is rounded into the downgrade for the
bulk of the load. This keeps the crest cut down without sacrificing much of the advantage of the slope.
Digging in the direction of the work is desirable. A loaded scraper moves more slowly, wears
more, uses more fuel, and may be less stable on turns than an empty one. If the load is picked up
heading toward the fill, it is able to take the shortest path to the dump, and to make the turns and
the longer run between them empty.
However, there is often sufficient reason for digging away from the dump. Digging downhill is
more important than direction. Occasionally a pusher can be best utilized if scrapers are loading in
both directions, which, in a single cut and fill, would require about half the unit’s load before turning
to go to the fill.

Pushed Dirt. The scraper knife usually pushes some dirt ahead of it, the amount increasing with
the size of the load. Loose material such as sand may be moved in considerable quantities. This
is left in low piles when the bowl is lifted.