Page 512 - Moving the Earth_ The Workbook of Excavation

PIT OPERATION

10.18 THE WORK

Wet working breaks down the soil particles into a structureless mud that dries into lumps and
sheets. The probability of damage increases with the amount of contained water, and its severity
decreases with increased proportions of sand or humus.
Packing under trucks or other heavy machinery may produce the same result by bringing water
out of small spaces between particles so that it will make a mud. Trucking on rather dry soils will
produce compression cakes, which are usually softer than the mud lumps.
A rough test of condition may be made by rubbing a sample of soil between thumb and finger. If it
smears, it is too wet, and if the particles remain separate, it is probably ready to work. The dirt turned
by the plow or dozer blade should be watched for smearing, which indicates that soil is too wet.
Topsoil should not be trucked over, but it is often impossible to avoid doing it. The stripped
areas may be too soft, or they may not offer enough space for maneuvering. If the latter, trucks
may be routed to drive empty across the topsoil and run on the subsoil with loads.
It is usually better to completely ruin a narrow strip of soil by using it as a haul road than to
damage a large area by allowing trucks to wander around on it.
Soil lumps are completely broken down by freezing and thawing, and usually disintegrate
slowly in wet seasons. If still on the field, the roots of a cover crop, or sometimes only a thorough
rolling or disking, will reduce them. If absolutely necessary, a hammer mill shredder of the type
used for humus can be used to pulverize them.

Piling. The standard tool for piling topsoil is the front-end loader. The old standby, the bulldozer,
does the same work but rather less efficiently, as it cannot make high piles without walking on
them. But it may separate topsoil more cleanly.
When piling is done in advance of loading, the standard practice is to heap the soil in windrows
(long piles). These may be run up and down the slope so as not to interfere with drainage; or
across it to keep in uniform soil types, or for convenience in trucking. It may be necessary to make
occasional breaks in windrows to prevent ponding of water above them. Piling should be started
at the entrance to avoid trucking over unstripped areas.
Windrow size will vary with loading requirements and soil depth, and the size of the bulldozer. Small,
closely spaced ridges are most easily piled, but loading machines work best in large, high piles. Large
dozers and deep soil favor building big piles. Building of the piles is described in Chap. 4.
Careful separation of the topsoil from subsoil is a requirement of most stripping. Inclusion of
even considerable amounts of loose fill in topsoil does not usually damage its usefulness, but it is a
type of adulteration that is unpopular with the buyer. The damage to appearance and value is espe-
cially severe if the subsoil is a conspicuously different color, or texture, or is in the form of lumps
or sheets. It is generally better to leave a thin sheet of topsoil on the field than to mix topsoil and fill.
However, if the stripping is done in order to get clean dirt or gravel for roads or other purposes,
it may be better to concentrate on cleaning the surface, even if some fill mixes with the topsoil.
Topsoil and subsoil are separated chiefly on the basis of color. If the difference is prominent,
the distinction is easy to make, but the results of a mistake are painfully obvious.
Light conditions may obscure the color difference. When the sun is very low, in the morning
or evening, or high in a clear sky, subsoil and topsoil may look the same. Cloudy days and inter-
mediate sun elevations give the clearest distinction.
Any shovel rig can be used for piling topsoil. The backhoe and dragline are best at it, and the
toothed clamshell slowest. Shovels work rather slowly in shallow soil. The hoe is adept at sal-
vaging soil along a wall or fence.
Shovels loosen and aerate the soil, and cause minimum damage in handling it when wet. Fill
dug with the topsoil can be concealed by mixing in. Teeth prevent absolutely accurate work, and
it is good practice to clean up afterward with a dozer.
Scrapers are used for piling topsoil chiefly when it has to be completely removed from a large
work area. They generally make wide, low piles that are more readily rehandled by scrapers than
by other excavators. They pack the soil heavily even when it is in good working condition.
If topsoil is stripped from one part of the job at the same time that it is spread on another, the
scraper can combine the two operations very efficiently.
Scrapers drawn by fast wheel tractors may be used to dig topsoil and deliver it by road to local
customers, who will usually appreciate having it spread.