Page 274 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

6.10 THE WORK

pure peats, however, are widely used to enrich soil, and command a fairly good market. These
have a very high water content, and may shrink 50 percent or more in the pile.
If the spoil is to be used for the pond banks, or similar unloaded fills, organic and inorganic
dirt is generally dug and handled together. If they are to be sold, the topsoil should be stripped and
piled to the side, and the subsoil dug afterward in a separate series of operations. As the surface of
the subsoil is often below water level, adequate drainage or a dependable pumping system should
be provided before putting the shovel on it.
Separation of materials in several layers may be very complex. Examples are digging gravel
interrupted by seams of unwanted clay and digging clay seams lying between sandy layers, the
decision as to what is desirable being a matter of local demand.
If the dragline is large, or has a long boom in proportion to the area to be dug, these layers may
be removed and piled separately, or they may be stripped back separately in the same manner as
topsoil. Often, however, a better solution is to truck away the least bulky or sometimes the most
valuable of the materials during the digging. Trucking may be possible only during a short period
late in the dry season, after a protracted period of artificial drying by drainage or pumping, or only
after construction of fills or timber roadways.
Selective digging should be done in the dry, so that the operator can see what he or she is doing.
It will be discussed further in connection with borrow pits in Chap. 10.

Predraining. Whenever the soil is to be trucked out, or when very sloppy conditions are found
in a swamp, it is advisable to investigate the possibility of drying up the area before work starts.
Almost any spot can be dewatered, at a price, by diversion of any streams or other surface
inflow, and well-point pumping.
Sump pumping may be more economical when surface water can be diverted, and when under-
ground reservoirs are small, or when relatively impervious soils cause groundwater movement to
be slow. The soil must be fairly stable.
The sump is a deep hole with a bottom below the proposed digging. One side should be sloped
gradually or terraced so that a pump can be set up wherever desired. Outlet hose, or a flume or
ditch, must be provided to lead the water out of the area being dried.
For best results, pumping should be started around the beginning of the dry season. If the swamp
is underlain by porous gravel or sand, most of the water can be removed from small basins in a
few days, and additional pumping will be required only occasionally. In large basins, continuous
pumping might be needed during the job. If the soil is tight, less water will be removed at first,
and seepage into the hole will be reduced very gradually.
This operation should cause the swamp to dry up, so that difficulties with mud will be reduced
or eliminated. However, the effect may be largely lost if heavy rains saturate it again before work
is started.
Both the speed and effectiveness of sump pumping can be greatly increased by drainage ditches
leading into the sump. A horseshoe-shaped trench enclosing the working area, but leaving an
undisturbed space for entrance of machinery, is a convenient layout. Such a trench may involve
piling hundreds or thousands of yards of spoil, but should be a good investment if it allows dry
digging for the bulk of the project.
Peat will burn except when saturated with water, and a peat deposit drained as suggested might
be entirely lost by fire. If this material had no value, this might be the quickest way of removing
it. The fire would also loosen any stumps, and if deep, might consume them.
A peat fire might have black or bad-smelling smoke (or be almost smokeless), burn for a long time,
and be difficult to prevent from spreading. Such a fire should be started only after consultation
with local fire and police officials.

DRAGLINE SIZE

Choosing the right size dragline for pond dredging in swamps involves consideration of a number
of factors. Small machines are more easily supported on soft ground, are easier to salvage if they
get in trouble, can work in restricted quarters, and usually have a faster digging cycle. However,