Page 4 - Moving the Earth_ The Workbook of Excavation

LAND CLEARING AND CONTROLS

1.4 THE WORK

prepared by the project’s consulting engineering or architectural firm. It is not unusual for the con-
tractors, who do earth moving on the site, to be co-permitees and their personnel required to receive
instruction about the NPDES regulations before they can do any work on an NPDES-permitted site.

DISPOSAL

Cut or uprooted vegetation must be processed or removed as part of most clearing jobs. Possible
ways include burial, allowing time to decay; burning, shredding, or chipping; removal from the
area; and various combinations of these methods.
Disposal methods will be discussed below and throughout the chapter. They are considered
first because of the extent to which they affect techniques of clearing.
Nitrogen. Disposal of vegetation by any method other than by burning is likely to be complicated
by absorption of nitrogen by decay processes. This element is essential for life of every kind.
Although abundant as a free gas in the air, its quantity in fixed or usable form in the soil is limited.
The problem of nitrogen deficiency is usually not a strong objection to disposal of vegetation
by any reasonable method such as surface decay, or mixing into soil or plowing it under. Nor does
it prohibit the use of wood chips for soil cover. But nitrogen deficiency must be considered in
assessing both the immediate and possible long-term effects of the work.
The deficiency can be largely corrected by addition of suitable amounts of nitrogen-rich fertilizer
throughout the period of decay or by planting legumes that can obtain nitrogen from the air. The end
result is usually substantial enrichment of the soil, as the presence of abundant nitrogen aids the
conversion of vegetation to soil humus.

Surface Decay. Cut or uprooted vegetation is sometimes left on the ground to rot. Soft material
such as grass and nonwoody plants may disappear in a few weeks, but trees will make the area
unusable for years. Prevention of regrowth is made difficult.
Disadvantages of disposal by surface decay are reduced, and may even be eliminated, if woody
material is reduced to small pieces as part of the clearing operation. This may be done by using a shred-
der or a heavy rotary mower for clearing, or a chipper to grind up pieces after cutting.
This method is suitable for construction only if clearing is done long in advance of removal of
topsoil.
Burial. In agricultural work, burial is often the preferred means of disposal if equipment of the
proper type is available, and is big enough to handle the growth density and trunk sizes involved,
and if the soil is soft enough to permit it.
Grass, weeds, brush, and sometimes saplings may be buried intact by a brush-breaker plow, or
slashed, chopped, and partially or wholly buried by a heavy disk harrow. Rolling choppers can
disintegrate and partially bury medium-size trees, including trunks.
Burial of this type is used in agricultural rather than construction clearing, and only when the ground
can be left undisturbed (except for planting of a cover crop) until a large part of the material has decayed.
This process may take weeks or years, depending on vegetation type and maturity, and weather.
In construction, about the only permissible burial is undisturbed, low-cut stumps under deep fills.
When allowed, this exposes the fill and its supported structures (usually road pavement) to even-
tual settlement as the wood decays. This danger is often outweighed by advantages in prevention
of sliding of fill down a slope, and in economy.
Loose stumps are often buried, but the operation is likely to be expensive and unsatisfactory.
Attached roots make them enormously bulky; cutting roots back to the buttresses is likely to ruin
saw chains by contact with clinging dirt. They are still awkward after cutting back.
Fill including stumps will almost always settle badly. Spaces left in and under irregularities
will gradually fill with soil, allowing the surface to sink. Rotting of the wood will cause slow,
long-term settlement. Both effects are at a minimum if burial is in permanently wet mud that will
flow around them immediately and preserve them against decay.