Page 266 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

6.2 THE WORK

of these forms of alteration needs to be planned with approval. Anyone filing with the Corps for
a dredge-and-fill permit under Section 404 must first attempt to avoid or drastically reduce the impact
on the wetland. Failing that, compensation through mitigation may be required for unavoidable
wetland loss, by either restoring an existing wetland or building a new one.

Wetland Evaluation Technique. The Federal Highway Administration is teaching state depart-
ments of transportation (DOTs) and their contractors how to determine the value of a wetland with
the wetland evaluation technique. The DOTs require the contractors to take a 4-day course before
contractors are allowed to bid on a job involving wetlands, because created wetlands fail when the
basics are not being done. If the site for a created wetland is too narrow and the slopes are too
steep, the wetland will remain barren. Once built, a newly created wetland must be monitored by
a state DOT or developer for 5 years to make sure it is self-sustaining or viable.

SWAMP RECLAMATION

Soil Conditions. Swamps which are wet all year are logical places to dig ponds. The spoil taken
out of the excavation can be used to build up the area around it so that the section worked is
changed from a bog into open water and dry land.
Swamps commonly have a top layer of soft peat or muck soil, which may be of any thickness
from a few inches to 100 feet or more. This organic material is easy to dig but provides very
treacherous support for machinery. Below the muck, any type of soil or rock may be found.
The reader is referred to Chap. 3 for techniques in handling the mud that is one of the usual prob-
lems of swamp work. Mud may be reduced or eliminated by working in a dry season; by diverting,
draining, or pumping out the water before or during the work; or drying up the area by sump or well-
point pumping. These techniques are discussed below and in Chap. 5.
It is very advantageous to get rid of as much water as is reasonably possible. Water prevents the
operator from seeing the bottom he or she is cutting, with resultant wasted passes and gouging. It
reduces the digging effectiveness of the bucket so that some soils which can easily be dug when
dry cannot be penetrated when under 1 or 2 feet of water. Even with skillful operation, water will
mix with soil in the bucket, making sloppy spoil piles that reduce the amount of digging at a stand,
and which sometimes will flow back into the excavation or cut off the shovel’s exit.
Excavators. For decades the dragline excavator has been the choice of equipment to excavate
ponds. It has good reach and can excavate the slopes for the pond very well. More recently, a full-
revolving hydraulic backhoe, also known as a hydraulic excavator, has been used to excavate ponds.
It does not have the reach of a dragline, but the hydraulic excavator has more power to dig hard
material. These forms of power shovels are described in detail in Chap. 13.
For this discussion of pond construction, the use of draglines will predominate so that there is
not much concern for the footing of the excavator. Footing is a problem for the hydraulic backhoe,
because it has to get closer to the area to be excavated than the dragline does.
Digging Plan. Figure 6.1 shows a general plan for digging a pond in a swamp and using the spoil
to build up the unexcavated parts. A drainage hole is dug at the downstream end, and the water level
lowered by a ditch drain or by pumping. A sill may be placed at the entrance to the ditch or the sump
hole to hold water back a few inches above the floor of the proposed pond.
If the swamp is fairly dry, and the digging is fast and continuous. the removal of water may be
unnecessary as the expanding excavation may keep the water at a low enough level that it will not
cause trouble. Surface water may be diverted around the excavation by shallow ditches or dikes
as shown, or allowed to flow into the hole.
If no obstructions prevent, the pond is dug from the center toward both sides, with the dragline
walking along the longest dimension, which is usually parallel to the direction of water flow, as
in Figs. 6.1 and 6.2. The machine keeps back far enough from the centerline that it can reach it
with an easy cast. It usually works on platforms or other artificial supports, but if the swamp has