Page 276 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

6.12 THE WORK

Islands may be built at spots most convenient for disposal of spoil, or according to other requirements.
A finished height of 2 or 3 feet above water is desirable. In humus, piles 6 feet or more above water level
may be needed because of loss through shrinkage, smoothing the top, and slumping under water.
Grading should be done by the dragline immediately after piling, or at least before the pond is filled.
Trees. Pond sites are usually not as open and regular as the examples discussed above. One of the
most common obstacles in the way of systematic work is a tree, or a group of trees. One tree can
cause an increase in dragline time of 300 or 400 percent for the digging in its immediate neighbor-
hood, and two trees may make the digging impossible without the use of trucks.
This is one reason why most pond diggers recommend making a clean sweep of all trees within
boom reach of the excavation area. Another factor is that if the water level is raised by construc-
tion of a dam, or if fill is placed around a tree, even with protection for the trunk, it is liable to die,
and large and old trees are particularly sensitive to any such changes around their roots.
Pond construction may cause injury or death to trees at some distance from the work. If a dam
is built, the water level may be raised not only at the pond edge but to a decreasing amount for
hundreds of feet back. Water level may be raised even when a dam is not built by groundwater
backing up behind impermeable fill from the pond, or as a result of raising the grade of the banks.
In addition, seepage may drown out trees below a dam. See Fig. 6.9.
Moderate lowering of the water table will only occasionally damage trees. Such lowering may
be caused by digging into and draining a water-bearing layer formerly having an outlet on higher
ground, or, in the case of a pond without a dam, by digging back into a bank.
Methods of protecting trees during construction, and from the effects of changes in ground and
water level, will be discussed in the chapter on landscaping.

FIGURE 6.9 Changes in level of groundwater.