Page 287 - Moving the Earth_ The Workbook of Excavation

PONDS AND EARTH DAMS

PONDS AND EARTH DAMS 6.23

Usable Materials. Small stumps can be used in a dam if the fill is muddy, so that it will form a
close bond and fill cavities. It is good practice to cut roots back close to the butt. Boulders may be
used in either wet or dry fills if the soil is carefully puddled or tamped around them, and they are
not close to each other.
If the fill is rich in organic matter, considerable shrinkage must be allowed for in both height
and thickness. Even after years of use, the dam may shrink still further if the pond is dry for an
extended period.
Cutoff Trench. If the soil on the dam site is porous, a trench should be dug down to better material,
approximately under the centerline of the dam. This should be filled with clay, well tamped or
puddled.
If a deep layer of peat is found at the dam site, it would be best to find another place for the
dam. If this is impractical, the peat may be dug or blasted out, or compacted by sand hole vertical
drainage. If the budget does not include funds for any of this work, the dam may be built on the
peat, and access for machinery provided so that it can be built up later if it sinks. If bulges appear
in the peat above or below the dam, they should be left, as they serve to partly counterbalance the
weight of the dam.

Settling and Cracking. In a dam, troubles to be guarded against are settling, cracking, slumping,
seepage, erosion, and damage by burrowing animals. Settling is prevented by building on a firm
base, using fill low in organic matter, and tamping or rolling it in thin layers if built dry. Cracking
may occur in a dam with a high clay content when the pond level is low, and may be avoided by
mixing in sandy soils. Such cracking rarely causes dam failure.

Slumping. Slumping may occur while building a dam with wet fill, and usually necessitates
stopping work on the affected section until it has partially dried. Much more serious slumping
may occur when water is impounded behind the dam before it is thoroughly consolidated. Wet
fills that have not dried, or uncompacted dry fills which have not stood long enough to settle
together, are apt to have this trouble. Seepage of pond water into the dam, softening it, and water
pressure giving it a push all act together.
Water in a pond exerts pressure against its shores, which tends to balance inward pressure from
the water they contain. If the pond is drained, removal of the water support may cause extensive
slumping, which may be disastrous if it occurs in a dam.
A dam or causeway separating two ponds is particularly vulnerable if the lower pond is drained.
For this reason, it is important to face dams with coarse, self-sustaining material that will resist
slumping.

Seepage. No earth dam is watertight, as there is a slow movement of water even through clay.
Water working its way from the pond through the dam is usually called seepage only when it is suf-
ficient in quantity to show on the downstream side, where it may make wet spots on the dam face
or marshy patches below it. Aside from the loss of water, such seepage may damage the dam by
liquefying it until it slumps; or by making channels of increasing size by washing out particles of
earth. Once definite channels have been established, the volume of flow may enable it to tunnel and
destroy the dam.
The seepage appearing below the dam may damage it by undermining, but more often merely
produces soft wet areas that may detract seriously from the value of the pond area.
Seepage may be largely prevented by cleaning and scarifying the subgrade, careful construc-
tion of the dam, using sufficient impervious material, compacting it well, and allowing it to set
before raising the water level. If suitable impervious, clay material is not available, a plastic mem-
brane can be used to reduce seepage. A 20-mil-thick PVC membrane placed on the upstream side
of the embankment and covered with a protective layer of granular fill will practically eliminate
the seepage.
The surest and most expensive cure for seepage in an existing dam is trenching along it, with
a backhoe shovel or clamshell, to solid foundation, and building or pouring a concrete core. This