15 AUGUST 2005
Earth Dam Embankments. Evaluate stability at three critical stages: the
end of construction stage, steady state seepage stage, and rapid drawdown stage. See
UFC 3-220-10N for pore pressure distribution at these stages. Seismic forces must be
included in the evaluation. Requirements for seepage cutoff and stability dictate design
of cross section and utilization of borrow materials.
Seepage Control. Normally the earthwork of an earth dam is zoned with
the least pervious, fine-grained soils in the central zone and coarsest, most stable
material in the shell. Analyze seepage by the methods of UFC 3-220-10N.
Consider the practicability of a positive cutoff trench extending to
impervious strata beneath the embankment and into the abutments.
For a properly designed and constructed zoned earth dam, there is little
danger from seepage through the embankment. Drainage design
generally is dictated by necessity for intercepting seepage through the
foundation or abutments. Downstream seepage conditions are more
critical for homogeneous fills. See UFC 3-220-10N for drainage and filter
Piping and Cracking. A great danger to earth dams, particularly those of
zoned construction, is the threat of cracking and piping. Serious cracking may result
from tension zones caused by differences in stress-strain properties of zoned material.
See Figure 1 (Influence of Soil Properties and Construction Methods on the
Performance of Homoaeneous Earth Dams) for classification of materials according to
zone development. Place an internal drainage layer immediately downstream of the
core to control seepage from possible cracking if foundation settlements are expected to
Dispersive soil. Dispersive clays should not be used in dam
embankments. Determine the dispersion potential using Table 8-1.6. A hole through a
dispersive clay will increase in size as water flows through (due to the breakdown of the
soil structure), whereas the size of a hole in a non-dispersive clay would remain
essentially constant. Therefore, dams constructed with dispersive clays are extremely
susceptible to piping.