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Living with Dams: Extreme Rainfall Events | 2015
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dam embankment in an accelerating
fashion, eventually developing into
a seepage pathway through the soil,
progressing from the downstream
toe in an upstream direction toward
the reservoir.
Eventually, water from the reservoir
flowing along this path through
the dam creates what is known as a
piping failure of the dam, releasing
the entire reservoir of stored water.
Seepage Along PipesWithin
Embankment Dams
Outlet pipes through earthfill
dams provide a potential seepage
path of water through earthfill
dams.
It is difficult to adequately
compact earthfill around the entire
perimeter of pipes through the
dam. Depending upon the age
of the dam and the design of the
pipe penetration, this may be an
issue. Modern design includes
construction details that provide
a continuous concrete footing
under the entire length of the
pipe. In addition, depending upon
the age of a dam, it may not have
provisions for collection of seepage
flowing along the pipe and for
the safe discharge of this seepage
downstream without removing soil
particles of the dam.
Seepage Through Dam
Foundation
The reservoir upstream of any
dammay seep continually
through the naturally occurring
materials of the dam’s foundation
in a downstream direction.
The occurrence and rate of water
movement through a dam’s
foundation is dependent on the
properties of the naturally occurring
materials. The foundation may
consist of sound durable bedrock
with little or no fractures and
seepage may be non-existent.
However, the foundation may consist
of fractured bedrock or bedrock
may be so deep that the dam is
built on the soil above the bedrock
with potentially pervious properties.
Depending on the age of the dam
and the sophistication of its design,
a cutoff may not exist through
any fractured rock or pervious
foundation materials. Regardless of
the design, foundation seepage may
exist to some extent under a dam.
During elevated reservoir
pool levels caused by extreme
rainfall events, the added
pressure on the seepage path
along pipe penetrations or
through foundation materials
may become so strong that soil
particles begin to be displaced
out of the dam.
Water flowing along either of these
seepage paths can create a piping
failure of the dam.