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Living with Dams: Extreme Rainfall Events | 2015

| 15

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


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


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.