SMALL DAMS PETITS BARRAGES

In some cases, the downstream face or foundation area downstream of the dam can be
treated to control piping, but not reduce seepage. Impervious materials should never be
placed against an area of seepage downstream from the impervious zone of a dam because
excessive pore pressures or uplift and a reduction in stability would result. To control piping,
filters must be placed against the affected areas to prevent migration of soil, and free-draining
materials should be placed against filters to convey seepage water. The grading (or particle
size distribution) of the filter and drain zones are determined by strict adherence to established
filter criteria.
In an emergency situation when large quantities of turbid water emerge from the dam or
foundation, the first response is to lower the reservoir as rapidly as possible. At the same time
three remedial actions should be attempted: (1) bulldoze large volumes of soil, gravel and rock
in the vicinity of the source of leakage in the reservoir if indicated by vortices, (2) add bales of
straw on the upstream side and (3) dump filter and free draining material onto the areas of
discharge.
Increasing flow through abutment or foundation materials should be viewed as a serious
problem that can worsen with time. Increasing flow, especially if turbid, indicates loss of
supporting materials. Erosion of pervious foundation/abutment layers, leaching of soluble
layers (e.g. calcium carbonate or gypsum), or subsurface collapsing due to breakthrough into
caverns can cause the overlying embankment to undergo differential settlement and cracking.
Seepage at the foundation embankment interface is especially serious as loss of embankment
material by piping could result.
If seepage rate is increasing and piping is suspected, the first follow-up action is to
trigger emergency plan and to start lowering the reservoir and follow the steps required by the
emergency action plan (Chapter 8).
7.3.4 Seepage Control Measures
Most embankment dams experience seepage to some degree. Under certain
conditions, excessive, or even moderate uncontrolled seepage can lead to progressive internal
erosion or piping of the embankment or foundation materials. There are many case histories
that describe seepage and internal erosion which went undetected for significant time periods
before the problems were recognized and seepage control measures implemented as
remedial measures (or rehabilitation).
It is sometimes difficult to determine how, or if, seepage is adversely affecting the safety
of a dam, and whether or not remedial measures are required to control seepage. Monitoring
tools and techniques for detecting potential seepage problems are covered in other sections of
this report. Once it has been determined that a dam requires modifications to control
seepage, the options that are available fall under two broad categories, as follows:
Seepage cutoff or barrier systems (reduce quantity of seepage);
Seepage interception and conveyance with engineered filters and drains (filters
prevent movement of soil particles under seepage forces; and drains relieve excessive
pressure, intercept seepage pathways, and provide for safe collection and conveyance of
seepage from the dam, its foundation and/or its abutments).
It is common to use those two categories of solutions in combination.
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