SMALL DAMS PETITS BARRAGES

7.3.4.2 Seepage Interception and Exit Control With Filters and Drains
Filters are typically the first line of defense against piping and internal erosion. Filters
and drains are often incorporated as remedial systems to control seepage in existing
embankment dams.
For small embankment dams that can be dewatered, it is possible to install central
vertical filters in open trenches. In the southwestern US, for example, hundreds of small
homogeneous earth dams were constructed for flood control under a federal government
watershed protection program that was initiated in 1954. Many of these flood control
structures were subsequently found to have developed severe cracking caused by desiccation
shrinkage, differential settlement, and collapse on inundation of metastable soils in the dam
foundations. However, recognizing the potential risks associated with piping and internal
erosion failure modes in cracked homogeneous embankment dams, NRCS retrofitted a large
number of the cracked dams with central sand and gravel filters, as shown on Figure 7.14.
NRCS also performed landmark research studies that advanced the general understanding of
the performance and design requirements for granular filters (Sherard et al., 1984a, 1984b;
Sherard and Dunnigan, 1985, 1989).
For water retention dams, or dams that are too high for open trenching from the crest,
the filter and drain zones can be constructed as weighted berms and wedges of fill on the
downstream side of the dam.
Fig. 7.14 – Central sand filter installed by open trenching from the crest of an existing
low-height flood control dam in southwestern USA
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