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9 Filter Design and Construction Considerations Introduction This article is intended to provide practical guidance for use by dam owners and engineers for the design and construction of filters for embankment dams, particularly small embankment dams. This article is not intended to be an all-inclusive guide for design of filter and drain systems. In many instances, the article directs readers to other references that provide more detailed information. In addition, an extensive list of references on the topic is provided at the end of this article. Why Filters? Although there are many existing dams that were constructed without filters and which have performed satisfactorily, filters offer substantial benefits with respect to dam safety. A well-designed filter provides protection against possible defects in an embankment core. If a core contains pervious layers or through-going transverse cracks, a filter (commonly referred to as a chimney filter) will safely collect seepage through these defects and prevent piping of the core, as illustrated in Figure 1. Figure 1: Filter collecting flow through core defect. Filters placed around conduits or other structural penetrations (commonly referred to as filter diaphragms) also provide protection against internal erosion or piping along the exterior walls of the penetration, where seepage is most likely to occur. Filters installed around conduits or structural penetrations should always include an outlet to prevent water pressure from building up in the filter, as shown in Figure 2. Figure 2: Filter diaphragm surrounding conduit. Additional benefits for incorporating a filter include: • Well-designed chimney filters provide positive control to produce a phreatic surface that is well within the embankment, improving stability. • Dam safety risk analyses have shown that a well- designed filter provides substantial benefits in risk reduction. Specifically, it is recommended that filters be included in all of the following cases: • All new dams over 25 feet high. • Existing dams with evidence of seepage above the toe on the downstream face. • Existing dams with likely defects through the core. • Existing dams in seismic areas with the likelihood of cracking under seismic loading. • Outlet works replacements or rehabilitations for existing dams. Now that we understand the importance of filters in embankment dams, let’s discuss some of the considerations that should be included during design. Designing Embankment Filters Three of the most important factors to consider during the design of an embankment filter are gradation, location, and size/thickness. The material gradation of the filter is important to ensure filter compatibility requirements are met for surrounding materials and to prevent piping or internal erosion of the embankment. The location of the filter is important to ensure it is 30% of all dam failures have been attributed to seepage or piping that could have likely been averted by a proper filter