Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsFigure 92.—Typical configuration for filter diaphragm used in design of anembankment dam. The filter diaphragm should extend into the foundationsoils, where an excavation is made for the conduit.• The type of equipment used to construct the filter diaphragm.—If a chimney filter is usedin the design of the embankment dam and it serves the coincidental purpose ofa filter diaphragm, then dimensions appropriate for a chimney filter should beused. Many times, chimney filters are sized for the width of equipment (about10 to 12 feet) to accommodate production equipment in placing andcompacting this zone in the embankment cross section. In small embankmentdams where the cost of the chimney filter may be excessive, a narrower widthmay be considered.• The method of constructing the filter zone.—If the chimney filter zone or filterdiaphragm is constructed concurrently with the adjoining fill zones, using awidth of 10 to 12 feet is suited to most construction equipment. A chimneyfilter designed with multiple filter zones complicates construction. Figure 87shows such a design. If the filter zone is placed in a trench cut in compactedfill, a width of 3 feet is often specified, because that is typically the width ofbackhoe buckets used for this method of construction. A paper by Hammer(2003) includes good discussions of the advantages and disadvantages ofvarious methods for constructing chimney filter zones. The discussions wouldapply equally well to sites where only a filter diaphragm was used instead of afull chimney filter section. The trenching method is illustrated in figures 6, 7,and 8 of that paper.• The predicted zones of embankment susceptible to hydraulic fracture resulting from thepresence of the conduit in the fill.—The filter diaphragm should extend vertically andlaterally far enough to intercept all zones of the fill and foundation that aresusceptible to hydraulic fracture attributable to the presence of the conduit inthe dam. Hydraulic fracture is discussed in section 5.2. Hydraulic fracturezones not caused by the conduit, but by other factors, such as steeply dipping142