Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 6—Filter Zones3. Upstream of a point where the top of the filter diaphragm has at least athickness of soil overlying it that is a minimum of one-half of thedifference in elevation between the top of the diaphragm and themaximum potential reservoir water level.The rationale for the third requirement is that if an open crack occurred withinthe embankment dam and the full reservoir water pressure was acting on thecrack, that pressure would be transmitted along the crack with little head loss tothe point where the crack intercepted the filter diaphragm. At that point, thecrack would presumably be sealed from sloughed particles carried along thecrack to the face of the filter diaphragm. Then, at that interface between theopen crack with a seal and the filter diaphragm (see figure 86 [bottom]), fullreservoir hydrostatic pressure could exist. The criterion is intended then toensure that the weight of overlying soil in the embankment counters thishydrostatic pressure. The rule requiring the thickness of one-half the reservoirhead is based on the simplification that the unit weight of moist earthfill isapproximately twice the unit weight of water.• Vertical/horizontal limits.—The filter diaphragm should extend below and toeither side of the conduit far enough to intercept potential flow alongexcavation/embankment interfaces. Usually, the filter diaphragm extends intothe foundation a dimension equal to at least 1.5 times the diameter of theconduit, unless bedrock is encountered at a shallower depth.• Lateral limits.—The filter diaphragm usually extends laterally a distance at leastequal to 3 times the diameter of the conduit or a minimum of 10 feet from thesides of the conduit. In some situations the filter diaphragm may need to bewider than these minimum suggested dimensions. For instance, if anexcavation has been made for the conduit, the filter diaphragm should notchinto the excavation slopes by at least 2 feet; see figure 92.A designer should consider several factors in determining the dimensions to use for afilter diaphragm, as follows:• Whether a filter diaphragm is a “stand-alone” element of the embankment dam’s design, or ifit is a coincidental part of a chimney filter in the dam.—Many embankment damsinclude a chimney filter that extends across the entire length of the dam. Whenthe chimney filter is located where it can also encompass the conduit passingthrough the embankment dam, a separate filter diaphragm is not required.Figure 88 shows two configurations commonly used for chimney filters thatwould serve the function of a filter diaphragm, as well as the functions of achimney filter.141