Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damscarried by the flowing water will accumulate on the surface of the diaphragm anddevelop a filter cake. The filter cake that develops on the upstream face of the filterdiaphragm reduces the flow and prevents further erosion of any cracks caused bythis flow. The filter diaphragm must extend far enough from the conduit that it canintercept all potential water flow paths associated with the conduit. A filterdiaphragm is typically installed during new construction or with conduit renovations.If the only postulated flow path is immediately along the contact between theearthfill and the conduit, the filter diaphragm may not need to extend far from theconduit. As an example, some agencies only use a filter diaphragm 18 inches thick,which is similar to a filter collar. In other cases, the embankment dam may besubject to hydraulic fracture in zones that are far above and on either side of theconduit. In the absence of a chimney filter, the filter diaphragm may need to bemuch wider and taller than the dimensions of the conduit to intercept those cracks.A filter diaphragm that extends farther from the conduit is often recommended fordesigns where significant differential settlement is associated with the conduit and/orthe trench used to install the conduit. This type of filter diaphragm is often used forembankment dams constructed without a chimney filter, when soils with a very lowresistance to internal erosion, such as dispersive clays, are used to construct the dam.This type of diaphragm is a zone of designed gradation filter sand that completelyencircles the conduit. The shape of the filter diaphragm is usually either rectangularor trapezoidal, and the diaphragm is typically 3 feet thick. Figure 91 shows a typicalconfiguration for a filter diaphragm. For further guidance on recommendeddimensions for a filter diaphragm, see NRCS’s Earth Dams and Reservoirs (1990) andDimensioning of Filter Drainage Diaphrams for Conduits According to TR60 (1989).The NRCS filter diaphragm typically has the following characteristics. Agency policyand the judgment of the individual designer may dictate different dimensions:140• Configuration.—The filter diaphragm is a rectangular or trapezoidally shapedzone of filter sand that is about 3 feet wide in a direction perpendicular to theconduit; see figure 91.• Location.—Locating the filter diaphragm along a conduit depends on several siteconditions. If the embankment is zoned, the filter is often located at thejuncture of the impervious core zone and downstream shell zone. In ahomogeneous embankment dam, the location of the filter diaphragm is usuallybased on the following requirements:1. Downstream of the cutoff trench.2. Downstream of the centerline of the embankment dam when no cutofftrench is used.