Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 6—Filter ZonesFigure 87.—Two stage chimney filter within an embankment dam locatedin Texas.Embankment dams with chimney filters do not normally require a separate feature tocontrol internal erosion or backward erosion piping along the conduit. The chimneyfilter will usually encompass the conduit and serve the function of a filter diaphragmas well as the other primary functions of a chimney filter, such as providingtransition filter capability and controlling the phreatic line. Figure 88 (top) shows azoned embankment dam with a chimney filter constructed along the interfacebetween the central core of the dam and the downstream shell. The chimney filterencompasses the conduit at the intersection and serves as a filter diaphragm as wellas a chimney filter.Figure 88 (bottom) illustrates a design where an embankment chimney filter alsoserves as a filter diaphragm around the conduit. This example is for a homogeneousembankment with a vertical chimney filter. Chimney filters are typically installed innew construction or within embankment dams undergoing extensive renovation.Figure 89 provides another example of a typical design used for a high hazardembankment dam. This design includes a chimney filter, a drain at the downstreamedge of the cutoff trench, and a foundation blanket drain. The chimney filter andcutoff drain satisfy the functions of a filter diaphragm around the conduit. Theblanket drain serves as an outlet for the collected seepage flow. The design shown istypical for embankment dams that have distinctly different materials in the core zoneand the exterior shells of the dam. The foundation blanket drain may also functionto collect seepage in bedrock or permeable foundation horizons, and function toconvey collected seepage to the downstream toe. Not all high hazard embankment135