Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 6—Filter Zonesfiltering the sand and providing a controlled outlet for the collected seepage. Thegravel should meet filter criteria for the sand filter.At the very downstream toe of the embankment dam, a short section of perforateddrain pipe (similar to a toe drain) is often provided to collect and convey any seepagewater out of the filter collar. This collector pipe will help keep the toe area frombecoming boggy. The pipe will also provide opportunity to measure seepage flowand monitor for sediment transport. Provisions should be made in the design of thecollector pipe to allow for inspection by CCTV equipment. See section 9.5.4.4 for adiscussion of CCTV inspection.Burying the collector pipe too far into the embankment should be avoided. Thegravel envelope drain should be capable of providing the needed drainage. Thedesigner should provide access provisions for the collector pipe to enable inspectionand cleaning. Also, should the collector pipe become damaged, it should be locatedsuch that it can be removed and replaced.6.6 Filter and drain gradation designDesigning the gradation of the sands used in the filter is important if they are tofunction properly. Standard filter design methods, such as the NRCS designprocedure shown in Gradation Design of Sand and Gravel Filters, National EngineeringHandbook, Part 633, Chapter 26 (1994), the USACE’s General Design and ConstructionConsiderations for Earth and Rock-Fll Dams (2004a), and Reclamation’s EmbankmentDams—Protective Filters (1999) are typical. Materials suitable for filters will rarely beavailable onsite and are usually purchased from concrete aggregate suppliers orprocessed from materials near the site. For many designs, ASTM C 33 fine concreteaggregate is found to meet criteria. However, designers should always determine arange of compatible filter gradations that will protect the soils used in thesurrounding embankment to increase construction flexibility. Sands used toconstruct filter diaphragms, filter collars, or chimney filters should be filtercompatible with the embankment zones being protected, and they must also be ableto deform and fill any cracks that may be propagated to the filter. If the filter doesnot have a property referred to as “self-healing,” the crack could propagate throughthe filter, and the filter would not satisfy its intended function. Vaughan and Soares(1982, p. 29) and the USACE (1993, p. 8-6) have described a simple test forevaluating the self-healing ability of a filter. Factors that influence the ability of afilter to be self-healing are the percentage of fines (percent finer than the No. 200sieve) and the plasticity of the fines. Filter designs usually require a low percentageof nonplastic fines (usually less than 5 percent as measured after placement in theembankment dam and compaction of the filter) to ensure that filters have adequatepermeability and self-healing characteristics. Fine, poorly graded filters have less147