Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams1950-1970. Most of these sites had concrete conduits that would slowly release thetemporarily impounded floodwaters. The design criterion used for most of thoseembankment dams arbitrarily required that the seepage path through the saturatedportion of the embankment be increased by 15 percent by adding antiseep collars.This requirement did not vary with the soil type in the embankment dam. Usually,collars were spaced along the conduit every 20 to 25 feet through the earth core ofzoned embankment dams or through the central portion of homogeneous dams.Figure A-1 shows a conduit with antiseep collars with hand compacting of earthfillnext to the conduit. Figure A-2 shows a failure of a conduit with antiseep collarsconstructed around the conduit.A.4 Changes in philosophyFrom 1960 to 1980, a number of small embankment dam failures occurred, eventhough antiseep collars were carefully installed in well constructed earthenembankments. Several of the failures were at structures designed by the NaturalResources Conservation Service. Sherard (1972) reported on a study of thosefailures. The study showed that intergranular seepage and associated backwarderosion piping was not the mode of failure for these embankment dams. Thefailures usually occurred almost exclusively when the completed embankments werefirst subjected to a pool, long before a phreatic surface had time to develop throughthe compacted earthfill. Other studies by Sherard (1973) on larger earthenembankment dams attributed failures and near-failures to internal erosion of claycores through hydraulic fractures in the embankment zones.The reasons why antiseep collars were ineffective in preventing failures near conduitsmay be summarized as follows:• The antiseep collars only influenced water flowing in the immediate vicinity ofthe conduit. The collars did not significantly affect the remainder of thesurrounding earthfill. Most of the failures were found to have occurred notimmediately along the conduit, but in compacted fill outside the zone of theantiseep collars.• The antiseep collars were designed to increase head loss in intergranularseepage, but most failures occurred long before steady seepage conditionsoccurred. Studies showed the failures occurred from water flowing alongcracks within the earthfill, and not through the soil immediately next to theconduit.Sherard, Decker, and Ryker (1972) discuss the mechanisms by which failures canoccur in the following quote from the reference. In the quote, the term piping isA-4