Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsUntil about the mid-1980s, the most common approaches for controlling seepagewere antiseep collars (also known as cutoff collars) and careful compaction (specialcompaction using small hand held compaction equipment) of backfill aroundconduits. Antiseep collars are impermeable diaphragms, usually of sheet metal orconcrete, constructed at intervals within the zone of saturation along the conduit.They increase the length of the seepage path along the conduit, which theoreticallylowers the hydraulic gradient and reduces the potential for backward erosion piping.Antiseep collars were designed primarily to address intergranular seepage (flowthrough the pore spaces of the intact soil). Antiseep collars did not fully address theoften more serious mechanism of failure (internal erosion), that occurs when waterflows through cracks and erodes the compacted earthfill near the conduit outside thezone of influence of the antiseep collars in the compacted earthfill near the conduits.In the 1980s, major embankment dam design agencies including the U.S. ArmyCorps of Engineers (USACE), and the Bureau of Reclamation (Reclamation)discontinued using antiseep collars on conduits for new dams. Reasons why antiseepcollars were abandoned include:• Antiseep collars impeded compaction of soils around the conduit.• Antiseep collars contributed to differential settlement and created potentialhydraulic fracture zones in the fill.• Designers realized that problems associated with conduits were more likely tobe caused from internal erosion mechanisms than from intergranular seepage.• Designers achieved increased confidence in the capability and reliability offilters to prevent internal erosion failures.• Antiseep collars can form a foundation discontinuity that could result indifferential settlement and cracking of the conduit.The Natural Resources Conservation Service (formerly Soil Conservation Service,SCS) also discontinued using antiseep collars on new embankment dams, butcontinues to allow them on small, low hazard dams and only under certain restrictiveconditions.Figures 14 through 16 show examples of the construction difficulties involved withcompaction around antiseep collars. Appendix A gives a detailed history of thedesign rationale used for antiseep collars and reasons for their being discontinued.Figures 17 and 18 show examples of the ineffectiveness of antiseep collars inpreventing embankment dam failure resulting from internal erosion near conduits.32