Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsAnaerobic: An environment or a condition which is free of oxygen or a organismwhich can grow in the absence of oxygen.Anchor: To fasten to prevent movement.Annulus: The space between an existing conduit and a newly installed slipliner.Antiseep collar: An impermeable diaphragm, usually of sheet metal or concrete,constructed at intervals within the zone of saturation along the conduit that passesthrough an embankment dam. In theory, antiseep collars were designed to increasethe seepage length along the conduit and thereby prevent backward erosion pipingby lowering the hydraulic gradient along the conduit.Approach channel: The channel upstream from an entrance structure. Thischannel is generally unlined, excavated in rock or soil, and with or without riprap,soil cement or other types of erosion protection.Appurtenant structure (FEMA, 2004): An ancillary feature of an embankmentdam, such as an outlet, spillway, powerplant, or tunnel.Auguring: A drilling technique that advances a hole into a soil material. The drillbit used can be one of a wide variety of helical style bits.Autogenous growth (ASCE, 2000): Self-generating growth produced withoutexternal influence.Auxiliary spillway: See Spillway, auxiliary.Backfill: Soil or concrete used to fill excavations.Backward erosion piping (piping): The term “piping” has often been usedgenerically in literature to describe various erosional processes, not all of which holdto the classic definition of the term piping. Piping in the classic sense ischaracterized by the formation of an open tunnel that starts at a downstream seepageexit point and progresses back upstream toward the reservoir. This classic type ofpiping is often termed “backward erosion piping,” and this term is used in thisdocument. Blowout (also known as heave or blowup) is another term used todescribe the condition where hydraulic head loosens a uniform body of cohesionlesssand to the point where the permeability of the sand increases and flow concentratesin that zone that is blown out. Failures by blowout may not be exactly the same as“backward erosion piping,” but for the purposes of this document, are groupedunder this blanket term. Backward erosion piping involves the following essentialconditions:390