Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 1GeneralConduits have been placed through embankments for centuries. However, placing aconduit within an embankment dam increases the potential for seepage and internalerosion or backward erosion piping. Water may seep through the earthfillsurrounding the conduit, through cracks in the embankment caused by the conduit,or into or out of defects (e.g., cracks, deterioration, or separated joints) in theconduit. If the conduit is flowing under pressure, and defects exist in the conduit,the water escaping the conduit can erode surrounding soils.Replacement of a conduit through an embankment dam is difficult, time consuming,and expensive. Designers should adopt a conservative approach for the design ofconduits. The purpose of this chapter is to provide guidance for both constructingnew conduits and renovating or replacing existing conduits in embankment dams.When evaluating existing conduits, designers should attempt to determine howclosely the design of the existing conduit complies with criteria for new conduits. Ifthe existing conduit lacks state-of-the-practice defensive design measures, it may beconsidered inadequate by modern standards. These design measures should provideboth primary and secondary defensive measures to reduce the probability of failures.Inadequate conduit designs, poor construction, and improper maintenance canadversely affect the safety of embankment dams.1.1 Historical perspectiveMost designers of embankment dams have attempted to include defensive designmeasures to address potential seepage along conduits extending through earthfill orearth- and rockfill embankments. Even so, many observed failures and accidents ofembankment dams have occurred, involving conduits or the earthfill near theconduits. For large embankment dams, about one-half of all failures are due tointernal erosion or backward erosion piping. In about one-half of these failures,internal erosion or backward erosion piping was known to have initiated around ornear a conduit (Foster, Fell, and Spannagle, 2000, p. 1032). This means that about25 percent of all embankment dam failures are a result of internal erosion orbackward erosion piping associated with conduits.31