Technical Manual: Conduits through Embankment Dams (FEMA 484)

Introductionbe observable at the surface of the embankment or foundation. Visual inspection,seepage and turbidity measurements, and pore pressure readings are useful indetecting whether problems like these may be developing in an embankment dam.Chapter 9 discusses inspection techniques in detail.Design and construction inadequacies are often to blame for internal erosion andbackward erosion piping incidents. Designers must understand which designmeasures are effective in preventing these mechanisms of failure.In nonpressurized conduits, water seeping through the embankment dam can enterthe conduit through defects. If the surrounding soils are susceptible to backwarderosion piping, cavities can develop in the embankment and foundation of theconduit. This problem is discussed in more detail in section 7.1.In pressure flow conduits, water under pressure can escape through defects anddamage the surrounding embankment and foundation. This problem is discussed inmore detail in section 7.2.In nonpressurized or pressurized conduits, water seeping along the interface betweenthe conduit and surrounding soil may be concentrated enough to result in backwarderosion piping, if the soils are susceptible. If the soils are resistant to backwarderosion piping, but a crack or potential flow path develops near the conduit, internalerosion can result. This problem is discussed in more detail in section 7.3.If the soils surrounding the conduit are resistant to backward erosion piping,hydraulic fracture may occur. The hydraulic fracture created can then erode and leadto a failure tunnel that is similar to that which develops in soils that are susceptible tobackward erosion piping. This problem is discussed in more detail in section 7.4.Internal erosion and backward erosion piping incidents are often associated withconduits through embankment dams. The following factors increase the likelihoodof these problems developing at a given site:• Conduits constructed across abruptly changing foundation conditions (i.e., aconcrete core wall or bedrock with a quickly changing profile) are more likely toexperience differential settlement. See section 1.2 for more discussion onfactors in locating conduits in the most favorable conditions.• Circular conduits constructed without concrete bedding or cradles are morelikely to experience problems than conduits in more favorable shapes (i.e.,horseshoe). See section 4.1 for more discussion on conduit shapes.• Conduits with an excessive number of joints are more likely to develop defectsthat can lead to problems. See section 4.3 for discussion on joints in conduits.17