Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 8—Potential Defects Associated with Conduits2,000 ohm/cm will be more likely to corrode than conduits surrounded bysands that typically have resistance values of 30,000 to 50,000 ohm/cm.• Acidity (pH).—Most soils fall into a pH range of 6 to 8, which is neutral. Waterand soils with lower pH values are acidic and can result in a more corrosiveenvironment.• Moisture content.—Soils that drain rapidly are less corrosive than soils that tend tohold water longer. Soils with high clay content are typically more corrosivethan sandy soils.• Soluble salts.—Salts that become ionized can decrease the resistivity of a soil.• Oxygen content.—Increasing levels of dissolved oxygen can accelerate corrosion.The process of corrosion can proceed either uniformly or in pitting of the surface.Uniform corrosion is where corrosion occurs evenly over the surface, resulting in alow rate of corrosion. Pitting corrosion is not uniform and is focused only on asmall surface area, resulting in a high rate of corrosion, until a perforation eventuallydevelops. Pitting can begin on surface imperfections, scratches, or surface deposits.Between pH 5 and 9, pitting is likely to occur, if no protective film is present.In the past, CMP has been a commonly used material for conduits throughembankment dams. Thousands of embankment dams in the United States and allover the world have CMP conduits installed in them. Corrosion is a commonproblem with CMP conduits (figure 109). Many State highway departments havemade extensive studies on the use and durability of CMP for culverts under highwayembankments. However, available information on the use of CMP for conduitsthrough embankment dams is limited. A study of 50 existing CMP conduits inwatershed dams located in the Midwestern United States was done in 1989(Koelliker and Lin, 1990). The study determined that the estimated average life ofthe sampled CMP conduits was 43 years, but the lifespan ranged from 24 to 72 years.This study also found that leakage and associated corrosion at pipe joints was mostoften the primary limiting aspect of life expectancy. Many spillway conduit systemsconstructed with CMP experience corrosion at the joint connection between theconduit section and the riser (the vertical pipe or inlet that connects to the outletpipe). Spillway risers are subject to deformation and movement (tilting) caused byice loadings or erosion, which can open the joint connection with the outlet pipe.The riser itself is also susceptible to corrosion.The most susceptible portion of a CMP to corrosion is the invert, since it is exposedto the flow of water for the longest length of time. CMPs that have inverts with sagscould trap water and further increase the potential for corrosion. Corrosion of183