Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 13Replacement of ConduitsGenerally, removal and replacement of an existing conduit through an embankmentdam consists of excavating the dam down to the existing conduit, stockpiling thematerial, removing the existing conduit, constructing a new conduit and possiblynew entrance and terminal structures, installing a filter diaphragm or collar aroundthe downstream portion of the conduit, and replacing the embankment material. Acofferdam may also be required if the reservoir cannot be drained duringconstruction.Removal and replacement of a deteriorating conduit can be time consuming andexpensive compared to other renovation methods. Typically, construction costs forremoval and replacement may be 5 to 10 times higher than for sliplining orcured-in-place conduit renovation methods. This cost difference depends upon theheight of the embankment dam. However, if the embankment dam is small and thedownstream impacts to users are acceptable; this method may be more advantageousthan renovation. Often, removal and replacement is the alternative of choice for lowhazard embankment dams, since it generally is less expensive. This is especially trueon older low hazard embankment dams, where they may have been built withoutadequate engineering. Few designers will want to try and guess how theembankment dam was built. The safer and more efficient solution would be toremove and replace the conduit and possibly the entire embankment dam..The advantages of removal and replacement of an existing conduit through anembankment dam include:• Evaluation.—The exposed foundation of the conduit can be fully evaluated.• Repairs.—Areas along the existing conduit that may have been damaged byinternal erosion or backward erosion piping can be repaired.• Seepage.—Extensive seepage control measures along the conduit can beinstalled.331