Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsThe advantages of grouting along an existing conduit through an embankment daminclude:• Impacts.—Construction impacts to downstream users are minimized.• Costs.—Construction costs are generally lower (initially) than for otherrenovation or replacement methods. However, grouting along conduits is notalways fully successful and should not be considered a permanent repair. Thismethod does allow for additional grouting attempts to be made in the future.The disadvantages of grouting along a conduit through an embankment dam include:• Not permanent.—This method should not be considered a permanent renovationmethod. This method will have limited effect on corrosion prevention orimprovement of structural integrity of the conduit.• Voids.—The filling of all voids surrounding the existing conduit cannot beverified.Grouting from the interior of man-entry accessible existing conduits generallyprovides a higher degree of success in filling voids outside the existing conduit andsealing joints. Geophysical methods can be used to identify areas of suspected voids.See chapter 10 for guidance on geophysical methods. These areas can be drilled anda small video camera inserted to determine the extent of the void. Accommodationswill need to be made for removal and control of water leaking into the existingconduit as the grout injection work is performed. Partial lowering of the reservoirwill reduce leakage into the existing conduit. While work is going on within theexisting conduit, a pumping system to discharge reservoir inflows may be required tokeep the reservoir at the desired elevation or to meet downstream requirements.Existing conduits that are not accessible to man-entry generally have limited successin filling voids outside the conduit and sealing joints. Drilling is usuallyaccomplished from the surface of the embankment dam. As the height of theembankment dam increases, so does the degree of difficulty for injecting grout at thedesired location along the existing conduit. Grout is advanced from the surfacethrough the drill hole to the void using pressure. The designer is cautioned that thismethod, unless carefully controlled, has the potential for causing hydraulic fracturewithin the embankment dam. Drilling from the surface of the embankment dam isnot advisable for situations where the reservoir water surface cannot be lowered. Ifthe reservoir cannot be lowered, grouting along the upstream portion of the conduitis not practicable. See section 14.3 for additional guidance on drilling inembankment dams.346