Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams• Loss of use.—A replacement means of providing downstream flow requirementsreservoir evaluation comparability, and flood discharge capacity will berequired.The most common way to abandon an existing conduit is by the injection of groutor concrete. Two methods are usually considered for grout injection:• From upstream or downstream access.—If conduit access is available from either anupstream or downstream location, this typically provides the simplest methodfor grout or concrete injection. Removal of a portion of the entrance orterminal structures may be required to attain sufficient access. Concrete (with aslump of about 6 or 7 inches) injection is typically more economical, since alarger diameter (about 5 inches) pump line can be used. This type of concreteis often called “backfill” concrete placed with a “slick” line. Injection of grouttypically uses a pump line diameter of about 1 to 1½ inches. Also, when usingconcrete, a pump truck can be used. Grout injection normally requires a mixer,which will deliver grout at a slower capacity and will require more time to fillthe existing conduit.• Through holes drilled from the surface of the embankment dam.— When the upstreamand downstream ends of the existing conduit are inaccessible, it may bepossible to fill the conduit with grout or concrete through holes drilled fromthe surface of the embankment dam (figure 197). In order to be successful, theprecise location of the existing conduit must be determined, and the drillermust be experienced and proceed with caution. For an example of this type ofconduit abandonment, see the Clair Peake Dam case history in appendix B.While completely filling the existing conduit is recommended, the need forcompletely or partially filling the entire conduit will need to be evaluated based onsafety concerns and costs. The indicated grouting and backfill procedures in thissection may require modification to adapt to given site conditions. The designer iscautioned that grout injection from the surface, unless carefully controlled, has thepotential for causing hydraulic fracture within the embankment dam. Drilling fromthe surface of the embankment dam is not advisable for situations where thereservoir water surface cannot be lowered.Another possible reason for abandoning an existing conduit would be when aproposed embankment dam raise would result in much higher embankment loadsover portions of the conduit. Structural analysis may determine that the higherembankment loads would fail the existing conduit and measures to strengthen it arenot feasible. To prevent failure and to reduce the potential for internal erosion andbackward piping erosion of embankment materials, the existing conduit would needto be abandoned and a new conduit constructed. The abandonment of the existingconduit may best be postponed until after the replacement conduit has been356