Technical Manual: Conduits through Embankment Dams (FEMA 484)

Problem orconditions Cause Response or emergency repair actionsExcessivesettlement oftheembankment ordam crestConduit failurecollapses. A small hole in thewall of a conduit can allowbackward erosion piping ofmaterials and develop asinkhole. Dirty water at theexit portal indicates erosion ofthe embankment dam materials.Lack of or loss of strength ofembankment materials. Loss ofstrength can be attributed toinfiltration of water into theembankment materials from acrack in the conduit or loss ofsupport by the dam foundation,causing a settlement or collapseof a conduit. Internal erosion orbackward erosion piping of theembankment dam materialsalong the conduit.Cracks, holes or joint offsets inthe conduit caused bysettlement, rust, erosion,cavitation and poorconstruction. Broken/bentsupport block or control stemand broken/missing stem guidesdue to concrete deterioration,rust, excessive force exertedwhen operating the outletgate/valve, and poormaintenance. Damage due torust, cavitation, erosion,vibration, wear, ice action, orexcessive stresses from forcinggate/valve closed when it isjammed.• Carefully inspect and record location and dimensions(depth, width, length) of the sinkhole. Stake out the sinkholeto monitor any growth and development of the sinkhole.Frequent monitoring of sinkholes and seepage.• Lower the reservoir level to a safe level or until theseepage stops. If the sinkhole results from backward erosionpiping of embankment materials into the conduit, alternativemeans to evacuate the reservoir may be required, such assiphoning, pumping, or controlled breach.• Excavate the sinkhole and plug the flow withwhatever material is available (e.g., hay bales, bentonite,or plastic sheeting), if the entrance to the internal erosioncan be located.• Place a protective filter of sand and gravel over theexit point(s) to prevent further migration of fineembankment materials.• Engage a qualified professional engineer to inspectthe conditions and recommend further corrective actions tobe taken.• Establish monuments along length of crest andselected locations on the embankment dam to determineexact amount, location, and extent of the settlement.• Engage a qualified professional engineer todetermine the cause of the settlement and to supervise allsteps necessary to reduce possible threat to the dam andcorrect the condition.• Re-establish lost freeboard, if required, by placingsandbags or backfilling in the top of the slide with suitableembankment materials. Caution should be exercised not tofurther increase slide potential.• Re-establish monuments across the crest andselected locations on the embankment dam and monitormonuments on a routine basis to detect possible futuresettlement.• If continued movement of the settlement of theembankment dam is seen, begin lowering the reservoir at arate and to an elevation considered safe given thesettlement condition.• Continue operating the reservoir at a reducedreservoir level until repairs can be made.• If internal erosion or backward erosion piping of theembankment materials through the conduit is the problem,close the outlet gate/valve to protect the embankmentdam from further erosional damage.• Lower the reservoir to a safe level. If the outletworks is inoperable or cannot be operable for some reason,alternative means to evacuate the reservoir may berequired, such siphoning, pumping, or controlled breach.• Monitor the conduit for settlement, development ofsinkholes, and muddy leakage.• Implement temporary measures to protect thedamaged structure, such as closing the outlet gate/valve.• Employ experienced professional divers, if necessaryto assess the problem and possibly implement repairs.• Engage a qualified professional engineer to inspectthe conditions and recommend further corrective actions tobe taken.270