Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 11—Appropriate Emergency Actionsexisting filters and drains were impacted, then these too should be replaced. Newfilters and drains should be added as needed.Several factors determine the extent of the excavation required to repair a sinkholethat was caused from embankment soil being eroded into a conduit defect. Onefactor is the method used to repair the defect in the conduit. In some cases, theconduit will be not be replaced, but will be repaired by one of the sliplining methods.In that case, the embankment does not have to be excavated to gain access to theconduit from the outside for repair operations. The extent and configuration of therequired excavation will then depend on how much embankment was damaged bythe sinkhole, and how the excavation must be prepared before subsequentreplacement of the sinkhole and excavated embankment can proceed.The configuration for the excavation made to repair the sinkhole must considerdifferential settlement that will occur between the excavation backfill and adjacentembankment and foundation soils that have already consolidated. The shape of theexcavation must also allow efficient operation of compaction equipment used in thereconstruction. Section 5.2 discusses the dangers of arching that can occur inbackfilled trenches that are overly steep, particularly when the trench is transverse tothe embankment centerline. Recommendations in that section suggest that anyexcavation made to repair sinkholes should probably be no steeper than 2H:1V forthis reason, and only that steep, if the embankment soils have favorable properties.Flatter slopes are recommended for less favorable conditions. The excavation mustalso be configured for use of appropriate compaction equipment. The slopes of theexcavation must be flat enough to operate equipment safely as the backfill of thesinkhole proceeds.If the repair of the sinkhole involves excavation and replacement of the damagedconduit, consider the recommendations for conduit replacement provided inchapter 13 together with the recommendations above. In most cases, the excavationrequired to replace the conduit will also remove the portion of the embankment damthat was damaged by the sinkhole.11.4 Alternative means of reservoir evacuationAlternative means of evacuating the reservoir should be specified in the event thatthe outlet works is inoperable, releases through the conduit are not possible/recommended due to internal erosion or backward erosion piping, or the outletcapacity has been reduced for some reason. The selection of a means that isappropriate depends on the size of the reservoir, the physical features of theparticular damsite, the availability of equipment and materials, the volume of waterthat could be released, and the required rate of release. Care should be employed indetermining the means of reservoir evacuation during a specific emergency, to277