Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams11.3.2 Initial remediationFigure 147.—Sinkhole over a spillway conduit.If the sinkholes are active and it appears that immediate remediation is needed tostabilize the situation, the placement of a well graded sand and gravel mix withnonplastic fines into the sinkhole can be attempted. The concept is that placementof these materials directly into the hole will cause the sand and gravel to betransported directly to the defect in the conduit. A well graded mix will hopefullycontain some particles that are larger than the defect and these will thus get trapped.Once this occurs, then other, smaller particles will be trapped. Eventually theprocess is capable of filtering the embankment dam’s core material, causing a seal toform, arresting the erosion completely. This type of solution should only beconsidered a temporary one, to be followed by a full investigation of the problem.11.3.3 InvestigationA full investigation should be conducted to determine the root cause of the sinkholeor subsidence area. This is absolutely necessary. No permanent solution can bedesigned until the problem is pinpointed.Should a sinkhole become visible on the surface of an embankment dam, it is likelythat an erosional failure mode is well underway. Emergency measures should beinstituted as described in this chapter. After the emergency conditions have beenstabilized, probably by lowering the reservoir level, a forensic investigation of thesinkhole is warranted. A carefully planned and executed investigation can provide274