Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case HistoriesProject name: Olufson DamLocation: WashingtonSummary: Outlet works conduit failureOlufson Dam was a privately owned embankment dam located in Pierce County,near Gig Harbor, Washington that experienced an outlet works conduit failure. Theembankment dam was 18 feet high, with a storage capacity of 15 acre-feet and21 acre-feet at the top of the dam. The principal spillway consisted of a 2-footsquare, concrete, drop inlet conduit. An open channel in the abutment served as anemergency spillway. The embankment dam was constructed in the 1960s withoutthe benefit of engineering plans. The owner did all the work himself, includingplacing earthfill and mixing his own concrete onsite. Conditions exposed by thefailure suggest that the elements of the construction that required skill weresubstandard. In particular, the concrete work suffered from inadequate cementcontent, poor overall mix gradation, and improper reinforcing. Thick steel cable wassubstituted, in part, for conventional reinforcing steel. Likewise, these cables wereimproperly positioned in the conduit section thus minimizing its enhancement of thetensile load capacity of the conduit. To limit concrete volumes, it appeared theowner had embedded bricks, rocks and concrete rubble into the walls as a fillerduring concrete pours. This practice, termed cyclopean concrete construction, hasbeen successfully used in large gravity structures, but was inappropriate for thinwalled, concrete box conduits.On December 11, 1996, a sinkhole 20 feet in diameter and 17 feet deep opened upin the crest of the embankment dam (figure B-56). At the time the sinkholedeveloped, the property on which the embankment dam sat was uninhabited due tothe recent death of the property owner. The sinkhole was discovered by neighborswalking the streambed to investigate the cause of muddy streamflows. This wasfortuitous in that the sinkhole was discovered before it lead to an embankmentfailure. The sinkhole appeared to have resulted from a collapse in the top section ofthe cast-in-place box culvert that served as the principal reservoir outlet. The failedsegment of the conduit allowed overlying masses of embankment soil, over time, torepeatedly drop into the conduit, where flows then flushed the soil downstream.This sequence of events was supported by the record of stream flows in adownstream gauging station. The gauge record shows normal flows interrupted by aseries of near zero creek flows immediately followed by short, abnormally highchannel discharges. The zero flows are interpreted as incidences of soil massesfalling into and plugging the conduit. The following anomalous high flows representa blowing out of the plug and a release of backwater in the conduit and inlet towerupstream of the plug.B-69