Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case HistoriesProject name: Little Chippewa Creek DamLocation: OhioSummary: Separation of spillway conduit joints due to foundation movementThe joints of the 48-inch reinforced concrete spillway pipe separated whenfoundation movement occurred during final stages of embankment construction.The failed spillway was removed, a new structure was constructed in a differentlocation, and stabilizing berms were added to the embankment design.Little Chippewa Creek Dam, known officially as “Chippewa Conservancy DistrictStructure VIIc” is a high-hazard dam located about 3 miles northwest of the city ofOrrville, Ohio. The 27-foot high embankment dam is a single-purpose, dry floodcontrol structure designed in 1971 by the Soil Conservation Service under theauthority of Public Law PL-566.The embankment dam was designed with an upstream slope of 3H:1V and adownstream slope of 2.5H:1V. The site lies on the glaciated, moderately rollingAllegheny Plateau. The site was glaciated during a series of advances and retreatsduring the Wisconsin Stage of the Pleistocene Epoch. The foundation soils consistof glacial outwash deposits of layered sand, silt and clay.Construction of the embankment dam started in July 1972. During spillwayconstruction, the inspector noted the presence of soft, gray silt at the bottom of theexcavation for the pipe. The foundation was overexcavated by 1 foot and backfilledwith AASHTO No. 46 coarse aggregate. The pipe was installed on top of theaggregate and bedded in concrete. After the pipe joints were covered with 12-inchwide sheet metal shields, embankment material was backfilled around the pipe.Construction was suspended in late 1972 due to winter weather.Construction of the embankment dam resumed in July 1973. Earthfill placementproceeded rapidly without incident until mid-August 1973. As the embankment damwas nearing completion, the downstream portion failed suddenly, severely damagingthe spillway conduit. The downstream end of the conduit moved about 2.4 feet inthe downstream direction. A 1.5-foot high bulge was observed in the streambottom, and some cracks were observed in the slope below the dam. However, nosettlement, cracking, or other distress of the embankment dam itself were observed.The bottom of the pipe under the maximum earthfill height settled by about 1.5 feet,and the joints of one section of the pipe separated by more than 1 foot (figure B-49).The engineers investigating of the failure reported that the foundation soils under theB-57