Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case HistoriesProject name: Upper Red Rock Site 20 DamLocation: OklahomaSummary: Failure of an embankment dam by internal erosion resulting fromhydraulic fracture of earthfill adjacent to the flood control conduitUpper Red Rock Site 20 Dam was a low hazard earthfill embankment structureconstructed by the NRCS for flood control in 1973. The embankment height wasabout 31 feet and it contained about 61,000 cubic yards of earthfill. The principalspillway conduit is a 36-inch diameter reinforced concrete pipe. The embankmentdam was constructed in an area of Oklahoma now known to have a highconcentration of dispersive clays. The embankment soils classify as CL in theUnified System with an LL of about 35 and a PI of about 15. The dispersive claysare produced from weathering of Permian or Pennsylvanian age shales of marineorigin. The sodium rich parent material produce dispersive clays that are highlyerodible.The embankment dam failed in 1986 by internal erosion when the reservoir filledsuddenly to a reservoir elevation higher than the reservoir had previously everimpounded water. At the time of the failure, the reservoir was about 1.6 feet belowthe top of the crest of the embankment dam. The embankment dam hadimpounded water continuously at a lower reservoir elevation for most of its history,until the rainfall event that filled the reservoir to this unprecedented higher elevation.Most often, failures similar to this one occur when the reservoir fills suddenly soonafter completion of the embankment dam. This failure occurred however when a“second first filling” type of event occurred. The site had previously filled to a poollevel corresponding to about one-third of the embankment dam height (the dam is aflood control single purpose reservoir), and maintained that pool for most of the13 years of its life. A large rainfall event caused water to rapidly fill the reservoir andwater flowed through the auxiliary spillway, but not over the crest of theembankment dam in 1986. Cracks in the earthfill in the upper part of theembankment dam allowed water to find a pathway through unsaturated dispersiveclay fill used to construct the embankment dam. The crack(s) in the embankmentdam quickly eroded and a breaching type failure of the dam resulted. The cracks inthe embankment dam were thought to be a result of a combination of hydraulicfracture and desiccation. The failure tunnel was located about 40 feet to the side ofthe conduit that penetrated the embankment dam, in the area of the old streamchannel.B-105