Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsProject name: Willow Creek DamLocation: MontanaSummary: Lining of an existing outlet works conduit using CIPPWillow Creek Dam is located in western Montana. The 84-foot high embankmentdam impounds a reservoir of 32,300 acre-feet, used primarily for irrigation. Theoutlet works consists of a 54-inch diameter concrete-lined tunnel through the rightabutment, with guard and regulating gates provided within a gate shaft upstream ofthe dam axis. The embankment dam and outlet works were originally constructedbetween 1907 and 1911, and were modified in 1917 and 1941. The Bureau ofReclamation owns the embankment dam, and the Greenfields Irrigation Districtoperates it.A large sinkhole was discovered on the crest of the embankment dam in June 1996.The sinkhole was located directly above the outlet works tunnel, about 50 feetdownstream from the gate shaft and near the dam axis. Earth materials were foundto be eroding periodically from a 1-inch weep hole in the tunnel sidewall. Four weepholes in the tunnel lining were sealed, and the sinkhole was temporarily stabilized bybackfilling with sand and gravel materials. The reservoir was gradually lowered usingthe outlet works, for a total drawdown of 27 feet.Excavation of the embankment dam revealed the sinkhole extended through 40 feetof bedrock to a large cavity surrounding the concrete tunnel lining. Tremie groutwas used to fill the voids around the tunnel, followed by the placement of backfillconcrete to the excavated bedrock surface at elevation. The embankment dam wasrestored by the placement of a filter blanket on the excavated foundation, followedby the placement of compacted glacial till materials to dam crest elevation, andreplacement of the slope protection on the downstream face.The outlet works tunnel was originally excavated in 1907 by hand-drilling andblasting, with considerable water and soft materials encountered. Heavy timberbeams and posts with timber lagging were used to support the tunnel excavationthroughout its length, resulting in a square, excavated opening for the circular tunnellining. Although it is unclear how the concrete tunnel lining was actuallyconstructed, the specifications called for a uniform concrete thickness of 8 inches,without reinforcing bars, and the placement of “puddled fill” outside the tunnellining to the excavated surface. Such construction could have resulted in asignificant quantity of fine grained backfill material along the outlet works tunnel.B-112