Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case HistoriesProject name: Turtle (Twin) Lake DamLocation: MontanaSummary: Sliplining of an existing outlet works conduit using HDPEConstructed in 1932 by the U.S. Indian Irrigation Service (now Bureau of IndianAffairs), Twin or Turtle Lake Dam is a 20-foot-high embankment dam, locatedabout 4 miles southeast of Polson, Montana on the Flathead Indian Reservation.The original outlet works at Turtle Lake Dam was constructed by cut and covermethods. The outlet works conduits were constructed of concrete pressure pipe inapproximately 4-foot long sections. The upstream conduit and the first 40 feet ofthe downstream conduit were 21-inch diameter pipe. The vertical wet well shaft atthe upstream edge of the dam crest separates the upstream and downstream conduitsand houses a 24-inch diameter slide gate, which regulates discharges. The remainderof the downstream conduit (340 feet, much of which extends downstream of the toeof the embankment dam) is 18-inch diameter.During the 1996-1997 winter, a sinkhole was discovered above the lower reaches ofthe downstream conduit. Investigations showed that a root ball had partially pluggedthe outlet, and portions of the pipe had partially collapsed. During the late spring of1997, the Bureau of Reclamation designed a temporary repair for the deterioratingoutlet works. Because of the small size of the conduit and the concern for seepagecoming into the conduit, it was decided that a watertight liner should be used. Theconduit downstream of the embankment dam toe was excavated, but it was notdesirable to excavate in the embankment dam itself. For this reason and becauseremotely CCTV inspection seemed to indicate potential for offsets and changes inthe conduit alignment, an HDPE sliplining was proposed.A 16-inch O.D. HDPE pipe was sliplined (figures B-75 and B-76) into the existingdownstream conduit beneath the dam up to the regulating gate, and the annulusbetween the existing and new pipes was grouted with a cement grout withsuperplasticizer. Downstream of the dam toe, the pipe transitions to a 22-inch O.D.pipe, which extends downstream to the original portal location.The HDPE-lined outlet works at Turtle Lake Dam has been in operation sincemodification without further incident. A CCTV inspection was conducted inApril 2001. The inspection indicated that the HDPE slipliner was performing well.B-103