4.2.6 Teton Dam

It was located on Teton River in S.E. Idaho. River diversion started in June 1973. Dam was topped out in November 1975. Maximum embankment height was 305 feet above valley floor and 405 feet above lowest excavated point in the foundation. Crest length was 3,100 feet. Crest elevation was 5,332 feet.

Failure was due to piping of Zone 1 material. Inner Zone 1 is composed of low-plasticity silt from windblown deposits, and outer Zone 2 was filled with sands & gravels from flood plain. Foundation was mostly rhyolite welded ash-flow tuff, with prominent and abundant joints, some basalt on left side, and Tuff underlain by sedimentary formations.

The actual mechanism causing piping is uncertain due to the complete erosion of the failed section.

Possible mechanisms leading to piping include:
- Seepage along the fill-to-rock interface, or along the top of the grout curtain, in the right key trench.
- Seepage along a low-density, high permeability lens within or adjacent to the right key trench.

Infractions of specifications were documented in:
- Preparation of rock surface on sidewalls of the key trench was inadequate.
- Grouting of the grout curtain, especially at shallow depths, was inadequate. Upper ends of some grout holes were not grouted.
- Inspection procedures for the Zone 1 fill were not adhered to early in construction. A "wet seam" of lower-density and higher permeability was identified in the lower portion of Zone 1.

Failure of Teton Dam, Early afternoon, June 5, 1976, Final Report by U.S. Dept. of the Interior, 1980

Sources:
- Ross Boulanger, Teton Dam failure, In-class handout, 2008
- Failure of Teton Dam – Final Report, by U.S. Department of the Interior, Teton Dam Failure Review Group, January 1980.
- Failure of Teton Dam – A Report of Findings, by U.S. Department of the Interior, Teton Dam Failure Review Group, April 1977.