Teton Dam Failure

Teton Dam was located on the eastern edge of the Snake River Plain about 44 miles northeast of Idaho Falls, Idaho. The dam was a central-core zoned earthfill structure, whose structural height was 405 feet and length was 3,100 feet (Figure 1). The height above streambed was 305 feet. The volume of embankment was about 10 million cubic yards. The project was designed to provide supplemental irrigation for 111,210 acres, power from two 10,000-kilowatt generators, flood control, recreation, and fish and wildlife benefits. The dam was constructed during the period from February 1972 to June 1976 by prime contractor Morrison-Knudsen Company, Inc., for the Bureau of Reclamation. At the site, the Teton River has eroded a steep-walled, narrow canyon some 400 feet deep into the foundation rock, a welded ashflow tuff (rhyolite in composition) that lies unconformably on a highly uneven surface sediments. Unconsolidated river alluvium occurred in the channel section to depths of 100 feet. Beneath the alluvium, on the left side of the channel, an intracanyon basalitc flow is present as an erosional remnant (Figure 2). Provisions for seepage control included a key trench in the foundation rock above elevation 5100 feet and a cutoff trench to foundation rock below this elevation. A grout curtain extended below these trenches. The volcanic rocks at the site were highly permeable and moderately to intensely fractured requiring large amounts of grout. As work progressed, several large fissures appeared on the north canyon wall, so large a person could walk into the bottom of several. Before all cracks were filled, approximately 500,000 cubic feet of grout was used, more than twice the original estimate. Due to the scarcity of more plastic materials, wind-deposited nonplastic to slightly plastic clayey silts were used for the core and key trench fill. The embankment was zoned, but no process filters or toe drains were incorporated in the designs. The embankment was topped out on November 26, 1975 as other work continued into the next year (Figure 3). Filling of the reservoir commenced on October 3, 1975 and continued until the dam failed on June 5, 1976. 12 pp., 10 figures, 9 references.