Teton Dam - History and Geology

History and Geology

There had been interest in building a dam in the eastern Snake River Plain for many years, to control spring runoff and provide a more constant water supply in the summer. The area had suffered a severe drought in 1961, followed by serious flooding in 1962. The Bureau of Reclamation proposed the Teton Dam in 1963, and Congress passed without opposition an authorizing bill the following year. The planned dam was to be an earthen structure 310 feet high and 0.6 miles long which would create a reservoir 17 miles long. The impounded water would be used to generate hydroelectric power. An environmental impact statement was issued for the dam in 1971, but it did not raise the possibility of a collapse.

The primary contractor for the dam was Morrison-Knudsen Co. of Boise, assisted by Peter Kiewet Sons Co. of Omaha, Nebraska. The $39 million contract was awarded in December 1971 and work began in 1972.

The eastern Snake River Plain is almost entirely underlain by basalt erupted from large shield volcanos on top of rhyolitic ash-flow tuff and ignimbrites. The tuff, a late-Cenozoic volcanic rock dates to about 1.9 million years. The dam site is composed of basalt and rhyolite, both of which are considered unsuitable for dam construction because of their high permeability. This was confirmed by long term pump-in tests at rates of 165 to 460 gallons per minute. Test cores, drilled by engineers and geologists employed by the Bureau of Reclamation, showed that the rock at the dam site is highly fissured and unstable, particularly on the right side of the canyon. The widest fissures were determined to be 1.7 inches wide. The Bureau planned to seal these fissures by injecting grout into the rock under high pressure to create a grout curtain in the rock.

In addition, an investigation of the area by geologist of the United States Geological Survey indicated that it was seismically active: five earthquakes had occurred within 30 miles of the dam site in the previous five years, two of which had been of significant magnitude. This information was provided to the Bureau of Reclamation in a memorandum, but the geologists' concerns were considerably watered down in the six-month re-drafting process before the USGS sent the final version of the memo to the BOR in July 1973.

In 1973, when the dam was only half-built, but almost $5 million had already been spent on the project, large open fissures were encountered during excavation of the key trench near the right end of the dam, about 700 feet from the canyon wall. The two largest, near-vertical fissures trend generally east-west and extend more than 100 feet below the bottom of the key trench. Some of the fissures are lined by calcite, and rubble fills some of them. Several voids, as much as 6 inches wide, were encountered 60-85 feet below the ground surface beyond the right end of the dam and grout curtain. The largest fissures were actually enterable caves. One of them was eleven feet wide and a hunded feet long. Another one was nine feet wide in places and 190 feet long. These were not grouted because they were beyond the keyway trench and beyond the area where the Bureau had decided grouting was required. This necessitated using twice as much grouting as had been originally anticipated – 118,000 linear feet were used in total. Later, the report of a committee of the House of Representatives which investigated the dam's collapse felt that the discovery of the caves should have been sufficient for the Bureau of Reclamation to doubt its ability to fill them in with grout, but this did not happen: the Bureau continued to insist, even after the dam had failed, that the grouting was appropriate.