The Teton Dam Collapse:
4A6(1)
Friday 28th October 2005
Group 22
The Teton Dam Collapse
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4A6(1):Group 22
The Teton Dam Collapse
Introduction:
The Teton Dam Project was first thought up in 1932, at a site 15 miles upstream
of the actual Teton Dam where work didn’t actually start ‘til early 1972.
It was an earth fill dam, 93meters in height and 975.5meters long at its crest, on
the Teton River, Madison County, south-eastern Idaho.
The dam was built and designed by the “United States Bureau of Reclamation”. It
was a multifunction waterworks dam, designed for irrigation, hydro-electricity
production, flood prevention and water-based recreation.
The dammed reservoir filled the steep walls of the Teton Canyon and lay
upstream of the city of Idaho Falls and the town of Rexburg.
On June 5th, 1976, while the reservoir was being filled for the first time, by an
early spring run-off, the dam failed and collapsed.
Aerial photo of the Teton Dam Collapse
Site Selection and Geology:
From 1932 to 1947 the U.S. Bureau of Reclamation, and the U.S. Geological
Service investigated several sites for a dam on the river Teton. A report in March 1947
showed all of these sites to be unsuitable economically and would incur high levels of
seepage.
The U.S. Corp. of Engineers selected the site at Teton Canyon and began site
investigation in 1957.
The Teton River is in the watershed of the Eastern Snake River Plain. This is a
wide tectonic depression underlain by rhyolitic and basaltic, late-Cenzoic volcanic rock.
This volcanic rock has a hard ash-flow turf, and sits on top of a thick sedimentary rock.
This geological configuration causes a high level of permeability in the
surrounding area.
The Teton Dam Collapse
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Construction:
Construction of the dam started in February 1972. Some 10 million cubic meters
of selected earth materials were need to build the multi-layer structure of the earth fill
dam.
The dam structure consisted of a centre zone made up of a clay, silt, gravel and
cobbles mixture, tightly compacting by tamping rollers to form a solid core, considered
impervious to water by the design engineers. This core was surrounded by 4 outer layers,
or zones, of additional rock material of various kinds (5-zone structure).
Zone 1:
Core - Moisture content 1.5% dry of optimum
Permeability 0.3meters per year
Zone 2:
Compacted Sandy Gravel –
Overlies zone1 and extends downstream into the floodplain
Zone 3:
Miscellaneous Fill –
Properties similar to zone 1
Zone4\5:
Rock FillCompacted on outer surface of dam
After excavation of the foundation trench, fissures were found in the bedrock,
some of a considerable size. These fissures had to be drilled and grout filled to make a
competent rock surface.
Collapse:
Before June 5th, 1976, no seepage conditions where seen upstream of the dam or
on the downstream face of the dam. On June 3rd, two small spring areas were noted by
engineers downstream of the dam, and another spring developed on June 4th. Engineers
were not alarmed at this stage as a certain amount of seepage is expected from an earth
fill dam.
However, on the morning of June 5th, 1976, a major leak was discovered, This
formed a large wet spot on the downstream face of the dam, which began eroding into the
embankment. At 10.30am two bulldozers began pushing rock material into the eroding
hole.
At 11.00am a whirlpool was observed forming 5meters from the edge of the
embankment. The whirlpool began to increase in diameter and depth. Two more
bulldozers began pushing lose rock and riprap into the whirlpool.
At 11.30am the bulldozers filling the hole on the embankment face where lost into the
growing hole. The embankment continued to erode rapidly, until the hole in the face was
in line with the whirlpool.
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At 11.55am the crest of the embankment fell into the water, and the dam was
finally breached at 11.57am!
At the time of failure the reservoir was almost full for the first time. The main
body of the reservoirs 251,000 acre-feet capacity was emptied within five hours.
Aftermath:
Downstream of the dam, the city of Idaho Falls had sufficient time to prepare and
evacuate; however the closer town of Rexburg did not have enough warning. In total 11
lives were lost and the cost of damages was estimated at $400 million.
Investigations showed there to be no sign of seismic activity around the time of
failure, or at any time during construction. Failure was put down to incomplete geological
surveying of the site and the US Bureau of Reclamation took most of the blame.
Enquires were made into accusations of mal-practice and corruption of political powers,
Teton Canyon in the aftermath of the collapse
References:
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
Wikipedia, free online encyclopedia
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http://en.wikipedia.org/wiki/Teton_Dam
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www.idahoptv.org/outdoors/shows/bofr/teton/brown.html
United States Bureau of Reclamation
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http://www.usbr.gov/pn/about/Teton.html
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http://www.geol.ucsb.edu/faculty/sylvester/Teton%20Dam/narrative.html
The Teton Dam Collapse
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The Teton Dam Collapse
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