Group 22
Teton Dam Failure
An investigation into the collapse of Teton Dam
and the events preceding its failure.
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Teton Dam Failure
Teton Dam
Introduction and Background
The Teton Dam was built on the eastern side of Snake River Plain in south-eastern Idaho
over the Teton River, and it was intended that the dam behave as a water retaining
structure to provide a reserve of water for irrigation and consumption as well as future
plans to generate hydro-electric power and to provide recreational facilities on the lake
reservoir, such as fishing. Some theories state that delivery of the dam was rushed by
senators seeking re-election and vital areas of concern regarding the location and
construction of the project were overlooked, resulting in a catastrophic failure in July 1976.
After the failure, a geological survey of the region showed that it sits atop a highly
permeable and fissured layer of volcanic rock through which allowed a large amount of
water seepage occurred, and that the method of grouting that was adopted to seal the
fissures was highly inefficient and was never a viable solution to problems posed by the
natural structure of the ground. Construction and maintenance was carried out by the
United States Bureau of Reclamation in 1975 at a cost of $100million, only to fail
spectacularly one year later upon its first filling.
The dam drained in less than six hours, emptying almost 3 million cubic metres of water
into the valley below with the result of taking the lives of 11 people and causing a great deal
of damage to property downstream of the site. One of the towns downstream of the dam,
Rexburg, suffered the loss of 80% of the towns’ existing structures. Buildings weren’t the
only loss as topsoil was wiped off of tens of thousands of acres of farmland rendering it
almost completely barren. The project was a great disaster for the Bureau, as not only was
there never an opportunity for it generate any sort of revenue but it cost untold millions
more in claims made against them for the loss of belongings that the local people suffered.
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Teton Dam Failure
Description of the Structure and the Geology of the Site
Geology
The Teton Dam was located in a steep walled canyon that the Teton River cut into over
millions of years. The present day site resembles a gently undulating, silt covered, volcanic
upland.
The immediate surrounding landscape is a broad tectonic fissure overlying a rhyolitic ash
flow tuff. The tuff itself is underlain by sedimentary rock. Along the axis of the dam, the tuff
ranges in thickness from about 15m to 152m. The area in general is highly porous and
fissured. There is evidence of volcanic activity in the past so it can be assumed that the site
geology is relatively volatile and unstable.
Structure
The dam was designed as a multi function facility which would generate electrical power as
well as provide irrigation water, flood protection, and water based recreational sports. The
total storage capacity was expected to be approximately 288,954m3.
The dam itself was an earthen dam comprising 7,645,500m3 of earth materials of which
contained clay, silt, sand, gravel and cobbles. The earth materials were overlain by four
additional layers of rock materials.
The dam was 93m high above the river bed with a crest length of 975m and width of 11m.
The slope of the upstream embankments was 2:5:1 and 3:5:1 downstream. The
embankment composed of five different specified zones as follows: Centre core (zone 1)
was a low permeability plastic to non-plastic silt. The centre core supported zone 2 which
consisted mainly of sandy gravel. Zone 3 was general fill and was placed between zone 2
and a protective rock fill surface on downstream side. Zone 4 was placed to enable stream
diversion upstream by acting as a cofferdam. Zone 5 consisted mainly of rock fill in the outer
parts of the embankment.
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Teton Dam Failure
Failure of the Teton Dam
On June 3rd, 1976, two springs appeared downstream of the dam. A day later, another
spring appeared. However, engineers were not concerned as they considered this to be a
normal occurrence for an embankment dam.
On the morning of June 5th, muddy water appeared to be flowing from the dam, indicating
the presence of sediment. The flow of this water steadily increased between 8am and 9am.
The engineers continued to believe this level of seepage was acceptable. However at
9.30am, seepage began to appear on the dam face at the right abutment. Embankment
material began to erode quickly, causing an emergency response by the engineers to fill the
hole using bulldozers.
By 11am, the bulldozers were struggling to prevent further erosion of embankment
material. A whirlpool began to develop in the upstream reservoir, indicating a rapid loss of
water. At 11.30am, the bulldozing was abandoned, the dam evacuated and downstream
residents were warned of the immediate failure of the dam.
At 11.55am, the crest of the dam collapsed, followed by the flood waters from the reservoir
two minutes later.
The floodwaters destroyed the downstream town of Wilford, and badly damaged the areas
of Sugar City and Rexburg. 11 people and over 15,000 livestock were killed. The area was
declared a Federal Disaster Area by President Gerald Ford. The total damage, including
claims, was estimated to be in excess of $ 2 billion.
The exact cause of the failure has been widely disputed. Some experts have argued that the
site selection played a key role, while others claim the dam failed due to inadequate
protection of the core material from erosion. Other theories include seismic stability, wet
seams in the embankment material and flaws in construction.
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