4A6(1) 2008-09 Disasters project- Group 22 – Teton Dam Failure
Introduction
The Teton dam was an embankment dam constructed on the Teton River in order to create a reservoir.
This reservoir would provide supplemental water to the Fremont-Madison Irrigation District during dry
years and the dam would alleviate floods caused by ice jams in the early spring. The water was also to be
used to operate a power plant at the base of the left abutment. The dam was 305 feet high and had a
crest length of 3100 feet, including the spillway.
Embankment dam
An embankment dam acts as a man made water barrier and is used to good effect in wide valleys to
create reservoirs. The Teton dam was constructed as an earthen dam which is a type of embankment
dam. It is essentially constructed of compact earth that is shaped like a bank generally with a core of
impermeable material to stop water from seeping through the soil. Two materials that are commonly
used in the impermeable core of this type of dam are clay and concrete and it can be built on either hard
rock or on soft soil as little force is exerted on to the foundations. The force that is transferred through
the dam from the water pressure along with the mass of the materials creates a downward pressure
effectively sealing off and waterproofing the underside between the dam and the ground underneath.
This pressure also increases the friction between the structure and foundations which helps to interlock
the particles making the dam a stable mass.
The embankment itself consisted of five main zones.
Zone 1 was the impervious centre core, which formed the water barrier of the dam.
Zone 2 overlaid Zone 1 and extended downstream to provide a layer to control seepage through the
foundation.
Zone 3 was downstream and its main function was to provide structural stability.
Zone 4 consisted of the storage areas downstream from the control structure and the temporary
enclosures built to permit the work to be done.
Finally, Zone 5 was the rock fill in the outer parts of the embankment (Independent Panel, 1976).
Geology
The area around the site consists of a broad tectonic depression on top of fissured rhyolitic consolidated
volcanic ash on sedimentary rock. The tests that were carried out on the region produced evidence of
high permeability and instability in the ground and on one side of the canyon regions of highly fissured
rock were discovered from the test boreholes. To remedy this situation the fissures had to be drilled and
grouted to stabilize the underlying ground. The dam itself was constructed in five layers with each of a
different permeability and structural significance, the main components of these layers were clay, silt and
gravel.
Failure Method
The Teton dam failed just as it was being completed and filled for the first time. At the time of the
collapse, spring runoff had almost filled the new reservoir to capacity. Failure was initiated by a large leak
near the northwest abutment of the dam, about 130 feet below the crest.
A recent study has blamed the collapse on the fissured Rhyolite in the foundations of the dam that
allowed water to seep under the dam and on the permeable Loess soil used in the core. The permeable
Loess was cracked. It is argued that the combination of these materials allowed water to seep through
the dam and led to internal erosion called piping that eventually caused the dam's collapse.
Water began seeping from the dam on the Thursday before the collapse, an event not unexpected for an
earthen dam. The only structures that had been initially prepared for releasing water were the
emergency outlet works, which could carry just 24 m3/s. Although the reservoir was still rising over 1.2 m
per day, the main outlet works and spillway gates were not yet in service. The spillway gates were
cordoned off by steel walls while they were being painted.
Prior to June 5, no seepage conditions of any type were noted upstream or on the downstream face of
the dam. On June 3, two small spring areas developed on the right abutment just about river level. They
were flowing clear water at approximately 40 and 60 gallons per minute.
On June 4, another small spring was found .The abutments and downstream face of the dam were
examined and no seepage conditions were reported on any part of the embankment.
After the major leaks were discovered on the morning of June 5, efforts were initiated to channel the
flow so it would not enter the powerhouse. At this time, safety of the dam was not believed to be in
jeopardy, however, a wet spot on the downstream face began eroding its way into the embankment. Two
bull dozers began pushing rock material into the hole on the downstream face of the dam. At 10:30 AM
the larger of the two dozers began to slide into the hole. At 11 AM a whirlpool was observed in the
reservoir 10 to 15 feet from the intersection of the reservoir surface with the embankment. The
whirlpool gradually increased in diameter and depth but stayed in a fixed location. Two dozers began to
push riprap into the whirlpool. At 11:30 AM both dozers on the downstream edge of the embankment
were lost in the eroded embankment hole. The embankment was now eroding rapidly. The erosion path
in the embankment followed the right abutment upstream along the contact until it was approximately in
line with the whirlpool. The crest of the embankment fell into the water at 11:55 AM, and the dam was
breached at 11:57 AM. The first observed embankment seepages had occurred only two to two and a half
hours earlier.
Results
This case demonstrates the importance of engineering geology and geotechnical engineering. In this
instance, both the rhyolite under the dam and the fine Aeolian silt used as a fill material were deficient.
As a result of the flood, eleven people lost their lives. The flood waters destroyed farmlands and towns
downstream of the dam, causing hundreds of millions of dollars worth of damage. The flood waters were
eventually contained by the All Americas dam further downstream.
The agency set up to investigate the failure found that the geological investigations had been sufficient
but that the design of the dam had failed to take the unusual geology of the area into account. The fill
used was also erodible and the dam was designed so that stresses were eased in some areas but
increased in others, encouraging the opening of channels at these points, which could then erode the fill.
They also discovered that the design had not included downstream protection in the case of leaks and
cracking in the dam. They concluded that it was the combination of the design choices and geological
features which caused the failure of the dam.
As a direct result of the Teton Dam failure, the United States Bureau of Reclamation, which had been
responsible for the construction of Teton dam, set up the “Safety of Dams” program. This program set
about reviewing every Dam in the Pacific Northwest. They now review them periodically, looking at their
resistance to “seismic stability, overtopping, internal stability, and physical deterioration.” These
“Comprehensive Facility Reviews” entail on site investigations as well as a look at the current dam loading
conditions, downstream populations and re-evaluation of the risks to the dams’ integrity. The aim of this
is to save lives by improved monitoring of the dams, so that should a problem come to light, it can been
rectified and the downstream residents can be given adequate warning.
References
http://matdl.org/failurecases/Dam%20Cases/Teton%20Dam.htm
http://www.usbr.gov/pn/about/dams/sod.html
http://www.usbr.gov/projects/Project.jsp?proj_Name=Teton+Basin+Project
http://en.wikipedia.org/wiki/Teton_Dam#cite_note-2