USC Dana and David Dornsife College of Letters, Arts & Sciences > Blog

April 16, 2013

Remember the Tetons

Filed under: Uncategorized — admin @ 10:22 pm

Why the Teton Dam was Built

The Teton Dam, in Eastern Idaho, was built after a back-to-back drought and flood between 1961 and 1962 (Reisner).  The Teton Dam was designed to function as a means for water storage, as well as a tool for flood control.  Furthermore, the dam served the purposes of irrigation, power generation, and recreation.  The plan for the dam was proposed in 1963 and passed in 1964, with an Environmental Impact Statement being released in 1971; however, this EIS did not account for the possibility of collapse.

Construction of the Teton Dam

            The dam was designed by the Office of Design and Construction of the U.S. Bureau of Reclamation and was built by Morrison-Knudsen-Kiewit in December of 1972 (Delatte).  The total cost of construction was $100 million (Reisner).  The foundation of the dam was the first defense against the unsuitable geology of the region.  The foundation “consisted of four basic elements: 21-m deep, steep-sided key trenches on the abutments above the elevation of 1,550 m; a cutoff trench to rock below the elevation of 1,550 m; a continuous grout curtain along the entire foundation; and the excavation of rock under the abutments(Delatte).The actual dam was built in five zonesZone 1: impervious center core.  In other words, a core impenetrable by water. Zone 2: atop and downstream of zone 1, allowed for controlled water seepage through foundationZone 3: downstream and provided structural stabilityZone 4: storage areas downstream from the control structure, temporary enclosures built to permit the work to be done.  Zone 5: rockfill in the outer parts of the embankment After construction the dam was:93m above the riverbed, and held 356m2 of water (Delatte).

Teton_Dam Teton1 Teton2

The Teton Dam Failure

            On June 3rd, 1976, inspectors found the first minor leaks in the Teton Dam.  As a result, the Bureau of Reclamation had the dam inspected daily (Delatte).  The following day, June 4th,The right abutment had small but visible springs.  The first major leak began on June 5th. The dam was leaking from the right abutment, and seepage was noticed 40m from the top of the dam (Delatte).  A whirlpool formed upstream of the dam.  In an effort to stop the leak, bulldozers attempted to push debris into the hole, but this failed, and two bulldozers were swallowed by the leak.  On the 5th, around 10am, the dam failed.  By 8pm the reservoir, which held approximately 300,000 acre-feet of water, was completely empty, and only two thirds of the dam wall remained.  Approximately 200 families from the towns of Wilford, Rexburg, Sugar City, and Roberts lost their homes.  Tragically the failure caused the loss of 14 lives, and $400 million, to $1 billion dollars of property damage (Delatte).

Environmental Impacts

 

            The failure of the Teton Dam destroyed the lower part of the river, reducing canyon walls and washing away the riparian zones.  This damaged stream ecology, endangering the native cutthroat trout population.  The force of the rushing water also damaged the stream habitat in the Snake River and some of its tributaries (Reisner).

Why the Teton Dam Failed

            The site of the Teton Dam proved to be unsuitable long before the actual start of construction.  For example, the soil made of basalt and rhyolite had high permeability (Reisner).  Furthermore, test holes absorbed water at a high rate, indicating serious leakage (Reisner).  Tests also showed that the rock was highly fissurable and unstable.  The largest fissures found were actually enterable caves (Reisner).  Lastly, according to the U.S. Geologic Survey, the dam’s proposed location was an area of high seismic activity.  Schleicher, a geologist, wrote a memorandum voicing his concerns about the seismic activity, but this part was never forwarded to the Bureau of Reclamation (Boffey).  However, a report signed by Schleicher and three other geologists was forwarded in June of 1973, discussing the seismic hazards but leaving out his “melodramatic” paragraph about anticipated catastrophic flooding (Boffey).  After the failure, an independent panel of experts analyzed the failure.  The panel came to fourteen conclusions:

” 1. The predesign site and geological studies were “appropriate and extensive.” 2. The design followed well-established USBR practices but without sufficient attention to the varied and unusual geological conditions of the site. 3. The volcanic rocks of the site are “highly permeable and moderately to intensely jointed.” 4. The fill soils used, “wind-deposited nonplastic to slightly plastic clayey silts,” are highly erodible. The soil classification was ML, low plasticity silt. 5. The construction was carried out properly and conformed to the design, except for scheduling. 6. The rapid rate of filling of the dam did not contribute to the failure. If the dam had been filled more slowly, “a similar failure would have occurred at some later date.” 7. Considerable effort was used to construct a grout curtain of high quality, but the rock under the grout cap was not geometry caused arching that reduced stresses in some areas and increased them in others and “favored the development of cracks that would open channels through the erodible fill.” 8. The dam’s combination of geological factors and design decisions that, taken together, permitted the failure to develop.”9. Finite element calculations suggested that hydraulic fracturing was possible. 10. There was no evidence of differential foundation settlement contributing to the failure. 11. Seismicity was not a factor. 12. There were not enough instruments in the dam to provide adequate information about changing conditions of the embankment and abutments. 13. The panel had quickly identified piping as the most probable cause of the failure, then focused its efforts on determining how the piping started. Two mechanisms were possible. The first was the flow of water under highly erodible and unprotected fill through joints in unsealed rock beneath the grout cap and thus development of an erosion tunnel. The second was “cracking caused by differential strains or hydraulic fracturing of the core material.” The panel was unable to determine whether one or the other mechanism occurred, or a combination of the two. 14. “The fundamental cause of failure may be regarded as a adequately sealed. The curtain was nevertheless subject to piping; “too much was expected of the grout curtain, and . . . the design should have provided measures to render the inevitable leakage” (Dellate).

Teton_4

Teton_5

Teton_6

Future Preventative Measures

As a result of the dam’s failure, analyses were completed to determine the potential causes.   Also, peer review of dams and frequent visits during construction of dams by the design engineer were institutionalized. Additionally, special treatment was given to fractured rock foundations and redundant measures were encouraged to control seepage, and prevent piping.  Moreover, a national dam safety program with annual dam inspections and instruments to monitor dams was implemented.  Lastly, the Reclamation Safety of Dams Act of 1978 was created to analyze and modify existing structures that were determined to be potentially unsafe (Dellate).

By Casey Frost & Carolin Meier

Works Cited

Boffey, Philip M. “Teton Dam Collapse: Was It a Predictable Disaster?” Science ns 193.4247 (1976):

30-32. Print.

 

Boffey, Philip M. “Teton Dam Verdict: A Foul-up by the Engineers.” Science ns 195.4275 (1977):

270-72. Print.

Delatte, Norbert J. Beyond Failure. Reston: American Society of Civil Engineers, 2008. Print.

“Teton Dam Failure Case Study.” MATDL. NSDL, 9 July 2012. Web. 27 Mar. 2013. <http://matdl.org/

failurecases/Dam_Failures/Teton>.

Failure of Teton Dam. Bureau of Reclamation, 18 Apr. 2011. Web. 27 Mar. 2013.

<http://www.usbr.gov/pn/about/Teton.html>.

Reisner, Marc. Cadillac Desert. New York: Penguin, 1993. Print

Teton Dam Failure. UCSB, n.d. Web. 27 Mar. 2013. <http://www.geol.ucsb.edu/faculty/sylvester/

Teton_Dam/welcome_dam.html>.

“Teton Dam Failure Case Study.” MATDL. NSDL, 9 July 2012. Web. 27 Mar. 2013. <http://matdl.org/

failurecases/Dam_Failures/Teton>.

Leave a Reply