Presentation on theme: "The Teton Dam Failure By Matthew Reddy and Robert Cundill."— Presentation transcript:
The Teton Dam Failure By Matthew Reddy and Robert Cundill
Contents Introduction Geology of the Surrounding area Geology of the Site Dam Composition The Failure Sequence Why it Happened Conclusion
Introduction The dam site was close to Rexburg in South East Idaho Constructed by the US Bureau of Reclamation Earth-fill dam 93 metres high 975 metres long With an active capacity of 250 million metres cubed. Failed abruptly in June 1976, the highest dam to have ever failed Loss of 14 lives directly or indirectly Cost of failure = $1billion
Geology of The Surrounding Area The dam was situated in a steep walled canyon in the eastern Snake River Plain, which is a broad, tectonic depression This depression is underlain with a rhyolitic variety of basalt (volcanic) Material is generally not considered to be acceptable for structural foundations These volcanic rocks are covered with a layer of windblown or fluvial sediments, loess.
Geology of The Site The dam’s abutments were comprised of a perversely fractured, welded ash flow tuff (rhyolite) with beds of basalt. Large voids associated with volcanic fumaroles were detected during construction Tuff: Volcanic ash of sand grain size; lithified or unlithified. Fumaroles are vents from which volcanic gas escapes into the atmosphere
The Dam Wind blown loess as the impervious core fill material The exposure of so much jointed and blocky rock in the abutments led to the excavation of, deep seepage cut-off trenches, or keyways in the abutments Only 1 grout curtain installed, instead of 3 because of higher than anticipated grout take due to the keyways and poor quality of the foundation rock. The single grout curtain A section through a keyway Loess as the core fill material
The Failure Sequence
Around 10:45 Around 11:20 Around 11:40 Around 11:50 Around 11:55 Around 12 noon
The final breaching is filmed from a helicopter. Amazingly, one of two men fishing a half mile downstream survived!
Why it Happened? Cause Piping or hydraulic fracturing of highly erodible loess core Deficiencies Geological: Poor quality fill, and numerous open joints in the abutments Engineering: Lack of filler or sealer between core loess and open fractures in the abutments Excessive steep sided walls in the abutment keyways, promoting stress concentration, arching and likelihood of piping Inadequate grout curtain
Conclusion The dam failed because of insufficient consideration for the varied and unusual geology. Better consideration for the safety was needed and the use of fail safe mechanisms. Alternatives?