1. The Teton Dam Failure By Matthew Reddy and Robert Cundill

2. Contents  Introduction  Geology of the Surrounding area  Geology of the Site  Dam Composition  The Failure Sequence  Why it Happened  Conclusion

3. 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

4. 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.

5. 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

6. 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

7. The Failure Sequence

8. Around 10:45 Around 11:20 Around 11:40 Around 11:50 Around 11:55 Around 12 noon

9. The final breaching is filmed from a helicopter. Amazingly, one of two men fishing a half mile downstream survived!

10. 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

11. 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?