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?