The Teton Dam Failure

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 dams 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

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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.

Loess as the core fill material
A section through a keyway
The single grout curtain
7
The Failure Sequence
8
Around 1120
Around 1045
Around 1150
Around 1140
Around 12 noon
Around 1155
9
The final breaching is filmed from a helicopter.
Amazingly, one of two men fishing a half mile
downstream survived!
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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

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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?