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THE FAILURE OF TETON DAM – A NEW THEORY BASED ON "STATE BASED SOIL MECHANICS". Paper: OSP-17. V.S. Pillai & B. Muhunthan. OUTLINE. Background Aspects Post failure investigations Focus of our investigations State dependent behavior of Teton core – Silt Analysis Conclusions.

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Paper osp 17 l.jpg

THE FAILURE OF TETON DAM – A NEW THEORY BASED

ON "STATE BASED SOIL MECHANICS"

Paper: OSP-17

V.S. Pillai & B. Muhunthan


Outline l.jpg

OUTLINE

  • Background Aspects

  • Post failure investigations

  • Focus of our investigations

  • State dependent behavior of Teton

  • core – Silt

  • Analysis

  • Conclusions


Background aspects l.jpg

Background Aspects


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Location of Teton Dam

  • 64 km Northeast of Idaho falls

  • Across Teton river

  • Near Wyoming-Idaho border in the Teton mountain range


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Design cross section of the dam at river valley section

( after IP, 1976)


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El.5300

El.5200

A typical cross section of the dam at the right abutment (after IP, 1976)


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  • Teton Dam-Zoned earth fill dam-405 ft. high

  • Part of a multi-purpose Irrigation and Power project (1972-75, USBR)

  • Construction of the dam completed and first filling started in November 1975

  • Dam failed suddenly on June 5, 1976 when the reservoir level rose to El.5301.7 ft.


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Leak

Dam Failure – First Leakage

  • Around 7:00 am on June 5, 1976 dam personnel discovered a leak about 30 m from the top of the dam


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The Dam Breaks (11:59 AM)


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Senator Frank Church (Idaho)

  • The anguished Senator Frank Church, flying over the disaster area, stated that:

    “This dam was built according to the latest state-of-the-art”

    “Nothing like this should have happened....Nothing like this could have happened, except for a fatal flaw either in the siting of the dam or in the design”


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Post Failure Investigations

  • Independent Panel (IP)

  • Interior Review Group (IRG)

    • Documented well in literature

  • General conclusions:

  • Seepage piping and internal erosion

  • Hydraulic fracture

  • Wet seams

  • Differential settlement and cracking

  • Settlement in bedrock

  • Seepage through rock openings


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Focus of Our Investigations

  • Low plasticity of the impervious core

  • Low placement liquidity index (LI) of the core

  • High compaction/Constrained modulus of the core

  • Crack potential of the core under low confining stresses/upper portion of the dam


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Design compaction curve

1.70

1.65

1.60

Dry density (g/cc)

1.55

1.50

1.45

10

15

20

Water content (%)

Teton core

25

Some properties of the impervious core – ZoneI


State based soil mechanics l.jpg

STATE BASED SOIL MECHANICS

  • State of soil is defined in a 3-D space (p, q, e or v)

p‘-mean effective stress - (s’1+2s’3)/3

q - shear stress - (s1-s3)

e - void ratio or v=(1+e) - specific volume

  • Limits to stable states of soil behavior –

  • SBS (p,q,e)

  • 2-D representation of the normalized state

  • boundary surface

  • Soils state in liquidity index-confining

  • stress space


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FL

CSL

Soil states in normalized stress-space


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Cam-clay regime

X1

Soft

X2

Dense

(Hvorslev regime)

X3

Possible soil states in v-lnp’ space


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B

A

LI5=LI+0.5log(p‘/5)

LI-lnp' diagram & q/p'-Equivalent liquidity diagram (after Schofield, 1980)


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Family of critical state lines (Modified after Schofield and Wroth, 1968)


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A1

A4

Typical element

An

Longitudinal section of the dam –(Schematic)

(Modified after IP, 1976)


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q

q/p' = 3

q/p'~2

CSL

q/p'~0.7

Crack Surface

A2

A4

Cam-clay yield surface

A3

A1

p'

Unstable

A1

Stable soft (Yield)

A2

A4

A3

NCL

Stable dense

CSL

-

Unstable

Crack line

(Fracture)

p'

q/p' = 0

q/p' = M

q/p'= 2

q/p' = 3

v

Stress path during construction - Conceptual


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Soil elements at different depths

El. 5300

El. 5200

Cross section of dam near right abutment


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q

q/p' = 3

q/p'~2

CSL

q/p'~0.7

Crack Surface

A2

A4

Cam-clay yield surface

A3

A1

p'

v

Unstable

A1

Stable soft (Yield)

A2

A4

A3

NCL

Stable dense

CSL

-

Unstable

Crack line

(Fracture)

lnp'

Stress path of a soil element during construction


Soil states in 3 d space l.jpg

Soil States in 3-D Space


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Material parameters

Critical State Parameter

Value

k

0.005

l

0.07

G

1.95

M

1.1

n

0.3

G (psf)

300000

p'c (psf)

12000


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FINITE ELEMENT ANALYSIS

  • ABAQUS – FE software developed by Hibbitt, Karlsson and Sorenson Inc.

  • Critical state plastic material model and Porous elastic material model

  • *MODEL CHANGE option was used to simulate the construction of dam

  • SURFACE was used to draw the contours of q/p‘ ratio as well as LI5 variation


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FE Analysis technique


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El.5301

Reservoir level

Contours of q/p‘ ratio


Slide28 l.jpg

CRACKED

3

Details of q/p ratio at the right abutment

( for q/p>3, Zone 1 cracked)


Slide29 l.jpg

Prone for cracking

Contours of equivalent liquidity LI5


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Conclusions

  • A transverse crack(s) or large opening(s) had developed in the core (Zone-1) to a maximum depth of 32 feet below the crest at the right abutment near Sta. 14+00

  • When the reservoir level rose to the level of the deepest crack, water flowed freely barreling downstream into the chimney drain (Zone- 2)

  • A combination of low plasticity, low LI, its variation under the subsequent confining stress condition, played a key role in the cracking of the core

  • State based soil mechanics also explains the flaws of the findings by others and are provided in the Paper


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Flooded City of Rexburg

Thank you for your patience !


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