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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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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


Background Aspects


Location of Teton Dam

  • 64 km Northeast of Idaho falls

  • Across Teton river

  • Near Wyoming-Idaho border in the Teton mountain range


Design cross section of the dam at river valley section

( after IP, 1976)


El.5300

El.5200

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


  • 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.


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


The Dam Breaks (11:59 AM)


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”


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


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


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

  • 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


FL

CSL

Soil states in normalized stress-space


Cam-clay regime

X1

Soft

X2

Dense

(Hvorslev regime)

X3

Possible soil states in v-lnp’ space


B

A

LI5=LI+0.5log(p‘/5)

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


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


A1

A4

Typical element

An

Longitudinal section of the dam –(Schematic)

(Modified after IP, 1976)


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


Soil elements at different depths

El. 5300

El. 5200

Cross section of dam near right abutment


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


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


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


FE Analysis technique


El.5301

Reservoir level

Contours of q/p‘ ratio


CRACKED

3

Details of q/p ratio at the right abutment

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


Prone for cracking

Contours of equivalent liquidity LI5


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


Flooded City of Rexburg

Thank you for your patience !


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