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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

THE FAILURE OF TETON DAM – A NEW THEORY BASED

ON "STATE BASED SOIL MECHANICS"

Paper: OSP-17

V.S. Pillai & B. Muhunthan

outline
OUTLINE
  • Background Aspects
  • Post failure investigations
  • Focus of our investigations
  • State dependent behavior of Teton
  • core – Silt
  • Analysis
  • Conclusions
location of teton dam
Location of Teton Dam
  • 64 km Northeast of Idaho falls
  • Across Teton river
  • Near Wyoming-Idaho border in the Teton mountain range
slide6

El.5300

El.5200

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

slide7
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.
dam failure first leakage

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
senator frank church idaho
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
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
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
slide13

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 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
slide15

FL

CSL

Soil states in normalized stress-space

slide16

Cam-clay regime

X1

Soft

X2

Dense

(Hvorslev regime)

X3

Possible soil states in v-lnp’ space

slide17

B

A

LI5=LI+0.5log(p‘/5)

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

slide19

A1

A4

Typical element

An

Longitudinal section of the dam –(Schematic)

(Modified after IP, 1976)

slide20

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
Soil elements at different depths

El. 5300

El. 5200

Cross section of dam near right abutment

slide22

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

material parameters
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
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
slide27

El.5301

Reservoir level

Contours of q/p‘ ratio

slide28

CRACKED

3

Details of q/p ratio at the right abutment

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

slide29

Prone for cracking

Contours of equivalent liquidity LI5

conclusions
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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