Paper: OSP-17 V.S. Pillai & B. Muhunthan THE FAILURE OF TETON DAM – A NEW THEORY BASED ON "STATE BASED SOIL MECHANICS"

Paper: OSP-17

V.S. Pillai & B. Muhunthan

THE FAILURE OF TETON DAM – A NEW THEORY BASED

ON "STATE BASED SOIL MECHANICS"

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 64 km Northeast of Northeast of Idaho fallsIdaho falls

• Across Teton Across Teton riverriver

• Near Near Wyoming-Wyoming-Idaho border Idaho border in the Teton in the Teton mountain mountain rangerange

Design cross section of the dam at river valley section

( after IP, 1976)

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

El.5200

El.5300

• Teton Dam-Zoned earth fill dam-Teton Dam-Zoned earth fill dam-405 ft. high405 ft. high

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

• Construction of the dam completed Construction of the dam completed and first filling started in November and first filling started in November 19751975

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

Dam Failure – First Leakage

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

Leak

The Dam Breaks (11:59 AM)

Senator Frank Church (Idaho)

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

““This dam was built according to the latest state-of-the-art”This dam was built according to the latest state-of-the-art”““Nothing like this should have happened....Nothing like Nothing like this should have happened....Nothing like

this could have happened, except for a this could have happened, except for a fatal flawfatal flaw either in the siting of the dam or in the either in the siting of the dam or in the designdesign””

Post Failure Investigations• Independent Panel (IP)Independent Panel (IP)• Interior Review Group (IRG)Interior Review Group (IRG)

• Documented well in literatureDocumented 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 coreLow plasticity of the impervious core• Low placement liquidity index (LI) of Low placement liquidity index (LI) of

the corethe core• High compaction/Constrained High compaction/Constrained

modulus of the coremodulus of the core• Crack potential of the core under low Crack potential of the core under low

confining stresses/upper portion of confining stresses/upper portion of the damthe dam

01020304050

60708090

100

0.001 0.01 0.1 1 10

Grain size (mm)

Per

cent

age

pass

ing

251.45

1.50

1.55

1.60

1.65

1.70

10 15 20

Water content (%)

Dry

den

sity

(g/

cc)

Design compaction curve

Some properties of the impervious core – Zone I

Teton core

STATE BASED SOIL MECHANICS• State of soil is defined in a 3-D space State of soil is defined in a 3-D space

(p(p, q, e or v), q, e or v)

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

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

CSLFL

Soil states in normalized stress-space

Possible soil states in v-lnp’ space

X1

X2

X3

Dense (Hvorslev regime)

Soft

Cam-clay regime

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

A

B

LI5=LI+0.5log(p‘/5)

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

Longitudinal section of the dam –(Schematic)

(Modified after IP, 1976)

A1

A4

Typical element

An

p'

-

–

Unstable

Unstable

(Fracture)

CSL

NCL

Stable dense

Stable soft (Yield)

Crack line

A1A2 A3

A4

p'

q

A1

A2 A3A4 Cam-clay

yield surface

Crack Surface

CSLq/p'~2

q/p'~0.7

q/p' = 3

Stress path during construction - Conceptual

q/p'= 2

q/p' = 3

q/p' = 0q/p' = M

v

Soil elements at different depths

A4

A2

A1

A3

Cross section of dam near right abutment

El. 5200

El. 5300

lnp'

v

-

–

Unstable

Unstable

(Fracture)

CSL

NCL

Stable dense

Stable soft (Yield)

Crack line

A1A2 A3

A4

p'

q

A1

A2 A3A4 Cam-clay

yield surface

Crack Surface

CSLq/p'~2

q/p'~0.7

q/p' = 3

Stress path of a soil element during construction

Soil States in 3-D Space

Material parameters

Critical State Parameter Value

0.005

0.07

1.95

1.1

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

Contours of q/p‘ ratio

El.5301

Reservoir level

3

Details of q/p ratio at the right abutment

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

Contours of equivalent liquidity LI5

Prone for cracking

Conclusions• A transverse A transverse crack(s) or large opening(s) (s) or large opening(s)

had developed in the core (Zone-1) to a had developed in the core (Zone-1) to a maximum depth of 32 feet below the crest maximum depth of 32 feet below the crest at the right abutment near Sta. 14+00at the right abutment near Sta. 14+00

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

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

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

Flooded City of Rexburg

Thank you for your patience !