dilative and contractive response

8/3/2019 Dilative and Contractive Response

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Undrained Contractive (Weak) vs. DilativeStron Res onse

Initial ConditionsLoose

SSL

Peak Undrained

Strength

Strength

Confinement


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1.05

1.1Contractive Behavior

Actual Stress

Loose

0.9

0.95

1

,e

e a a

Average Void

on ons or40 to 90 feet of Ash

0.8

0.85

Void

Ratio a o .

0.65

0.7

.

SSL

0.6

10 100 1000 10000 100000

Major Principal Stress, psf

Dilative Behavior

``

Confining Stress


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Weak Thin Layer of Silt and Ash

Slimes at Bottom of Loose Ash Under the Cell 2 and Old Cell 3 there is a layer

of high void ratio laminated silt mixed with fineash that has low undrained shear strength

The silt and fine ash is sensitive,

overconsolidated and behaves in a brittlemanner initially, becomes plastic and thensoftens to low strength

The silt and ash slime is hi hl structured andlooses strength when disturbed

Unusual Latent Condition Likely Trigger


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Old Spillway


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Soft Laminated Silt and Ash

,potential for more slimes between 1958 to mid

1960s due to reat distance from delta to s illwain Ash Pond

Thin Laminated Deposit of Silt Size and RoundedAsh Slime Particles in 1/2 to 6 inches thick layers

Highly Structured, Sensitive Slime Material

Slimes are Hi hl Unusual Material Havin Low

Shear Strength Properties


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Unusual Properties of Slimes High Water Content 40% to 140% (Wc = Ww/Ws)

High Liquidity Indices 3.5 to 7 (Rarely found in nature)

Relatively low permeability 6.5x10-4 feet/day

Specific Gravity Gs = 2.2 to 2.3, Wet Density is 90 pcf

CKUDSS Shear Strengths range from 600 to 1,250ps w on rac ve n ra ne e av or

CKUDSS Creep test shows failure at 85% of peakshear strength


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Photos and Selected Tests on TubeSamples of Sensitive Silt and

Holes at toe of Cell 2


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Intact Interfaces of Laminated Slimes & Ash

09-100B S5 33.0-35.5 ftMicroscopePhotograph

09-100B S5 33.0-35.5 ft

FAILED ASH

LAMINATED HIGH WATER CONTENT SLIMES


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Unfailed Clay and Laminated Slimes Interfaces at

09-100B

SLIMES

. .


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Photo of Laminated Slimes at 09-


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Photos of Unfailed Clay and Failed

-


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Laminated Silt/Ash Interface at 09-108B

e ane

UnfailedAsh

FailedAsh

09-108B OST 223.5 to 26.0 ft


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DSS at 09-108B, Su = 690 psf


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CKUDSS Creep Test on Slimes


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Interface of Laminated Slimes and

Unfailed Flyash at 09-6005B

Ash

Dredge Cut

09-605B S7 47.0 to 49.5 ft


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Medium to Stiff Alluvial Silts & Clay

Found Below Sensitive Silts and Ash, ,

Medium Plasticity Moderate to High Sensitivity

R l iv l l w rm ili k =4x1 -3 f

Overconsolidated due to Desiccation

v y . .

Detected in >90% of the Borings and Probes Undrained Shear Strength from Vanes can

be correlated to CPTu ti resistance usin N

Minor Impact Retards Drainage of Silts and the Clay is Stronger than the Slimes


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09-301B OST 6 87.5 to 90.0 ft

INTACT UNFAILEDCLAY/ASH

INTERFACECLAY

- . o .


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PLANE


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Failure Slide PlaneW E

Cross Section Looking North at Toe of Cell 2, Dike A

Dike CLine

Toe of Dike A


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Photos of Slide Plane in Slimes at 09-104B

Failure Plane

Unfailed Clay Failed Ash

Unfailed Slimes


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Failed Clay/Ash Interface at 09-101B

n a eClay

a ure ane

Liquefied Ash

4-INCH SHEAR PLANE OFTOTAL LIQUEFACTION

PARTIAL LIQUEFACTION


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Clay/Laminated Slimes/Failed Ash

at 09-500B, OST5

Tortured AshTotallyLiquefied Ash

Failure Planeon top of Slimes

Clay Slimes

09-500B S5 29.0 to 31.5 ft


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Clay/Failed Ash Interface at 09-201B

Failure Plane

CLAY

-CLAY

LIQUEFIED ASH

25.5 to 28.0 ft


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Clay/Ash Interface at 09-202B

09-202B S2 28.5 to 31.0 ft

dilative and contractive response

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