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Soil Properties - II

Amit Prashant

Indian Institute of Technology Gandhinagar

Short Course on

Geotechnical Aspects of Earthquake Engineering 04 – 08 March, 2013

Seismic Waves

2

Near the ground surface, most of the seismic waves arrive vertically

Earthquake

Rock

2

Important Properties Propagation of Shear Waves

Density = Mass per unit volume

Shear Modulus

Damping Characteristics

3

Shear Modulus

4

StrainShear

StressShear

Strain,Shear

Stress,Shear oG

1

Initial Shear Modulus secG

1

Secant Shear Modulus

tanG1

Tangent Shear Modulus

Useful in Equivalent Linear Analysis

Used in Nonlinear Analysis

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Secant Modulus

5

1G2G

3G

Strain,Shear

Stress,Shear

G

oG

Cyclic Loading – Secant Shear Modulus

6

Equivalent Linear Analysis

G

Nonlinear Analysis (step by step) G

Branch curve (Hysteresis loop)

Skeleton curve

oG

4

Hysteretic Damping

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G

W

W

4 Ratio, Damping

W = Strain Energy

W = Loss of Energy per cycle

Modulus Reduction Curve

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Threshold Strain (Below this strain the behaviour is linear)

%

Pla

stic

ity

ind

ex

After Vucetic, 1994

G

oG

scale log

Modulus Reduction Curve

5

Typical Values of Initial Shear Modulus

9 (Source: FHWA-SA-97-076)

Initial Shear Modulus

Increasing Factor Go

Effective Stress Increases

Void Ratio Decreases

Geologic age Increases

Cementation Increases

Overconsolidation Increases

Plasticity Index Negligible to small increase

Strain Rate No effect on sand Increases for clay

Number of loading cycles Increases for sand Decreases for clay

10

6

Correlations of initial shear Modulus

11 (Source: FHWA-SA-97-076)

Modulus Reduction Curve Effect of Confining Pressure

12

Non-plastic soil

(After Iwasaki et al., 1978)

oGG

7

Modulus Reduction Curve Effect of Confining Pressure

13

(After Ishibashi, 1992)

oGG

Non-plastic soil Plastic soil

Modulus Ratio, G/Go

Increasing Factor G/Go

Cyclic Strain Decreases

Effective Stress Increases

Void Ratio Increases

Geologic age May Increase

Cementation May Increase

Overconsolidation No effect

Plasticity Index Increases

Strain Rate No effect

Number of loading cycles Increases for drained sand Decreases for undrained sand Decreases for clay

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8

Modulus Reduction Curve Effect of Soil Type

Clay Sand

Gravel

σ’m0 (kPa)

Clay 100

Sand 50 ～300

Gravel 50～830

(Imazu & Fukutake, 1986)

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oGG

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Shear Modulus and Damping with Cyclic Strain

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0.5

1.0

γ

τ

10－6 10－1

scale log

oGG

G

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Modulus Reduction Curve with Hysteresis and Damping along Depth

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τ

γ

τ

γ τ

γ

G

oG

scale log

Increasing Overburden Deeper Strata τ

γ

τ

γ

Liquefaction Reduction on effective overburden?

Shear Modulus and Damping Effect of Plasticity Index

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oGG

(%)

(After Vucetic and Dobry, 1991)

(%)

Ratio Damping

For sand

10

Damping Ratio,

Increasing Factor

Cyclic Strain Increases

Effective Stress Decreases

Void Ratio Decreases

Geologic age Decreases

Cementation May decrease

Overconsolidation No effect

Plasticity Index Decreases

Strain Rate May Increase

Number of loading cycles No significant change

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Typical values of Poisson’s Ratio

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11

Initially loose configuration Contractive

Dilative Initially Dense configuration

Increase in Pore Water Pressure

Drained Shearing

Slow Loading

Undrained Shearing

Fast Loading

Decrease in Pore Water Pressure

Volume Change or Evolution of Pore Water Pressure During Shearing

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Settlement

Reduced effective stress

Sand Behavior during Cyclic Loading

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γ

τ

γ

Pore water pressure, p

N (cycle)

γ

τ

Drained/Slow Loading:

Compression

Undrained/Fast Loading:

o

Liquefaction

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Stress Strain Curve for soils

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q

q

Due to Initial Overburden

Peak Shear Strength

Zone of instability

Steady State Shear Strength OR Residual Shear Strength

Cyclic loading

Strength reduced to 80% of Undrained shear strength is often considered to avoid large deformations and Instabilities ?

Residual Shear Strength

Difficult to measure in the laboratory Triaxial test not reliable at very high strains

Ring Shear Test Sampling issues

Time consuming

Field Test Correlation with SPT, N60 corrected for fines content.

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Residual Strength and SPT

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(After Seed and Harder, 1990)

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Thank You