soil properties - ii properties - ii ... considered to avoid large deformations and instabilities ?...
TRANSCRIPT
1
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
3
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
7
G
W
W
4 Ratio, Damping
W = Strain Energy
W = Loss of Energy per cycle
Modulus Reduction Curve
8
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
14
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)
28
oGG
15
Shear Modulus and Damping with Cyclic Strain
16
0.5
1.0
γ
τ
10-6 10-1
scale log
oGG
G
9
Modulus Reduction Curve with Hysteresis and Damping along Depth
17
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
18
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
19
Typical values of Poisson’s Ratio
20
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
21
Settlement
Reduced effective stress
Sand Behavior during Cyclic Loading
22
γ
τ
γ
Pore water pressure, p
N (cycle)
γ
τ
Drained/Slow Loading:
Compression
Undrained/Fast Loading:
o
Liquefaction
12
Stress Strain Curve for soils
23
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.
24