Soil Properties - II

Transcription

1 Soil Properties - II Amit Prashant Indian Institute of Technology andhinagar Short Course on eotechnical Aspects of Earthquake Engineering March, 2013 Seismic Waves Earthquake Rock Near the ground surface, most of the seismic waves arrive vertically 2 1

2 Important Properties Propagation of Shear Waves Density = Mass per unit volume Shear Modulus Damping Characteristics 3 Shear Modulus Shear Stress Shear Strain Useful in Equivalent Linear Analysis Secant Shear Modulus Initial Shear Modulus Shear Stress, o 1 sec 1 1 tan Tangent Shear Modulus Shear Strain, Used in Nonlinear Analysis 4 2

3 Secant Modulus 2 1 Shear Stress, o 3 Shear Strain, 5 Cyclic Loading Secant Shear Modulus Nonlinear Analysis (step by step) o Skeleton curve Equivalent Linear Analysis Branch curve (Hysteresis loop) 6 3

4 Plasticity index Hysteretic Damping W = Loss of Energy per cycle W = Strain Energy W Damping Ratio, 4 W 7 Modulus Reduction Curve o Modulus Reduction Curve log scale Threshold Strain (Below this strain the behaviour is linear) % After Vucetic,

5 Typical Values of Initial Shear Modulus (Source: FHWA-SA ) 9 Initial Shear Modulus Increasing Factor Effective Stress Void Ratio eologic age Cementation Overconsolidation Plasticity Index Strain Rate Number of loading cycles o Decreases Negligible to small increase No effect on sand for clay for sand Decreases for clay 10 5

6 Correlations of initial shear Modulus (Source: FHWA-SA ) 11 Modulus Reduction Curve Effect of Confining Pressure Non-plastic soil o (After Iwasaki et al., 1978) 12 6

7 Modulus Reduction Curve Effect of Confining Pressure Non-plastic soil Plastic soil o (After Ishibashi, 1992) 13 Modulus Ratio, / o Increasing Factor Cyclic Strain Effective Stress Void Ratio eologic age Cementation Overconsolidation Plasticity Index Strain Rate Number of loading cycles / o Decreases May Increase May Increase No effect No effect for drained sand Decreases for undrained sand Decreases for clay 14 7

8 Modulus Reduction Curve Effect of Soil Type 28 o ravel Sand Clay σ m0 (kpa) Clay 100 Sand ravel 50 ~300 50~830 (Imazu & Fukutake, 1986) 15 Shear Modulus and Damping with Cyclic Strain 1.0 o log scale

9 Modulus Reduction Curve with Hysteresis and Damping along Depth Liquefaction Reduction on effective overburden? Increasing Overburden Deeper Strata o log scale 17 Shear Modulus and Damping Effect of Plasticity Index o For sand Damping Ratio (%) (%) (After Vucetic and Dobry, 1991) 18 9

10 Damping Ratio, Increasing Factor Cyclic Strain Effective Stress Void Ratio eologic age Cementation Overconsolidation Plasticity Index Strain Rate Number of loading cycles Decreases Decreases Decreases May decrease No effect Decreases May Increase No significant change 19 Typical values of Poisson s Ratio 20 10

11 Volume Change or Evolution of Pore Water Pressure During Shearing Initially loose configuration Initially Dense configuration Drained Shearing Slow Loading Contractive Dilative Settlement Undrained Shearing Fast Loading Increase in Pore Water Pressure Reduced effective stress Decrease in Pore Water Pressure 21 Sand Behavior during Cyclic Loading Drained/Slow Loading: Compression Undrained/Fast Loading: Pore water pressure, p o Liquefaction N (cycle) 22 11

12 Stress Strain Curve for soils q Peak Shear Strength Zone of instability Due to Initial Overburden Cyclic loading Steady State Shear Strength OR Residual Shear Strength q Strength reduced to 80% of Undrained shear strength is often considered to avoid large deformations and Instabilities? 23 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, N 60 corrected for fines content

13 Residual Strength and SPT (After Seed and Harder, 1990) 25 Thank You 26 13