Soil Properties - II Amit Prashant Indian Institute of Technology Gandhinagar Short Course on Geotechnical Aspects of Earthquake Engineering 04 – 08 March, 2013 Seismic Waves Earthquake Rock Near the ground surface, most of the seismic waves arrive vertically 2 1 Important Properties Propagation of Shear Waves Density = Mass per unit volume Shear Modulus Damping Characteristics 3 Shear Modulus Shear Stress Shear Strain Secant Shear Modulus Useful in Equivalent Linear Analysis Initial Shear Modulus Shear Stress, 1 Go 1 Gsec 1 Gtan Tangent Shear Modulus Used in Nonlinear Analysis Shear Strain, 4 2 Secant Modulus Go G1 G2 G Shear Stress, G3 Shear Strain, 5 Cyclic Loading – Secant Shear Modulus Go Equivalent Linear Analysis G Nonlinear Analysis (step by step) G Skeleton curve Branch curve (Hysteresis loop) 6 3 Hysteretic Damping G W = Loss of Energy per cycle W = Strain Energy Damping Ratio, W 4W 7 Modulus Reduction Curve Plasticity index Go Modulus Reduction Curve G log scale % After Vucetic, 1994 Threshold Strain (Below this strain the behaviour is linear) 8 4 Typical Values of Initial Shear Modulus (Source: FHWA-SA-97-076) 9 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 5 Correlations of initial shear Modulus (Source: FHWA-SA-97-076) 11 Modulus Reduction Curve Effect of Confining Pressure Non-plastic soil G Go (After Iwasaki et al., 1978) 12 6 Modulus Reduction Curve Effect of Confining Pressure Non-plastic soil Plastic soil G Go (After Ishibashi, 1992) 13 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 7 28 Modulus Reduction Curve Effect of Soil Type Clay Gravel G Go Sand σ’m0 (kPa) Clay 100 Sand 50 ～300 Gravel 50～830 (Imazu & Fukutake, 1986) 15 Shear Modulus and Damping with Cyclic Strain G τ γ 1.0 G Go 0.5 10－6 log scale 10－1 16 8 Modulus Reduction Curve with Hysteresis and Damping along Depth Liquefaction Reduction on effective overburden? τ τ G Increasing Overburden Deeper Strata γ τ γ γ Go τ γ τ γ log scale 17 Shear Modulus and Damping Effect of Plasticity Index G Go For sand Damping Ratio (%) (After Vucetic and Dobry, 1991) (%) 18 9 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 10 Volume Change or Evolution of Pore Water Pressure During Shearing Drained Shearing Slow Loading Undrained Shearing Fast Loading Contractive Increase in Pore Water Pressure Initially loose configuration Settlement Initially Dense configuration Dilative Reduced effective stress Decrease in Pore Water Pressure 21 Sand Behavior during Cyclic Loading Drained/Slow Loading: τ γ γ Compression Undrained/Fast Loading: Pore water pressure, p τ o γ Liquefaction N (cycle) 22 11 Stress Strain Curve for soils Peak Shear Strength q 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 Ring Shear Test Triaxial test not reliable at very high strains Sampling issues Time consuming Field Test Correlation with SPT, N60 corrected for fines content. 24 12 Residual Strength and SPT (After Seed and Harder, 1990) 25 Thank You 26 13
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