Dynamic Soil Properties- Geotechnical Engineering.pptx

Dynamic Soil properties;
Seismic Waves;
Shear modulus;
Secant Modulus;
Cyclic lodaing;
Shear test;
Cyclic shear test;
Cyclic triaxial test;
bender element system;
Modulus reduction curve;
Damping ratio;

1. Dynamic Soil Properties Prof. Samirsinh P Parmar Department of Civil Engineering, Faculty of Technology Dharmsinh Desai University, Nadiad-387001 E-mail: [email protected]

2. Content of the presentation Seismic Waves Shear Modulus Secant Modulus Cyclic loading Dynamic Properties of soils Shear tests (Shear, triaxial etc ) Bender Element System Modulus reduction curve/ ratio Damping ratio. 2

3. Seismic Waves 3 Near the ground surface, most of the seismic waves arrive vertically Fault Line Earthquake Rock

4. Important Properties Propagation of Shear Waves Density = Mass per unit volume Shear Modulus Damping Characteristics 4

5. Stress Strain Curve for soils 5 2G max 2G First yield point Elastic zone Peak Shear Strength Strain hardening zone Softening zone Zone of instability Steady State Shear Strength G max = Maximum Shear Modulus G = Secant Shear Modulus

6. Shear Modulus 6 1 Initial Shear Modulus 1 Secant Shear Modulus 1 Tangent Shear Modulus Useful in Equivalent Linear Analysis Used in Nonlinear Analysis

7. Secant Modulus 7 Modulus Reduction Curve

8. Cyclic Loading – Secant Shear Modulus 8 Equivalent Linear Analysis Nonlinear Analysis (step by step) Branch curve (Hysteresis loop) Skeleton curve

9. Hysteretic Damping 9 W = Strain Energy D W = Loss of Energy per cycle

10. Dynamic properties of Soil Shear Modulus, G = r .V S 2 Shear wave velocity = V S (m/sec) Mass density = r (g /g) (Kg/m 3 ) Unit weight of soil = g (KN/m 3 ) Acceleration of gravity = g (m/sec 2 ) Damping, D = decay in energy Shear Modulus (G) is measured in KN/m 2 & Damping (D) in %

11. Dynamic properties of soil Low Strain Amplitude test For very low strains (10 -6 % to 10 -4 %) Frequency range: 10 Hz to 200Hz Vibratory loading (Rotating Machinery etc) High Strain Amplitude test For very low strains (10 -4 % to 10 -2 %) Frequency range: 0.1 Hz to 2 Hz ( in general ) Blast loading, Earthquake

12. Dynamic properties (Lab test) High Strain Amplitude test Cyclic Triaxial Test Cyclic Direct Simple Shear Test Low Strain Amplitude test Resonant Column Test Bender Element Test

13. Cyclic Triaxial Test ( High strain amplitude test) Measures dynamic properties of soil under almost similar conditions as Triaxial test Dynamic Properties: 1. Shear Modulus (G) 2. Damping ratio (D) Loading conditions : Most commonly used cyclic loading = 1Hz frequency; where radial stress kept constant and axial stress is cycled.

14. Cyclic Triaxial Test

15. Cyclic Simple Shear Test ( High strain amplitude test) Digitally controlled Electro-mechanical actuators are used to apply the stress or strain controlled loading Output : Shear modulus (G), Damping (D)

16. Bender Element System (BES) 16 Signal Sensors http://www.sciencedirect.com/science/article/pii/S0267726104001563

17. BES Measurements 17 P-Wave velocity: S-Wave velocity: L = Distance between source and receiver element l = Length of the element

18. Damping Ratio using Half-Power Method 18 By varying the frequency with constant input voltage amplitude Or, sometimes it is preferred to use

19. Resonant Column Test ( Low strain amplitude test) The basic principle of the resonant column device is to excite one end of a confined cylindrical soil specimen in a fundamental mode of vibration by means of torsional or longitudinal excitation. Once the fundamental mode of resonance frequency is established, measurements are made of the resonance frequency and amplitude of vibration from which wave propagation velocities and strain amplitudes are calculated using the theory of elasticity. The Resonant Column Test provides laboratory values of Shear modulus (G) and Damping ratio (D) .

20. Resonant Column Test: Determination of Shear Modulus of soil (G)

21. Resonant Column Test: Determination of Damping properties of soil ( x) Decay of Free Vibration x = 1/2 p · D 1

22. Modulus Reduction Curve 22 Threshold Strain (Below this strain the behaviour is linear) Threshold Strain Plasticity index After Vucetic , 1994 Modulus Reduction Curve

23. Typical Values of Initial Shear Modulus 23 (Source: FHWA-SA-97-076)

24. Initial Shear Modulus Increasing Factor G o Effective Stress Increases Void Ratio Decreases Geologic age Increases Cementation Increases Over consolidation 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 24

25. Correlations of initial shear Modulus 25 (Source: FHWA-SA-97-076)

26. Initial Shear Modulus Predictions 26

27. Modulus Reduction Curve Effect of Confining Pressure 27 Non-plastic soil (After Iwasaki et al., 1978)

28. Modulus Reduction Curve Effect of Confining Pressure 28 (After Ishibashi , 1992) Non-plastic soil Plastic soil

29. Modulus Ratio, G/G o Increasing Factor G/G o Cyclic Strain Decreases Effective Stress Increases Void Ratio Increases Geologic age May Increase Cementation May Increase Overconsolidation No effect Plasticity Index I ncreases Strain Rate No effect Number of loading cycles Increases for drained sand Decreases for undrained sand Decreases for clay 29

30. Modulus Reduction Curve Effect of Soil Type 30 Clay Sand Gravel 　　　　 σ’ m0 ( kPa )　　　 　 Clay 　　　100　　　　 　 　 Sand 　 　　50 ～300 　　　 　 Gravel 　 　50～830　　 　 ( Imazu & Fukutake , 1986 ) 28

31. Strain Dependent Shear Modulus 31 Linear Elastic Model Nonlinear Elastic Model % Include Plasticity

32. Shear Modulus and Damping with Cyclic Strain 32 0.5 1.0 γ τ 10 － 6 10 － 1

33. Modulus Reduction Curve with Hysteresis and Damping along Depth 33 τ γ τ γ τ γ Increasing Overburden Deeper Strata τ γ τ γ Liquefaction  Reduction on effective overburden?

34. Shear Modulus and Damping Effect of Plasticity Index 34 (After Vucetic and Dobry , 1991) For sand

35. Damping Ratio, x Increasing Factor x Cyclic Strain Increases Effective Stress Decreases Void Ratio Decreases Geologic age Decreases Cementation May decrease Overconsolidation No effect Plasticity Index De creases Strain Rate May Increase Number of loading cycles No significant change 35

36. Variation of Shear Modulus with Strain Amplitude at Different Loading Cycle 36 Sand Clay

37. Effect of Loading Cycles on Damping 37 Sand Clay

38. Effect of Loading Frequency on Shear Modulus 38 Sand Clay

39. 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 39 Settlement Reduced effective stress

40. Sand Behavior during Cyclic Loading 40 γ τ γ Pore water pressure, p N (cycle) γ τ Drained/Slow Loading: Compression Undrained /Fast Loading: Liquefaction

41. 41 Thank You