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An Overview of Seismic Design of Earth Dams and Embankments Sudhir K Jain October 2013

An Overview of Seismic Design of Earth Dams and Embankments Sudhir K Jain October 2013. Outline. Performance of Earth Dams in 2001 Bhuj earthquake Introduction to Seismic Design Principle Dynamic Soil Properties Site Effects Liquefaction Embankment Analysis.

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An Overview of Seismic Design of Earth Dams and Embankments Sudhir K Jain October 2013

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  1. An Overview of Seismic Design of Earth Dams and Embankments Sudhir K Jain October 2013 Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  2. Outline • Performance of Earth Dams in 2001 Bhuj earthquake • Introduction to Seismic Design Principle • Dynamic Soil Properties • Site Effects • Liquefaction • Embankment Analysis Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  3. Performance of Earth Dams in 2001 Bhuj earthquake Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  4. 2001 Bhuj Earthquake • Magnitude 7.7 • Maximum MSK Intensity X • Bhuj in Seismic Zone V of Indian seismic map • 8.46 am on 26 January 2001 • More than 13,805 dead; 1,67,000 injured • 300,000 houses destroyed; 700,000 houses damaged • Numerous multistorey RC buildings collapsed • 130 such buildings collapsed in Ahmedabad ~225km from epicenter Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  5. 2001 BhujEarthquake (contd…) • 2 year drought; reservoirs at very low levels • 16 (out of 20) medium dams damaged • 5 severely • 80 (out of 165) minor dams damaged • 14 severely • Damage consisted of • Liquefaction beneath engineered fill • Slope displacements • Longitudinal cracking at crests, cracking and movements on upstream faces • Damage to outlet towers, spillways, parapet walls, … Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  6. Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  7. Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  8. Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  9. Tappar Dam Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  10. Tappar Dam Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  11. Tappar Dam Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  12. Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  13. Fatehgadh Dam Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  14. Fatehgadh Dam Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  15. Fatehgadh Dam Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  16. Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  17. Kaswati Dam Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  18. Kaswati Dam Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  19. Kaswati Dam Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  20. Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  21. Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  22. Rudramata Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  23. Rudramata Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  24. Intake tower Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  25. Spillway Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  26. Piers Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  27. Some Remarks on Seismic Design Principles Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  28. Seismic Design Principle • Large earthquakes are infrequent as compared to smaller earthquakes • Should a structure meant for 50 years be designed to remain undamaged for an earthquake that may occur once in 500 years? • The criteria is: • Minor (and frequent) earthquakes should not cause damage • Moderate earthquakes should not cause significant structural damage (but could have some non-structural damage) • Major (and infrequent) earthquakes should not cause collapse Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  29. Seismic Design Principle … • A well designed structure can withstand a horizontal force several times the design force due to: • Energy dissipation in a variety of ways, e.g., ductility • Overstrength • Redundancy • In many cases, limited deformation may be acceptable, e.g., slopes, retaining walls. Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  30. Site Specific Design Criteria • Seismic design codes meant for ordinary projects • For important projects, such as nuclear power plants, dams and major bridges site-specific seismic design criteria are developed • These take into account geology, seismicity, geotechnical conditions and nature of project • Site specific criteria are developed by experts and usually reviewed by independent peers • A good reference to read on this: • Housnerand Jennings, “Seismic Design Criteria”, Earthquake Engineering Research Institute, USA, 1982. Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  31. Shaking is not the only issue! • Ground shaking can affect the safety of structure in a number of ways: • Shaking induces inertia force • Soil may liquefy • Sliding failure of founding strata may take place • Fire or flood may be caused as secondary effect of the earthquake. • Fault rupture may pass through the structure Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  32. Direction of Ground Motion • During earthquake shaking, ground shakes in all possible directions. • Direction of resultant shaking changes from instant to instant. • Structure must withstand maximum ground motion occurring in any direction. • Peak ground acceleration may not occur at the same instant in two perpendicular directions. • Hence for design, maximum seismic force is not applied in the two horizontal directions simultaneously. Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  33. Direction of Ground Motion … • On average, peak vertical acceleration is one-half to two-thirds of the peak horizontal acceleration. • Structures experience vertical acceleration equal to gravity (g) at all times. • Vertical acceleration is a concern for: • Stability issues (e.g., slopes) • Large span structures • Cantilever members • Prestressed horizontal members Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  34. Dynamic Soil Properties Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  35. Dynamic Soil Properties • Behaviour of soil complex under static loads. Even more complex under dynamic loads • Need for simple methods to characterize complex behaviour • Analysis techniques: • Equivalent linear models • Cyclic non-linear models • Advanced constitutive models Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  36. Shear Modulus • Soil stiffness depends on strain amplitude, void ratio, mean principal effective stress, plasticity index, over consolidation ratio, and number of loading cycles • Shear Modulus • Tangent modulus • Secant modulus • Shear modulus varies with strain level. It is high at low strains Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  37. Shear Modulus … Figure: Hysteretic stress-strain response of soil subjected to cyclic loading Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  38. Dynamic Properties • Shear modulus decreases with strain increase • Damping increases with strain increase Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  39. Maximum Shear Modulus (Gmax) • Can be obtained in a number of ways: shear wave velocity, laboratory tests, and empirical relationships • Shear wave velocity obtained from geophysical tests at strains lower than about 3x10-4% Gmax= ρvs2 Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  40. Soil Properties Exploration data converted to shear modulus: Gmax = 65N [Seed, and Idris 1983] Gmax = 1000[35(N60)0.34] (σ’)0.4 [Seed,Wong,andIdris, 1986] Gmax= 1000[20(N1,60)0.33] (σ’)0.5 [Seed,Wong,andIdris, 1986] Gmax= 325(N60)0.68 [Imai, and Tonouchi, 1982] Gmax= K (N60)0.66 [PWRI, 1998] Where, N60 = SPT value, uncorrected for over-burden pressure N1,60 = SPT value, corrected for over-burden pressure σ’ = Effective soil pressure Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  41. Soil Properties … Eqn (1) Eqn (5) Eqn (2) Eqn (4) Eqn (3) • Small strain Shear Modulus (Gmax) • Tends to vary significantly, depending on which relationship is used Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  42. Ground Motion Along Depth • Peak amplitude of underground motion is smaller than that at the surface • Variation of amplitude depends on • Earthquake characteristics • Frequency content • Type of soil and its distribution along depth Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  43. Ground Motions Along Depth … Vertical distribution PGV Vertical distribution PGA Vertical distribution PGD Figure: Distribution of peak amplitude of ground motion along depth, (Kanade, 2000) Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  44. Ground Motions Along Depth … Artificially generated time history [SIMQKE -1] Known Spectrum Back calculated time history [SHAKE 2000] Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  45. Ground Motions Along Depth … Response time history [SHAKE 2000] Response time history [SHAKE 2000] Response time history [SHAKE 2000] Assumed earthquake Corresponding response spectrum [SMSIM] Artificially generated time history [SIMQKE-1] Figure: Schematic representation of procedure used for artificially generated time histories for earthquake motion Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  46. Underground Structures • When seismic waves hit the ground surface, these are reflected back into ground • The reflection mechanics is such that the amplitude of vibration at the free surface is much higher (almost double) than that under the ground • Codes allow the design spectrum to be one-half if the structure is at depth of 30m or below. • Linear interpolation for structures and foundations if depth is less than 30m. Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  47. Site Effects Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  48. Site Effects • Motion at the base rock different from that at the top of soil. • Local amplification of the earthquake motion due to the soil profile at the site. • Site Effect. Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  49. Mexico Earthquake of 1985 • Earthquake occurred 400 km from Mexico City • Great variation in damages in Mexico City • Some parts had very strong shaking • In some parts of city, motion was hardly felt • Ground motion records from two sites: • UNAM site: Foothill Zone with 3-5m of basaltic rock underlain by softer strata • SCT site: soft soils of the Lake Zone Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

  50. Mexico Earthquake of 1985 … Time (sec) • PGA at SCT site about 5 times higher than that at UNAM site • Epicentral distance is same at both locations Figure from Kramer, 1996 Short course on Seismic Design of Earth and Rock-fill Dams / October 15-18, 2013

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