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Geotechnical Aspects of EQ Resistant Design


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Geotechnical Aspects of EQ Resistant Design

  1. 1. Geotechnical Aspects in Earthquake Resistant Design <ul><li>by </li></ul><ul><li>Dr. J.N.Jha </li></ul><ul><li>Professor & Head </li></ul><ul><li>Department of Civil Engineering </li></ul><ul><li>Guru Nanak Dev Engineering College </li></ul><ul><li>Ludhiana-141006 </li></ul><ul><li>Email: </li></ul>
  2. 2. Conception or rather Misconception of Structure Designer <ul><li>Structure Designer believes erroneously most of the time that superstructure needs to be strengthened more in poor soil than in a good soil. </li></ul><ul><li>Dynamic behaviour of soil rarely receives any attention of structure engineer as compared to dynamic behaviour of structure. </li></ul>
  3. 3. Niigatta Earthquake: 1964, Mag. 7.5
  4. 4. Japan earthquake 1964: Niigata- Mag. 7.5
  5. 5. Tokachi-oki Earthquake: 2003 The Damage of Sewerage Structures kushiro (Town) Lifted up manhole and gushed soil during liquefaction Lifted up manhole
  6. 6. Caracas Earthquake 1967: Mag. 6.6
  7. 7. Chile Earthquake 1960 : Island nearValdivia- Mag. 9.5
  8. 8. Alaska Earthquake 1964:Mag. 9.2
  9. 9. Observed Damage from Earthquakes <ul><li>Chile earthquake 1960 : An island near Valdivia- Mag.- 9.5 </li></ul><ul><li>Large settlements and differential settlements of the ground surface - Compaction of loose granular soil by EQ </li></ul><ul><li>Alaska earthquake 1964: Turnagain heights landslide- Mag.- 9.2 </li></ul><ul><li>Major landslide - combination of dynamic stresses and induced pore water pressur e </li></ul><ul><li>Japan earthquake 1964: Niigata -Mag.- 7.5 </li></ul><ul><li>Settlement and tilting of structures - liquefaction of soil </li></ul><ul><li>Caracas earthquake 1967- Mag.- 6.6 </li></ul><ul><li>Response of building during EQ found to depend on the thickness of soil under the building . </li></ul>
  10. 10. Inference and Attention: <ul><li>Influence of local soil condition on shaking and damage intensity during earthquake affects the stability of structure . </li></ul><ul><li>Large scale tilting of well built houses in </li></ul><ul><li>the Niigatta earthquake of 1964 focussed </li></ul><ul><li>the attention of profession on problems </li></ul><ul><li>of soil dynamics: Requiring a careful </li></ul><ul><li>attention by Engineers. </li></ul>
  11. 11. Objective of Earthquake Resistant Design <ul><li>Ensuring structural reliability without structural damage due to ground shaking of moderate intensities. </li></ul><ul><li>Special structures like high level bridges, tall dams , Nuclear Power Plant need specific design procedures (Site specific) supported by detailed investigation. </li></ul>
  12. 12. Geotechnical Aspects of Earthquake Resistant Design <ul><li>Geotechnical Aspects of Earthquake Resistant Design of foundation of structures and other soil retaining structures of soil depends on: </li></ul><ul><li>Dynamic stress deformation and strength characteristics </li></ul><ul><li>Earth pressure problems and retaining wall </li></ul><ul><li>Dynamic bearinng capacity and design of shallow foundation </li></ul><ul><li>Liquefaction of soil </li></ul><ul><li>Machine foundations </li></ul>
  13. 13. Influence of local soil conditions on Acceleration(Cause for damage during EQ) <ul><li>Some Basic Information : </li></ul><ul><li>Acceleration Response Spectrum </li></ul><ul><li>A graph showing the maximum accelerations induced in structures with fundamental period ranging from 0 to several seconds </li></ul><ul><li>Velocity Response Spectrum </li></ul><ul><li>A plot showing relationship between maximum velocity with fundamental period of the structure </li></ul><ul><li>Relation between Velocity Spectrum ( S v )and Acceleration Spectrum ( S a ) </li></ul><ul><li>S v ≈ (T/2 π )S a T = fundamental period of the structure </li></ul><ul><li>Value of horizontal peak ground acceleration = 0.6g ? </li></ul><ul><li>Physical meaning :Movement of the ground can cause a maximum horizontal force on a rigid structure equal to 60% of its weight </li></ul>
  14. 14. Site approximately same distance from the zone of energy release – 1957 San Francisco Earthquake
  15. 15. Site approximately same distance from the zone of energy release – 1957 San Francisco Earthquake
  16. 16. Effect of Soil Conditions on form of Response Spectra –Site A
  17. 17. Effect of Soil Conditions on form of Response Spectra –Site B
  18. 18. Effect of Soil Conditions on form of Response Spectra – Site C
  19. 19. Effect of Soil Conditions on form of Response Spectra – Site D
  20. 20. Effect of Soil Conditions on form of Response Spectra – Site E
  21. 21. Effect of Soil Conditions on form of Response Spectra – Site F
  22. 22. Effect of Soil Conditions on form of Response Spectra – Site A - F
  23. 23. Development of Peak/Max. Acceleration <ul><li>Clay (soft): Moulded easily at natural water content and readily excavated </li></ul><ul><li>Clay (firm): Moulded by substantial pressure at natural water content and excavated </li></ul><ul><li>with a spade </li></ul><ul><li>Out of six four spectra obtained from the same city in the same EQ at a considerable </li></ul><ul><li>distance from the epicenter </li></ul><ul><li>To eliminate the influence of different amplitudes of surface acceleration, plot made </li></ul><ul><li>between period and normalized acceleration (Spectral Acceleration/Maximum </li></ul><ul><li>Ground Acceleration </li></ul>Sites (Increasing order of softness) Period (sec) (Maximum spectral acceleration) A 0.3 B 0.5 C 0.6 D 0.8 E 1.3 F 2.5
  24. 24. <ul><li>Clay layer : Amplified seismic shock of glacial till (Both event) </li></ul><ul><li>Peat deposits : Amplified seismic shocks (Only for distant shock) </li></ul>
  25. 25. <ul><li>Time taken for each complete cycle of oscillation is called FUNDAMETNAL NATURAL PERIOD (T) of the building </li></ul><ul><li>Time taken by the wave to complete one cycle of motion is called PERIOD OF EQ WAVE (0.03 to 33 seconds) </li></ul><ul><li>Short EQ wave have large response on short period buildings </li></ul><ul><li>Long EQ wave have large response on long period buildings </li></ul>Building Response variation during Earthquake
  26. 26. <ul><li>T (Inherent property of the building) depends on the building flexibility and mass,Any alteration made to the building will change its “T” </li></ul><ul><li>Bldg (3-5 storey); damage intensity higher in area with underlying soil cover 40-60 m thick and minimal in areas with larger thickness of soil cover </li></ul><ul><li>Bldg (10-14 storey); damage intensity maximum when soil cover in the range of 150-300m and small for lower thickness of soil cover </li></ul><ul><li>Soil plays the role of filter allowing some ground waves to pass through and filtering the rest. </li></ul>
  27. 27. Damage potential coefficient varies with building characteristics and soil depth
  28. 28. Relationship between building characteristics, soil depth and damage potential coefficient (S v /k) Structure Fundamental period Damage intensity (D r ) 2 to 3 storey 0.2 sec Remains same regardless of soil depth 4 to 5 storey 0.4 sec Max. damage intensity expected at soil depth of about 20 to 30 m 10 to 12 storey 1.0 sec Damage intensity expected to increase with soil depth up to 150 m or so 15 to 20 storey Damage intensity even greater for soil depth of 150 to 250 m & relatively low for soil depth up to 80 m or so
  29. 29. Dynamic bearing capacity <ul><li>Soil dynamics: Engineering properties and behaviour of soil under dynamic stress </li></ul><ul><li>Factors to be considered for estimation of bearing capacity under dynamic condition: </li></ul><ul><li>Nature of variation of the magnitude of the loading pulse </li></ul><ul><li>Duration of the pulse </li></ul><ul><li>Strain rate deformation of soil during deformation </li></ul>
  30. 30. <ul><li>For both static, dynamic and seismic cases bearing capacity failure of foundation grouped into three categories: </li></ul><ul><li>General shear failure </li></ul><ul><li>Punching shear failure </li></ul><ul><li>Local shear failure </li></ul><ul><li>Various theories concerning dynamic bearing capacity have been presented in last 50 years but definition of dynamic bearing capacity is yet to evolve till date </li></ul>
  31. 31. Ultimate bearing capacity of continuous shallow foundation (static case)
  32. 32. Seismic bearing capacity
  33. 33. <ul><li>Static case </li></ul><ul><li>Seismic case </li></ul><ul><li>q u = q N q + ½( γB N γ ) </li></ul><ul><li>q = γD f </li></ul><ul><li>N q ,N γ = bearing capacity </li></ul><ul><li>factors </li></ul><ul><li>N q and N γ = f (Ø’) </li></ul><ul><li>Ø’= tan-1[(0.67+ R D - 0.75R 2 D )tan Ø] </li></ul><ul><li>q uE = q N qE + ½( γB N γE ) </li></ul><ul><li>q = γD f </li></ul><ul><li>N qE , N γE = bearing capacity </li></ul><ul><li>factors </li></ul><ul><li>N qE and N γE = f (Ø’, tanθ) </li></ul><ul><li>tanθ = k h /(1- k v ) </li></ul><ul><li>k h &k v =horizontal and vertical </li></ul><ul><li>coefficient of acceleration </li></ul><ul><li>α AE & α PE = Inclination angles for </li></ul><ul><li>active and passive pressure conditions </li></ul>
  34. 34. <ul><li>Static bearing capacity factors </li></ul><ul><li>Seismic bearing capacity factors </li></ul>Ø N q N γ 0 1 0 10 2.47 1.22 20 6.40 5.39 30 18.40 22.40 40 64.2 109.41 Ø N q N γ 0 1 0 10 2.4 1.4 20 5.9 6.4 30 16.5 23.8 40 59.0 112.0
  35. 35. <ul><li>Variation of N q and N γ with Ø’ </li></ul><ul><li>Variation of N γE /N γ and N qE / N q with tanθ and Ø’ </li></ul>
  36. 36. Prediction of dynamic load- settlement relationship for foundations on clay (Jackson and Halada,1964) <ul><li>For any given value of </li></ul><ul><li>S stat /B, multiply Q/B 2 c u </li></ul><ul><li>by the strain rate factor </li></ul><ul><li>(=1.5) and plot it in the </li></ul><ul><li>same graph </li></ul>
  37. 37. Settlement of strip footing due to an earthquake <ul><li>S Eq (m) = </li></ul><ul><li>(0.174)(V 2 /Ag)[k h * /A] -4 tan α AE </li></ul><ul><li>α AE = Inclination angles for </li></ul><ul><li>active pressure conditions </li></ul><ul><li>V= peak velocity for the </li></ul><ul><li>design earthquake (m/sec) </li></ul><ul><li>A = acceleration coefficient </li></ul><ul><li>for the design earthquake </li></ul><ul><li>g = acceleration due to gravity (9.81 m/sec 2 ) </li></ul><ul><li>k h /(1- k v ) = k h * , if k v = 0 </li></ul>
  38. 39. Geotechnical Aspects of Earthquake resistant design <ul><li>Liquefaction of Soil </li></ul><ul><li>Soils behave like a liquid. How and why? </li></ul><ul><li>To understand the above phenomenon: Some understanding of basics required : </li></ul><ul><li>Total stress, </li></ul><ul><li>Pore water pressure </li></ul><ul><li>Effective stress </li></ul>
  39. 40. Total stress, Pore water pressure and Effective stress Figure-1 Figure-2 Case Total Pressure Pore Pressure Effective Pressure Figure- 1 475 150 325 Figure- 2 475 250 225
  40. 42. Liquefaction of Soil <ul><li>Shear stress, τ = c + σ n tanø </li></ul><ul><li>Effective stress gives more realistic behaviour of soil, </li></ul><ul><li>and can be expressed as τ = c’ + ( σ n –u)tanø ’ </li></ul><ul><li>During the ground motion due to an earthquake, </li></ul><ul><li>static pore pressure may increase by an amount u dyn </li></ul><ul><li>τ = c’ + ( σ n – u + u dyn )tanø’ </li></ul><ul><li>Let us consider a situation when u + u dyn = σ n , then τ = c’ </li></ul><ul><li>In cohesionless soil, c’= 0, hence τ = 0 </li></ul><ul><li>Soil loose its strength because of loss of effective stress </li></ul><ul><li>Saturated sand when subjected to ground vibration, it tends to compact and decrease in volume ; if drainage is unable to occur, the tendency to decrease in volume results in an increase in pore water pressure and when this becomes equal to the overburden pressure effective stress becomes equal to zero, sand looses its strength completely and it develops a liquefied state. </li></ul><ul><li>φΦγ </li></ul>
  41. 44. Influence of soil conditions on liquefaction potential
  42. 45. The Damage of Embankment Structures Toyokoro Collapsed Embankment
  43. 46. Place where Embankment was collapsed Abashiri River (1) Shibetsu River (6) Kushiro River (5) Kiyomappu River (2) Tokachi River (66) Under investigation Lateral Spread was observed ( ) : the number of collapsed points Tokachi River The Damage of Embankment Structures
  44. 47. Toyokoro Liquefied Soil Collapsed Embankment The Damage of Embankment Structures Liquefied Soil
  45. 48. Failure Mode   (notice : this is only concept) Liquefied Stratum Embankment Settlement Land Slide Lateral Spread The Damage of Embankment Structures
  46. 49. The Damage of Port Structures (at Kushiro Port) Kushiro Settlement behind Quay Wall Trace of Sand Boiling
  47. 50. Alaska Earthquake ( 1964 )
  48. 52. Caracas ( 1967 )
  49. 53. Alaska 2002 Boca del Tocuyo, Venezuela, 1989
  50. 54. Lateral spread at Budharmora (Bhuj, 2001)
  51. 55. Arial view of kandla port, Marked line sows ground crack and sand ejection (Gujrat Earthquake 2001)
  52. 56. Adverse effects of liquefaction <ul><li>Most catastrophic ground failure </li></ul><ul><li>Lateral displacement of large masses of soil </li></ul><ul><li>Mass comprised of completely liquefied soil or blocks of </li></ul><ul><li>intact material riding on a layer of liquefied soil </li></ul><ul><li>Flow develop in loose saturated sand or silts or relatively </li></ul><ul><li>steep slope (>3 degree) </li></ul>Flow failure
  53. 57. Lateral spread and Ground oscillation <ul><li>Liquefaction at depth-decouple overlaying soil layer from the underlying </li></ul><ul><li>ground </li></ul><ul><li>Lateral displacement of large superficial blocks of soil as a result of </li></ul><ul><li>liquefaction of subsurface layer and Allowing upper soil to oscillate </li></ul><ul><li>back and forth / up and down in the form of ground wave </li></ul><ul><li>Displaced ground-Break up internally causing fissures, scarps etc in the </li></ul><ul><li>form of surface failure </li></ul>
  54. 58. Loss of bearing strength <ul><li>Large deformation occur within the soil allowing the structure to settle & tip </li></ul><ul><li>e.g, 1964 Niigata earthquake, Japan-Most spectacular bearing failure </li></ul>
  55. 59. Soil conditions in Areas where Liquefaction has occurred : Case Study: Niigata Earthquake Kawangishicho apartment complex, tipped by 60 degree
  56. 60. Survey of damaged structure (Liquefaction Zone) Zone Damage Soil Characteristics Water table Remark A No damage (Coastal dune area) Dense Sand soil up to depth of 100 ft At great depth from ground level <ul><li>Type of structure: Same </li></ul><ul><li>Extent of damage: Different </li></ul><ul><li>Reason: </li></ul><ul><li>Characteristics of under lying sand : Different </li></ul><ul><li>ii) Type of foundation : Different </li></ul>B Relatively light damage (Low land area) Medium to light Sand soil up to depth of 100 ft Depth of water table less than ‘A’ C Damage and Liquefaction (Low land area) Medium to light Sand soil up to depth of 100 ft Depth of water table less than ‘A’ But similar to ‘B’
  57. 61. Standard Penetration Resistance Test (Zone-B & C-Comparison of soil condition) <ul><li>Average Penetration Resistance: Same up to 15 ft in zone B & C </li></ul><ul><li>Average Penetration Resistance: More below 15 ft depth in zone-B (Sand in zone-B are denser than those in zone- C) </li></ul><ul><li>Sand below 45 ft in both zone: Relatively dense & unlikely to be involved in liquefaction </li></ul><ul><li>Difference in Penetration resistance of sand in depth range from 15 ft to 45 ft is responsible for the major difference in foundation and liquefaction behaviour in two zones </li></ul>
  58. 62. Soil Foundation Condition and Building Performance (Zone-C-Range of penetration resistance:heavy damage zone ) <ul><li>Variation of Penetration resistance with depth falls within shaded area </li></ul><ul><li>Standard Penetration Resistance: Top 25 ft: Generally less than 15 and sometimes less than 5 </li></ul><ul><li>Conclusion: N =28 at the base of the foundation , required, To prevent major damage </li></ul>
  59. 63. Classification of Extent of Damage for each Building (Zone-C) <ul><li>Buildings Foundation </li></ul><ul><li>Supported on Shallow spread footing foundations </li></ul><ul><li>(Number: 63) </li></ul><ul><li>Supported on Piles </li></ul><ul><li>(Number: 122) </li></ul><ul><li>Extent of damage due to foundation failure </li></ul><ul><li>( Category-I to Category-IV) </li></ul><ul><li>Category-I:No damage to Building </li></ul><ul><li>(Tilt: upto 20 min, Settlement: upto 8 inch) </li></ul><ul><li>Category-IV: Heavy damage to Building </li></ul><ul><li>(Tilt: upto 2.3 degree, Settlement: upto 3 ft) </li></ul>
  60. 65. Relationship between N at the base of Foundation and Extent of Damage
  61. 66. Relationship between depth of pile, ‘N’ of sand at pile tip and Extent of Damage (Zone-C)
  62. 69. Case Study : Gujrat Earthquake, 2001 <ul><li>Soil Condition </li></ul>S.No. Region Type of Soil 1 Ahmedabad and Surrounding region Alluvial belt 2 Bhuj and Surrounding region Silty sand 3 Coastal area (Kandla) Soft clay 4 South Gujrat Expansive Clay
  63. 70. Condition of soil before and after earthquake (Relative density of sand with depth) <ul><li>Change in density observed </li></ul><ul><li>Increase in density observed upto 5m depth from ground surface </li></ul><ul><li>Decrease in density from 10-15m depth from ground surface </li></ul><ul><li>Change in density of sand under saturation during vibration cause for liqufaction and possible reason for large differential settlement at Ahmedabad </li></ul>
  64. 71. D vs depth of layer of three section charaterized by predominant period T p of microseismic vibrations <ul><li>Damage </li></ul><ul><li>Zone A-Minor, </li></ul><ul><li>Zone B- Moderate, </li></ul><ul><li>Zone C- Heavy </li></ul><ul><li>Direct co-relation exists between quality of ground, dynamic characteristics and anticipated consequences of earthquake </li></ul>
  65. 72. Liquefaction Analysis <ul><li>Objective : To ascertain if the soil has the ability or potential to liquefy during an earthquake </li></ul><ul><li>Assumption : Soil Column move horizontally as a rigid body in response to maximum horizontal acceleration a max exerted by the earthquake at ground surface </li></ul>
  66. 73. <ul><li>At equilibrium: </li></ul><ul><li>Horizontal seismic force = Max. </li></ul><ul><li>shear force at the base of column ( τ max ) </li></ul><ul><li>Horizontal seismic force = Mass x Accl. </li></ul><ul><li>= [( γ t .z)/g]a max = σ vo (a max /g) = τ max </li></ul><ul><li>Mass = W/g = ( γ t .z)/g = σ vo /g </li></ul><ul><li>If effective vertical stress = σ ’ vo , </li></ul><ul><li>Then ( τ max / σ ’ vo ) = ( σ vo / σ ’ vo )(a max /g) </li></ul><ul><li>In reality, during an earthquake, soil </li></ul><ul><li>column does not act as a rigid body </li></ul><ul><li>( τ max / σ ’ vo ) = r d ( σ vo / σ ’ vo )(a max /g) </li></ul><ul><li>r d ~ 1- 0.012z , also depends upon the magnitude of the earthquake </li></ul>
  67. 74. <ul><li>Conversion of irregular earthquake record to an equivalent series of </li></ul><ul><li>uniform stress cycle by assuming the following: </li></ul><ul><li>τ av = τ cyc = 0.65 τ max = 0.65 r d ( σ vo / σ ’ vo )(a max /g) </li></ul><ul><li>To felicitate liquefaction analysis, define a dimensionless parameter </li></ul><ul><li>CSR or SSR = τ cyc / σ ’ vo = 0.65 r d ( σ vo / σ ’ vo )(a max /g) </li></ul><ul><li>CSR = Cyclic stress ratio, SSR = Seismic stress ratio </li></ul><ul><li>FS = Factor of safety against liquefaction = CRR/CSR </li></ul><ul><li>CRR= Cyclic resistance ratio </li></ul>Time history of shear stress during earthquake for liquefaction analysis
  68. 75. Cyclic resistance ratio <ul><li>Represents liquefaction </li></ul><ul><li>resistance of soil </li></ul><ul><li>Data used: EQ ~ 7.5, </li></ul><ul><li>Line represents dividing line </li></ul><ul><li>Three lines contain- 35, 15 or ≤ 5 % fine </li></ul><ul><li>Data to the left of each line indicate field liquefaction </li></ul><ul><li>Data to the right of each line indicate no liquefaction </li></ul><ul><li>FS = CRR/CSR </li></ul><ul><li>FS = Factor of safety against liquefaction </li></ul>
  69. 76. Evaluation of liquefaction potential <ul><li>CSR = Cyclic stress ratio, CRR= Cyclic resistance ratio </li></ul><ul><li>FS = Factor of safety against liquefaction = CRR/CSR= CSR L /CSR = τ cyc,L / τ cyc </li></ul>
  70. 77. Liquefaction Analysis: Niigata 1964 EQ <ul><li>A site in Japan had the measured SPT resistance as given in the table. The </li></ul><ul><li>SPT procedure used in Japan delivered about 72% of the theoretical free </li></ul><ul><li>fall energy of the sampler. Assuming that the sands have an average void </li></ul><ul><li>ratio of 0.44 and that the water table is at a depth of 1.5m, determine the </li></ul><ul><li>extent to which the liquefaction would have been expected in 1964 Niigata </li></ul><ul><li>EQ (M=7.5) if the peak horizontal acceleration in the ground surface </li></ul><ul><li>was 0.16g. </li></ul>D (m) N m D N m D N m D N m D N m 1.2 7 5.2 5 9.2 14 13.2 11 17.2 5 2.2 4 6.2 9 10.2 9 14.2 11 18.2 6 3.2 3 7.2 12 11.2 23 15.2 24 19.2 4 4.2 3 8.2 12 12.2 13 16.2 27 20.2 38
  71. 78. Standard penetration test (SPT) <ul><li>(N 1 ) 60 = (N m C N )(E m /0.60E ff ) </li></ul><ul><li>(N 1 ) 60 = Corrected standard penetration resistance (N):To normalize the N value to an overburden pressure of 100kPa and to correct it to an energy ratio of 60% </li></ul><ul><li>Energy ratio is the average ratio of the actual energy delivered by safety hammer to the theoretical free fall energy </li></ul><ul><li>N m = Measured penetration resistance </li></ul><ul><li>C N = Overburden correction factor </li></ul><ul><li>E m = Actual hammer energy </li></ul><ul><li>E ff = Theoretical free fall hammer energy </li></ul>
  72. 79. Void ratio = 0.44, Gs = 2.7, Dry and submerged densities 1.874 Mg/m 3 1.180 Mg/m 3 <ul><li>σ vo ’ = Vertical Effective stress at 6.2 m depth = </li></ul><ul><li>[(1.5m)(1.874 Mg/m 3 )+(4.7m)(1.180Mg/m 3 )](9.81 m/sec 2 ) </li></ul><ul><li>= 82kPa </li></ul><ul><li>σ vo = Total Vertical stress at 6.2 m depth = </li></ul><ul><li>[(1.5m)(1.874 Mg/m 3 )+(4.7m)(2.180Mg/m 3 )](9.81 m/sec 2 ) = 128.1kPa </li></ul><ul><li>C N = {1/(82kPa)[(0.01044 ton/ft 2 )/kPa]} 0.5 = 1.08 </li></ul><ul><li>(N 1 ) 60 = N m C N (E m /0.60 E ff ) </li></ul><ul><li>= (9)(1.08)(0.72 E ff /0.60 E ff )= 11.7 at 6.2 m depth </li></ul>
  73. 80. SPT overburden correction factor & values of (N 1 ) 60
  74. 81. τ av = τ cyc = 0.65 τ max = 0.65 r d ( σ vo / σ ’ vo )(a max /g) r d = Stress reduction factor = 0.960 at 6.2m depth=C D
  75. 82. Cyclic Shear Stress τ cyc = 0.65 r d ( σ vo )(a max /g) (6.2m depth) <ul><li>=0.65(0.960)(128.1)(0.16g/g) </li></ul><ul><li>= 12.8 kPa </li></ul><ul><li>cyclic shear stress required </li></ul><ul><li>to initiate liquefaction </li></ul><ul><li>depends on liquefaction </li></ul><ul><li>resistance of the soil = </li></ul><ul><li>Liquefaction resistance </li></ul><ul><li>characterized by (N 1 ) 60 </li></ul><ul><li>CSR M=7.5 = 0.130 </li></ul><ul><li>when (N 1 ) 60 = 11.7 at 6.2m depth </li></ul>
  76. 83. <ul><li>Magnitude Correction Factor for Cyclic Stress approach: </li></ul><ul><li>CSR M /CSR M=7.5 </li></ul><ul><li>Magnitude of Niigata EQ = 7.5 </li></ul><ul><li>CSR M /CSR M=7.5 =1 </li></ul><ul><li>CSR = τ cyc / σ ’ vo = 0.65 r d ( σ vo / σ ’ vo )(a max /g) </li></ul><ul><li>Cyclic stress ratio required for liquefaction </li></ul><ul><li>τ cyc,L = CSR L ( σ ’ vo ) = (0.130)(82.0)=10.7 kPa </li></ul><ul><li>[CSR L = (CSR M=7.5 )(1.0)= 0.130(1.0)= 0.130] </li></ul><ul><li>FS = Factor of safety against liquefaction </li></ul><ul><li>= CSR L /CSR = τ cyc,L / τ cyc = 10.7/12.8= 0.84 </li></ul><ul><li>[At a depth of 6.2 m] </li></ul>
  77. 84. Details of calculation
  78. 85. Extensive liquefaction observed for upper 8-10m & at greater depth Peak horizontal acceleration at Niigata 0.2g to 0.3g (> 0.16g ) Extensive liquefaction predicted in this problem is consistent with actual observation in 1964 Niigata earthquake
  79. 86. Modified Chinese Criteria for liquefaction Assessment <ul><li>Criteria for Soil to liquefy: </li></ul><ul><li>Clay Fraction < 15 %, </li></ul><ul><li>Liquid Limit < 35 %, </li></ul><ul><li>Moisture Content > 0.9 LL </li></ul>
  80. 87. SAFETY AGAINST LIQUEFACTION Zone Depth below ground level ‘ N’ value III, II Up to 5 m 15 III, II Up to 10 m 25 II (For important structure) Up to 5 m 10 II (For important structure) Up to 10 m 20
  81. 88. Liquefaction Potential Damage Range of SPT N corrected Potential Damage 0-20 High 20-30 Medium > 30 No Significant Damage
  82. 89. What are the options for liquefaction mitigations? <ul><li>Strengthen structures to resist predicted ground movements (if small) </li></ul><ul><li>Select appropriate foundation type and depth including foundation modification of existing structure </li></ul><ul><li>Stabilize soil to eliminate the potential for liquefaction or to control its effects </li></ul>
  83. 90. Counter measures against Liquefaction <ul><li>Densification </li></ul><ul><li>Vibro-floatation </li></ul><ul><li>Blasting </li></ul><ul><li>Stabilization of soils </li></ul><ul><li>Filtration (drainage) </li></ul><ul><li>Lowering of Ground water Table </li></ul><ul><li>Application of dead weight </li></ul><ul><li>Mitigation of lateral flow by providing baffle walls </li></ul><ul><li>Reinforced Earth </li></ul>
  84. 91. Uttarkashi Earthquake, 1991 <ul><li>Site: National Highway (NH-58)at Byasi (30 km from Rishkesh towards Badrinath in Garhwal Himalaya): Agency: BRO </li></ul><ul><li>Geo-synthetic Retaining Wall (Height-11m, Length- 19.5 m) </li></ul><ul><li>Location of Existing Retaining Wall of the area </li></ul>
  85. 92. Cross-section(Retaining Geogrid Reinforced cohesionless backfill)
  86. 93. Field Performance of wall 4 O.P. Fixed in the Wall: To monitor the lateral movement of wall top away from backfill using Electronic Distance Meter for a period of 36 months
  87. 94. Average Lateral Deflection of wall with time <ul><li>Stable equilibrium: 700 days </li></ul><ul><li>Major part of total lateral movement (60~70%) : Short span of 45 days </li></ul><ul><li>Active earth pressure exerted on wall due to ground shaking by </li></ul><ul><li>Uttarkashi earth quake 1991 </li></ul><ul><li>Cost: 79% of the cost of retaining wall with conventional earth fill </li></ul>
  88. 95. Hyogoken Nambu Earthquake 1995 Height of wall – 4 to 8 m Conventional Retaining Wall – suffered maximum damage Geo-synthetic reinforced soil retaining wall –Performed very well (due to relatively high ductility of the wall)
  89. 96. Preloading for oil tanks <ul><li>Site:500 km from the sea shore on a coastal alluvial Plain, 5 km south west of THESSALONIKI in Northern Greece, area moderately seismic active </li></ul><ul><li>Pre-loading-Aug. 1979 to June, 1980 </li></ul><ul><li>Table: Change in the Variation caused by Pre Loading </li></ul>B- Before Preloading, A – After Preloading Depth Range (Metre) SPT Resistance (Bloe/0.3m) B A 0 - 5.5 6 22 5.5 - 8.0 22 34 8.0 - 26.0 10 39
  90. 97. <ul><li>Preloading for Grain Silos at city THESSALONIKI in north Greece </li></ul><ul><li>The continuity of settlement time curve was not upset , atleast not appreciably earthquake in 1978 </li></ul><ul><li>The time rate of settlement versus time curves does not show however a kink </li></ul>
  91. 98. Rokko & Port (Kobe) <ul><li>Ground improvement : Pre loading /Vertical drain/Sand compaction pile </li></ul><ul><li>Untreated ground: N - 8 to 15 </li></ul><ul><li>Subsidence: 30 to 100 cm. (Avg. 50) </li></ul><ul><li>Treated ground: N - 25 or more </li></ul><ul><li>Subsidence: less than 5 cm. </li></ul>
  92. 99. Foundation of modern building that survived earthquake <ul><li>Concrete slab without piles (b) The same at deeper depth </li></ul><ul><li>(c ) Concrete foundation grillage on wood built up piles </li></ul><ul><li>The same but with concrete piles </li></ul><ul><li>Foundation grillage suspended by bolts on concrete piles </li></ul><ul><li>1 – Street Level; 2 – Level of compacted soil </li></ul>
  93. 100. Can Liquefaction be predicted? <ul><li>Occurrence of liquefaction can’t be predicted </li></ul><ul><li>Possible to identify areas giving detailed information that have the potential for liquefaction </li></ul><ul><li>Mapping of liquefaction potential on a regional scale </li></ul><ul><li>Maps exists for many regions in USA and Japan </li></ul><ul><li>Liquefaction potential map : complied by superimposing a liquefaction susceptibility map with liquefaction opportunity map </li></ul><ul><li>liquefaction susceptibility: capacity of soil to resist liquefaction </li></ul><ul><li>(Controlling factor: soil type, density and water table) </li></ul><ul><li>liquefaction opportunity: A function of the intensity of seismic shaking or demand placed on the soil </li></ul><ul><li>(factor affecting opportunity: Frequency of earthquake occurrence, intensity of seismic ground shaking) </li></ul>
  94. 101. Criteria for liquefaction potential map <ul><li>Area known to have experienced liquefaction during historic earthquakes </li></ul><ul><li>Area containing liquefaction susceptible material that are saturated , nearly saturated or expected to become saturated </li></ul><ul><li>Area having sufficient existing geotechnical data indicating the soil are potentially liquefiable </li></ul><ul><li>Area underlain with saturated geologically young sediments (< 1000 to 15000 year old) </li></ul>
  95. 102. Is it possible to prepare for liquefaction ? <ul><li>Possible to identify areas potentially subject to liquefaction with hazard zone map </li></ul><ul><li>Emphasis in terms of developing appropriate public policy or selecting mitigation technique in area of major concern </li></ul><ul><li>Use of hazard map by public and private owners the seriousness of expected damage and most vulnerable structure </li></ul><ul><li>Using this map local government could designate liquefaction potential areas, and require by ordinance, site investigation and possible mitigation techniques for properties in these area particularly underground pipes and critical transportation routes </li></ul>
  96. 103. Acknowledgements <ul><li>The author wishes to acknowledge all the various sources used and especially Principle of Soil Dynamics (by Das and Ramana, CENGAGE Learning), and paper published by Seed et al. in various Journal/Book at different times during the preparation of this presentation which aided and enhanced the quality either in the form of information, data, figure, photo, graph or table. </li></ul><ul><li>Thanks for your attention </li></ul>