1.Geotechnical Aspects in Earthquake Resistant Design

• by

Dr. J.N.Jha

Professor & Head

Department of Civil Engineering

Guru Nanak Dev Engineering College

Ludhiana-141006

Email: jagadanand@gmail.com

2. Conception or rather Misconception of Structure Designer

Structure Designer believes erroneously most of the time that superstructure needs to be strengthened more in poor soil than in a good soil.

Dynamic behaviour of soil rarely receives any attentionof structure engineer as compared to dynamic behaviour of structure.

3. Niigatta Earthquake: 1964, Mag. 7.5 4. Japan earthquake 1964:Niigata- Mag. 7.5 5. Tokachi-oki Earthquake: 2003 The Damage of Sewerage Structures kushiro (Town) Lifted up manhole and gushed soilduring liquefaction Lifted up manhole 6. Caracas Earthquake1967: Mag. 6.6 7. Chile Earthquake1960 :Island nearValdivia- Mag. 9.5 8. Alaska Earthquake1964:Mag. 9.2 9. Observed Damage from Earthquakes

Chile earthquake 1960 :An island near Valdivia- Mag.- 9.5

Large settlements and differential settlements of the ground surface - Compaction of loose granular soil by EQ

Alaska earthquake 1964:Turnagain heights landslide- Mag.- 9.2

Major landslide - combination of dynamic stresses and induced pore water pressur e

Japan earthquake 1964:Niigata -Mag.- 7.5

Settlement and tilting of structures - liquefaction of soil

Caracas earthquake 1967- Mag.- 6.6

Response of buildingduring EQ found to depend on thethickness of soil under the building .

10. Inference and Attention:

Influence of local soil conditionon shaking and damage intensity during earthquakeaffects the stability of structure .

Large scale tilting of well built houses in

the Niigatta earthquake of 1964 focussed

the attention of profession onproblems

of soil dynamics:Requiring a careful

attention by Engineers.

11. Objective of Earthquake Resistant Design

Ensuring structural reliability without structural damagedue to ground shaking ofmoderate intensities.

Special structureslike high level bridges, tall dams , Nuclear Power Plantneed specific designprocedures (Site specific) supported by detailed investigation.

12. Geotechnical Aspects of Earthquake Resistant Design

Geotechnical Aspects of Earthquake Resistant Design of foundation of structures and other soil retaining structuresof soil depends on:

Dynamic stress deformation and strength characteristics

Earth pressure problems and retaining wall

Dynamic bearinng capacity and design of shallow foundation

Liquefaction of soil

Machine foundations

13. Influence of local soil conditions on Acceleration(Cause for damage during EQ)

Some Basic Information :

Acceleration Response Spectrum

A graph showing the maximum accelerations induced in structures with fundamental period ranging from 0 to several seconds

Velocity Response Spectrum

A plot showing relationship between maximum velocity with fundamental period of the structure

Relation between Velocity Spectrum ( S v)and Acceleration Spectrum ( S a)

S v (T/2 )S a T = fundamental period of the structure

Valueof horizontal peak ground acceleration = 0.6g ?

Physical meaning :Movement of the ground can cause a maximum horizontal forceon a rigid structure equal to60% of its weight

14. Site approximately same distance from the zone of energy release 1957 San Francisco Earthquake 15. Site approximately same distance from the zone of energy release 1957 San Francisco Earthquake 16. Effect of Soil Conditions on form of Response Spectra Site A 17. Effect of Soil Conditions on form of Response Spectra Site B 18. Effect of Soil Conditions on form of Response Spectra Site C 19. Effect of Soil Conditions on form of Response Spectra Site D 20. Effect of Soil Conditions on form of Response Spectra Site E 21. Effect of Soil Conditions on form of Response Spectra Site F 22. Effect of Soil Conditions on form of Response Spectra Site A - F 23. Development of Peak/Max. Acceleration

Clay (soft): Moulded easily at natural water content and readily excavated

Clay (firm): Moulded by substantial pressure at natural water content and excavated

with a spade

Out of six four spectra obtained from the same city in the same EQ at a considerable

distance from the epicenter

To eliminate the influence of different amplitudes of surface acceleration, plot made

between period and normalized acceleration (Spectral Acceleration/Maximum

Ground Acceleration

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.

Clay layer : Amplified seismic shock of glacial till (Both event)

Peat deposits : Amplified seismic shocks (Only for distant shock)

25.

Time taken for each complete cycle of oscillation is called FUNDAMETNAL NATURAL PERIOD (T) of the building

Time taken by the wave to complete one cycle of motion is called PERIOD OF EQ WAVE (0.03 to 33 seconds)

Short EQ wave have large response on short period buildings

Long EQ wave have large response on long period buildings

BuildingResponse variation during Earthquake 26.

T (Inherent property of the building) depends on the building flexibility and mass,Any alteration made to the building will change its T

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

Bldg (10-14 storey); damage intensity maximum when soil cover in the range of 150-300m and small for lower thickness of soil cover

Soil plays the role of filter allowing some ground wavesto passthroughand filtering the rest.

27. Damage potential coefficient varies with building characteristics and soil depth 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. Dynamic bearing capacity

Soil dynamics: Engineering properties and behaviour of soil under dynamic stress

Factors to be considered for estimation of bearing capacity under dynamic condition:

Nature of variation of the magnitude of the loading pulse

Duration of the pulse

Strain rate deformation of soil during deformation

30.

For both static, dynamic and seismic cases bearing capacity failure of foundation grouped into three categories:

General shear failure

Punching shear failure

Local shear failure

Various theories concerning dynamic bearing capacity have been presented in last 50 years butdefinition of dynamic bearing capacity is yet to evolve till date

31. Ultimate bearing capacity of continuous shallow foundation (static case) 32. Seismic bearing capacity 33.

Static case

Seismic case

q u= q N q+ ( B N )

q = D f

N q ,N = bearing capacity

factors

N qand N = f ()

= tan-1[(0.67+ R D- 0.75R 2 D )tan ]

q uE = q N qE+ ( B N E )

q = D f

N qE , N E= bearing capacity

factors

N qEandN E= f (, tan)

tan = k h /(1- k v )

k h &k v=horizontal and vertical

coefficient of acceleration

AE & PE = Inclination angles for

active and passive pressure conditions

34.

Static bearing capacity factors

Seismic bearing capacity factors

N q N 0 1 0 10 2.47 1.22 20 6.40 5.39 30 18.40 22.40 40 64.2 109.41 N q N 0 1 0 10 2.4 1.4 20 5.9 6.4 30 16.5 23.8 40 59.0 112.0 35.

Variation of N qand N with

Variation of N E /N and N qE / N qwith tan and

36. Prediction of dynamic load- settlement relationship for foundations on clay (Jackson and Halada,1964)

For any given value of

S stat /B, multiply Q/B 2 c u

by the strain rate factor

(=1.5) and plot it in the

same graph

37. Settlement of strip footing due to an earthquake

S Eq (m) =

(0.174)(V 2 /Ag)[k h * /A] -4 tan AE

AE= Inclination angles for

active pressure conditions

V= peak velocity for the

design earthquake (m/sec)

A = acceleration coefficient

for the design earthquake

g = acceleration due to gravity (9.81 m/sec 2 )

k h /(1- k v ) = k h * , if k v = 0

38. 39. Geotechnical Aspects of Earthquake resistant design

Liquefaction of Soil

Soils behave like a liquid.How and why?

To understand the above phenomenon: Some understandingof basics required :

Total stress,

Pore water pressure

Effective stress

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 41. 42. Liquefaction of Soil

Shear stress,= c + ntan

Effective stress gives more realistic behaviour of soil,

and can be expressed as= c + ( nu)tan

During the ground motion due to an earthquake,

static pore pressure may increase by an amount u dyn

= c + ( n u + u dyn )tan

Let us consider a situationwhenu + u dyn = n , then= c

In cohesionless soil,c= 0, hence= 0

Soil loose its strength becauseofloss of effective stress

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 completelyand it develops a liquefied state.

43. 44. Influence of soil conditions on liquefaction potential 45. The Damage of Embankment Structures Toyokoro Collapsed Embankment 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 47. Toyokoro Liquefied Soil Collapsed Embankment The Damage of Embankment Structures Liquefied Soil 48. Failure Mode (notice : this is only concept) Liquefied Stratum Embankment Settlement Land Slide Lateral Spread The Damage of Embankment Structures 49. The Damage of Port Structures(at Kushiro Port) Kushiro Settlement behind Quay Wall Trace of Sand Boiling 50. Alaska Earthquake ( 1964 ) 51. 52. Caracas ( 1967 ) 53. Alaska2002 Boca del Tocuyo, Venezuela,1989 54. Lateral spread at Budharmora (Bhuj, 2001) 55. Arial view of kandla port, Marked line sows ground crack and sand ejection (Gujrat Earthquake 2001) 56. Adverse effects of liquefaction

Most catastrophic ground failure

Lateral displacement of large masses of soil

Mass comprised of completely liquefied soil or blocks of

intact material riding on a layer of liquefied soil

Flow develop in loose saturated sand or silts or relatively

steep slope (>3 degree)

Flow failure 57. Lateral spread and Ground oscillation

Liquefaction at depth-decouple overlaying soil layer from the underlying

ground

Lateral displacement of large superficial blocks ofsoil as a result of

liquefaction of subsurface layer and Allowingupper soil to oscillate

back and forth / up and down in the form ofground wave

Displaced ground-Break up internallycausing fissures, scarps etc in the

form ofsurface failure

58. Loss of bearing strength

Large deformation occur within the soil allowing the structure to settle & tip

e.g, 1964 Niigata earthquake, Japan-Most spectacular bearing failure

59. Soil conditionsin Areas whereLiquefactionhasoccurred : Case Study:Niigata EarthquakeKawangishicho apartment complex, tipped by 60 degree 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

Type of structure: Same

Extent of damage: Different

Reason:

Characteristics ofunder lying sand :Different

ii)Type offoundation :Different

B Relatively light damage (Low land area)Medium to lightSand soil up to depth of 100 ft Depth of water table less than A C Damage and Liquefaction (Low land area) Medium to lightSand soil up to depth of 100 ft Depth of water table less than A But similar to B 61. Standard Penetration Resistance Test(Zone-B & C-Comparison of soil condition)

Average Penetration Resistance:Sameup to15 ftin zoneB & C

Average Penetration Resistance:More below 15 ftdepth inzone-B(Sand in zone-B are denser than those in zone- C)

Sand below 45 ft in both zone: Relatively dense & unlikely to beinvolved in liquefaction

Difference in Penetration resistance ofsand in depth range from15 ft to 45 ftisresponsible for the major difference infoundation and liquefaction behaviour in two zones

62. Soil Foundation Condition and Building Performance(Zone-C-Range of penetration resistance:heavy damage zone )

Variationof Penetration resistance with depthfalls within shaded area

Standard Penetration Resistance: Top 25 ft: Generally less than 15 and sometimes less than 5

Conclusion:N =28at the base of the foundation , required, Toprevent major damage

63. Classification of Extent of Damage for each Building (Zone-C)

Buildings Foundation

Supported on Shallow spread footing foundations

(Number: 63)

Supported on Piles

(Number: 122)

Extent of damage due to foundation failure

( Category-I to Category-IV)

Category-I:No damage to Building

(Tilt: upto 20 min, Settlement: upto 8 inch)

Category-IV: Heavy damage to Building

(Tilt: upto 2.3 degree, Settlement: upto 3 ft)

64. 65. Relationship between N at the base of Foundation and Extent of Damage 66. Relationship between depth of pile, Nof sand at pile tip and Extent of Damage (Zone-C) 67. 68. 69. Case Study :Gujrat Earthquake, 2001

Soil Condition

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 70. Condition of soil before and after earthquake (Relative density of sand with depth)

Change in density observed

Increase in density observed upto 5m depth from ground surface

Decrease in density from 10-15m depth from ground surface

Change in density of sand under saturation during vibration cause for liqufaction and possible reason for large differential settlement at Ahmedabad

71. D vs depth of layer of three section charaterized by predominant period T pof microseismic vibrations

Damage

Zone A-Minor,

Zone B- Moderate,

Zone C- Heavy

Direct co-relation exists between quality of ground, dynamic characteristics and anticipated consequences of earthquake

72. Liquefaction Analysis

Objective : To ascertain if the soil has the ability or potential to liquefy during an earthquake

Assumption : Soil Column move horizontally as a rigid body in response to maximum horizontal acceleration a maxexerted by the earthquakeat ground surface

73.

At equilibrium:

Horizontal seismic force= Max.

shear force at the base of column ( max )

Horizontal seismic force = Mass x Accl.

= [( t.z)/g]a max= vo(a max /g) = max

Mass = W/g = ( t.z)/g = vo/g

If effectivevertical stress = vo,

Then( max/ vo) =( vo/ vo)(a max /g)

Inreality, during an earthquake, soil

column does not act as a rigid body

( max/ vo)= r d( vo/ vo)(a max /g)

r d~ 1- 0.012z ,also depends upon the magnitude of the earthquake

74.

Conversion of irregular earthquake record to an equivalent seriesof

uniform stress cycleby assuming the following:

av= cyc= 0.65 max= 0.65 r d( vo/ vo)(a max /g)

To felicitate liquefaction analysis, define a dimensionless parameter

CSR or SSR = cyc/ vo= 0.65 r d( vo/ vo)(a max /g)

CSR= Cyclic stress ratio,SSR = Seismic stress ratio

FS= Factor of safety against liquefaction= CRR/CSR

CRR=Cyclic resistance ratio

Time history of shear stress during earthquake for liquefaction analysis 75. Cyclic resistance ratio

Represents liquefaction

resistance of soil

Data used: EQ ~ 7.5,

Line represents dividing line

Three lines contain- 35, 15 or 5 % fine

Data to the left of each lineindicate field liquefaction

Data to the right of eachline indicate no liquefaction

FS = CRR/CSR

FS = Factor of safety against liquefaction

76. Evaluation of liquefaction potential

CSR= Cyclic stress ratio,CRR=Cyclic resistance ratio

FS= Factor of safety against liquefaction= CRR/CSR=CSR L /CSR = cyc,L/ cyc

77. Liquefaction Analysis: Niigata 1964 EQ

Asite in Japan had the measured SPT resistance as given in the table. The

SPT procedure used in Japan delivered about 72% of the theoretical free

fall energy of the sampler. Assuming that the sands have an average void

ratio of 0.44 and that the water table is at a depth of 1.5m, determine the

extent to which the liquefaction would have been expected in 1964 Niigata

EQ (M=7.5) if thepeak horizontal acceleration in the ground surface

was 0.16g.

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 78. Standard penetration test (SPT)

(N 1 ) 60= (N mC N )(E m /0.60E ff )

(N 1 ) 60= Corrected standard penetration resistance (N):To normalize the N value to an overburden pressure of 100kPa and to correct it to anenergy ratio of 60%

Energy ratiois the average ratio of theactual energydelivered by safety hammer to thetheoreticalfree fallenergy

N m= Measured penetration resistance

C N= Overburden correction factor

E m= Actual hammer energy

E ff= Theoretical free fall hammer energy

79. Void ratio = 0.44, Gs = 2.7, Dry and submerged densities 1.874Mg/m 31.180 Mg/m 3

vo = Vertical Effective stress at6.2 m depth =

[(1.5m)(1.874 Mg/m 3 )+(4.7m)(1.180Mg/m 3 )](9.81 m/sec 2 )

= 82kPa

vo= Total Vertical stress at6.2 m depth =

[(1.5m)(1.874 Mg/m 3 )+(4.7m)(2.180Mg/m 3 )](9.81 m/sec 2 ) = 128.1kPa

C N = {1/(82kPa)[(0.01044 ton/ft 2 )/kPa]} 0.5= 1.08

(N 1 ) 60 = N mC N(E m /0.60 E ff )

= (9)(1.08)(0.72 E ff /0.60 E ff )= 11.7 at6.2 m depth

80. SPT overburden correction factor & values of (N 1 ) 60 81. av= cyc= 0.65 max= 0.65 r d( vo/ vo)(a max /g)r d= Stress reduction factor = 0.960 at 6.2m depth=C D 82. Cyclic Shear Stress cyc= 0.65 r d( vo)(a max /g)(6.2m depth)

=0.65(0.960)(128.1)(0.16g/g)

= 12.8 kPa

cyclic shear stress required

to initiate liquefaction

depends on liquefaction

resistance of the soil =

Liquefaction resistance

characterized by (N 1 ) 60

CSR M=7.5 = 0.130

when (N 1 ) 60= 11.7 at6.2m depth

83.

Magnitude Correction Factor for Cyclic Stress approach:

CSR M/CSR M=7.5

Magnitude of Niigata EQ = 7.5

CSR M/CSR M=7.5=1

CSR = cyc/ vo= 0.65 r d( vo/ vo)(a max /g)

Cyclic stress ratio required for liquefaction

cyc,L= CSR L ( vo ) = (0.130)(82.0)=10.7 kPa

[CSR L = (CSR M=7.5 )(1.0)= 0.130(1.0)= 0.130]

FS = Factor of safety against liquefaction

=CSR L /CSR = cyc,L/ cyc= 10.7/12.8= 0.84

[At a depth of 6.2 m]

84. Details of calculation 85. Extensive liquefaction observed for upper 8-10m & at greater depthPeak horizontal acceleration at Niigata0.2g to 0.3g(> 0.16g) Extensive liquefaction predictedin this problem is consistentwith actual observation in 1964 Niigata earthquake 86. Modified Chinese Criteria for liquefactionAssessment

Criteria for Soil to liquefy:

Clay Fraction < 15 %,

Liquid Limit < 35 %,

Moisture Content > 0.9 LL

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 88. Liquefaction Potential Damage Range of SPT N corrected Potential Damage 0-20 High 20-30 Medium > 30 No Significant Damage 89. What are the options for liquefaction mitigations?

Strengthen structures to resist predicted ground movements (if small)

Select appropriate foundation type and depth including foundation modification of existing structure

Stabilize soil to eliminate the potential for liquefaction or to control its effects

90. Counter measures against Liquefaction

Densification

Vibro-floatation

Blasting

Stabilizationof soils

Filtration (drainage)

Lowering of Ground water Table

Application of dead weight

Mitigation of lateral flow by providing baffle walls

Reinforced Earth

91. Uttarkashi Earthquake, 1991

Site: National Highway (NH-58)at Byasi (30 km from Rishkesh towards Badrinath in Garhwal Himalaya): Agency: BRO

Geo-syntheticRetaining Wall (Height-11m, Length- 19.5 m)

Location of Existing Retaining Wall of the area

92. Cross-section(Retaining Geogrid Reinforced cohesionless backfill) 93. Field Performance of wall 4 O.P. Fixed in the Wall: To monitor thelateral movement of wall top away from backfillusing Electronic Distance Meter for aperiod of 36 months 94. Average Lateral Deflection of wall with time

Stable equilibrium: 700 days

Major part of total lateral movement (60~70%) : Short span of 45 days

Active earth pressure exerted on wall due to ground shakingby

Uttarkashi earth quake 1991

Cost: 79% of the cost of retaining wall with conventional earth fill

95. Hyogoken Nambu Earthquake1995 Height of wall 4 to 8 m Conventional Retaining Wall suffered maximum damageGeo-synthetic reinforced soilretaining wall Performed very well(due to relatively high ductilityof the wall) 96. Preloading for oil tanks

Site:500 km from the sea shore on a coastal alluvial Plain, 5 km south west of THESSALONIKIin Northern Greece, area moderately seismic active

Pre-loading-Aug. 1979 to June, 1980

Table: Change in the Variation caused by Pre Loading

B- Before Preloading, A After Preloading Depth Range (Metre)SPT Resistance (Bloe/0.3m) BA 0-5.5 622 5.5 -8.0 2234 8.0 -26.0 1039 97.

Preloading for Grain Silos at city THESSALONIKI in north Greece

The continuity of settlement time curve was not upset , atleast not appreciably earthquake in 1978

The time rate of settlementversus time curves does not show however a kink

98. Rokko & Port (Kobe)

Ground improvement : Pre loading /Vertical drain/Sand compaction pile

Untreated ground:N-8 to 15

Subsidence:30 to 100 cm. (Avg. 50)

Treated ground:N-25 or more

Subsidence:less than 5 cm.

99. Foundation of modern building that survived earthquake

Concrete slab without piles (b) The same at deeper depth

(c ) Concrete foundationgrillage on wood built up piles

The same but with concrete piles

Foundation grillagesuspended by boltson concrete piles

1 Street Level; 2 Level of compacted soil

100. Can Liquefaction be predicted?

Occurrence of liquefaction cant be predicted

Possible to identify areas giving detailed information that have the potential for liquefaction

Mapping of liquefaction potential on a regional scale

Maps exists for many regions in USA and Japan

Liquefaction potential map : complied bysuperimposingaliquefaction susceptibilitymap withliquefaction opportunity map

liquefaction susceptibility: capacity of soil to resist liquefaction

(Controlling factor: soil type, density and water table)

liquefaction opportunity: A function of the intensity of seismic shaking or demand placed on the soil

(factor affecting opportunity: Frequency of earthquake occurrence, intensity of seismic ground shaking)

101. Criteria for liquefaction potential map

Area known to have experienced liquefaction during historic earthquakes

Area containing liquefaction susceptible material that are saturated , nearly saturated or expected to become saturated

Area having sufficient existing geotechnical data indicating the soil are potentially liquefiable

Area underlain with saturated geologically young sediments (< 1000 to 15000 year old)

102. Is it possible to prepare for liquefaction ?

Possible to identify areas potentially subject to liquefaction with hazard zone map

Emphasis in terms of developing appropriate public policy or selecting mitigation technique in area of major concern

Use of hazard map by public and private owners the seriousness of expected damage and most vulnerable structure

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

103. Acknowledgements

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.

Thanks for your attention