General Principles and Design Criteria

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Slide 1

An Overview of Geotechnical Earthquake Engineering

Sudhir K Jain

Sudhir K. Jain Course on Geotech Earthq Engg / March 2013

Slide 2

Outline

Introduction to Seismic Design Principle

Dynamic Soil Properties

Site Effects

Soil Structure Interaction

Issues for Foundation Design

Liquefaction

Embankment Analysis


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Slide 3

Some Remarks on

Seismic Design Principles


Slide 4

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


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Slide 5

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.


Slide 6

Response Reduction Factor

Hence, structure is designed for seismic force much less than what is expected under strong shaking if the structure were to remain linear elastic

Earlier codes just provided the required design force

It gave no direct indication that the real force may be much larger

Now, the codes provide for realistic force for elastic structure and then divides that force by some factor.

This gives the designer a more realistic picture of the design philosophy.


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Slide 7

Increase in Permissible Stresses

Applicable for Working Stress Design

Permits the designer to increase allowable stresses in materials

For instance, 33% - 50% for load cases including seismic loads.


Slide 8

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:

Housner and Jennings, “Seismic Design Criteria”, Earthquake Engineering Research Institute, USA, 1982.


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Slide 9

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


Slide 10

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.


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Slide 11

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


Slide 12

Load Combination 0.9DL 1.5EL

Horizontal loads are reversible in direction.

In many situations, design is governed by effect of horizontal load minus effect of gravity loads.

In such situations, a load factor higher than 1.0

on gravity loads will be unconservative.

Hence, a load factor of 0.9 specified on gravity

loads.

Many designs of footings, columns, and positive steel in beams at the ends in frame structures are governed by this load combination


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Slide 13



Slide 14


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


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Slide 15

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


Slide 16

Shear Modulus …

Figure: Hysteretic stress-strain response of soil subjected to cyclic loading


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Slide 17

Dynamic Properties

Shear modulus decreases with strain increase

Damping increases with strain increase


Slide 18

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


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Slide 19

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,and Idris, 1986]

Gmax = 1000[20(N1,60)0.33] (σ’)0.5 [Seed,Wong,and Idris, 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 Sudhir K. Jain Course on Geotech Earthq Engg / March 2013

Slide 20

0

50

100

150

200

250

300

0.E+00 1.E+05 2.E+05 3.E+05 4.E+05 5.E+05 6.E+05 7.E+05 8.E+05

Small-strain Shear Modulus(G max) (kN/m2)

Dep

th (

m)

G max (Eqn 2) G max (Eqn 3) G max (Eqn 5)

Gmax (Eqn 1) G max (Eqn 4)

Eqn (5)

Eqn (4)

Eqn (1)

Eqn (2)

Eqn (3)

Soil Properties …

Small strain Shear Modulus (Gmax)

Tends to vary

significantly,

depending on

which relationship

is used


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Slide 21

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


Slide 22

Ground Motions Along Depth …

Vertical distribution PGD

Vertical distribution PGV

Vertical distribution PGA

Figure: Distribution of peak amplitude of ground motion along depth, (Kanade, 2000)


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Slide 23

Known Spectrum

Artificially generated time history [SIMQKE -1]

Back calculated time history [SHAKE 2000]

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.01 0.1 1 10Time perios (s)

Sp

ectr

al A

ccele

rati

on

(g

)

Functional - Target response spectrum Functional - Generated response spectrum

Safety - Target response spectrum Safety - Generated response spectrum



Slide 24

Artificially generated time history [SIMQKE-1]

Response time history [SHAKE 2000]



Assumed earthquake

Corresponding response

spectrum [SMSIM]

Figure: Schematic representation of procedure used for artificially generated time histories for earthquake motion



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Slide 25

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.


Slide 26

Site Effects


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Slide 27

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.


Slide 28

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


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Slide 29

Mexico Earthquake of 1985 …

PGA at SCT site about 5 times higher than that at UNAM site

Epicentral distance is same at both locations

Time (sec)

Figure from Kramer, 1996


Slide 30

Extremely soft soils in Lake Zone amplified weak long-period waves Natural period of soft clay layers happened to

be close to the dominant period of incident seismic waves

This lead to resonance-like conditions

Buildings between 7 and 18 storeys suffered extensive damage Natural period of such buildings close to the

period of seismic waves.

Mexico Earthquake of 1985 …


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Slide 31

Site Response Amplification

Depends on:

Soil properties (shear modulus, damping)

Soil depth

Contrast in soil properties

More amplification if greater contrast

Intensity of ground motion

Soil is elastic at low strains

Shear modulus decreases and damping

increases as soil strain increases

More amplification for weak motion


Slide 32

Site Response Amplification …

Figure: Relationship between maximum acceleration on rock and on soft sites (Idriss, 1990, 1991).


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Slide 33



Slide 34

Figure: Procedure for modifying ground motion parameters from a seismic hazard analysis to account for the effects of local site conditions



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Slide 35

Figure: Two hypothetical soil deposit overlying rigid bedrock: (a) site A; (b) site B. Soils are identical, except the s- wave velocity of the soil at site B is four times greater than that at site A.



Slide 36

Figure: Amplification functions for sites A and B. Note that the softer soil at site A will amplify low-frequency input motions much more strongly than will the stiffer soils of site B. At higher frequencies, the opposite behaviour would be expected.



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Slide 37

Soil Effect

Recorded earthquake motions show that response spectrum shape differs for different type of soil profile at the site

Period (sec)

Fig. from Geotechnical Earthquake Engineering, by Kramer, 1996


Slide 38

This variation in ground motion characteristic for different sites is now accounted for through different shapes of response spectrum for three types of sites.

Spect

ral Acc

ele

ration C

oeff

icie

nt

(Sa/g

)

Period (s)

Fig. from IS:1893-2002

Soil Effect …


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Slide 39

Modern Codes (e.g., IBC) classify the soil type based on weighted average (in top 30m) of:

Soil Shear Wave Velocity, or

Standard Penetration Resistance, or

Soil Undrained Shear Strength

Soil Effect …


Slide 40

Shape of Design Spectrum

The three curves in IS1893 have been drawn based on general trends of average response spectra shapes.

In recent years, the US codes (UBC, NEHRP and IBC) have provided more sophistication wherein the shape of design spectrum varies from area to area depending on the ground motion characteristics expected.


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Slide 41



Slide 42


If there is no structure, motion of the ground surface is termed as Free Field Ground Motion

Normal practice is to apply the free field motion to the structure base assuming that the base is fixed.

This is valid for structures located on rock sites.

For soft soil sites, this may not always be a good

assumption.


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Slide 43

Presence of structure modifies the free field motion since the soil and the structure interact. Hence, foundation of the structure experiences

a motion different from the free field ground motion.

The difference between the two motions is accounted for by Soil Structure Interaction (SSI)

SSI is not the same as Site Effects Site Effect refers to the fact that free field motion

at a site due to a given earthquake depends on the properties and geological features of the subsurface soils also.

Soil Structure Interaction …


Slide 44

Consideration of SSI generally

Decreases lateral seismic forces on the structure

Increases lateral displacements

Increases secondary forces associated with P-

delta effect.

For ordinary structures, one usually ignores SSI.

Soil Structure Interaction …


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Slide 45

Radiation Damping

Structure

Artificial Boundary

Analytical Region

Scattering Wave

Transmitted Wave Reflected Wave

Infinite Soil Medium

Ground Surface

Radiation of energy to infinity

Also called geometric damping or geometric

attenuation


Slide 46

Issues for Foundation Design


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Slide 47

Foundations

Lessons from Past Earthquakes:

Seismic damage, particularly to low height bridges, often caused by foundation failures.

Foundation failures could be due to:

Excessive ground deformation

Loss of stability

Bearing capacity problem

Large foundation displacements may cause:

Relative shifting of and damage to the superstructures,

Damage to the bearings

Backfills cause large forces on abutments. Sudhir K. Jain Course on Geotech Earthq Engg / March 2013

Slide 48

Wing walls may break loose from the abutments due to excessive backfill forces.

Poor soils (soft soil and a high water table) contributed to a lot of damage to bridges in the past earthquakes.

Examples:

Nigata earthquake of 1964

Alaska earthquake of 1964.

Foundations …


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Slide 49

Modes of Foundations Failure


Slide 50

Modes of Failure for Spread Footings


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Slide 51

Modes of Failure for Pile Footings


Slide 52

Shallow Foundations in Clay

Cyclic loading during earthquake

Generally, clay does not loose much strength during cyclic undrained loading

There may be some settlement, lateral movement or rotation, depending on

Factor of safety under static condition

Generally, good seismic performance due to adequate factor of safety in static loading

Concern: site effects


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Slide 53

Shallow Foundations in Dry Sand

Under cyclic shear, dry sand reduces in volume

Settlement of ground (and hence the foundation) during earthquake motion

Settlement more significant for loose sand than for dense sand

Structure: part on shallow footing and part on end bearing piles:

Part on footing may undergo settlement relative

to the other part


Slide 54

Shallow Foundation in Saturated Sand

Liquefaction is a major concern

Liquefaction

Geologically young sand

Saturated sand

Loose sand

Right particle size distribution

Liquefaction can occur at the same site in subsequent earthquakes


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Slide 55

Shallow Foundations … (Contd…)

Lateral flow (lateral spreading) of liquefied sand can occur

Liquefaction leads to surface settlement after the water pressure dissipates

Liquefaction occurs first adjacent to the spread footing and then under it

Liquefaction may occur after the earthquake motion has stopped


Slide 56

Deep Foundations in Clay

Gapping may occur near the ground surface

Pile foundation failures in buildings in the Mexico 1985 earthquake

Due to low factors of

safety

Development of a gap adjacent to a pile subject to cyclic lateral load in clay (after Swane and Poulos, 1984).


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Slide 57

Deep Foundations in Sand

Bridge performance in Alaska (1964) earthquake (Ross et al 1969) shows:

Large foundation displacements for

foundations in saturated sands

Foundations in gravel and gravelly sands

have much less damage

Foundations on rock not damaged

Foundations by piling through sands to

bedrock have minor damage


Slide 58

Foundations on Raked Piles

Particularly vulnerable to severe damage during earthquakes


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Slide 59

Liquefaction


Slide 60

Liquefaction

In case of loose or medium dense saturated soils, liquefaction may take place.

Sites vulnerable to liquefaction require

Liquefaction potential analysis.

Remedial measures to prevent liquefaction.

Else, deep piles are designed assuming that soil

layers liable to liquefy will not provide lateral

support to the pile during ground shaking.


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Slide 61

Difficult to obtain undisturbed samples

Approach based on in-situ tests preferred

SPT and CPT based procedures are popular

Simplified procedure of Seed and Idriss used with SPT values

Liquefaction Analysis


Slide 62

North Bank: Rail and Road : At Center of Embankment : Water Level 1.0 m below GL

amax/g 0.60 h w 1.00 g w 0.98 h emb 21.00 g emb 1.85 g sub 0.85 M 7.00

D Rng Depth %Fine g sat s v u 0 s v ' Avg.N C N C 60 (N l ) 60 r d K m K a K s CSR eq CSR 7.5 CSR L FS liq CSR eql % e D V

0.0-1.5 0.75 8.20 1.85 40.24 0.00 40.24 6.22 2.00 1.00 12.44 0.67 1.09 1.00 0.66 0.26 0.15 0.11 0.42 0.37 1.95 0.03

1.5-3.0 2.25 8.00 1.85 43.01 1.23 41.79 8.80 2.00 1.00 17.60 0.65 1.09 1.00 0.64 0.26 0.24 0.17 0.64 0.38 1.95 0.03

3.0-4.5 3.75 8.30 1.85 45.79 2.70 43.09 12.08 1.50 1.00 18.16 0.63 1.09 1.00 0.63 0.26 0.21 0.15 0.56 0.38 1.93 0.03

4.5-6.0 5.25 8.50 1.85 48.56 4.17 44.40 16.30 1.31 1.00 21.43 0.61 1.09 1.00 0.62 0.26 0.26 0.18 0.68 0.38 1.50 0.02

6.0-7.5 6.75 6.15 1.85 51.34 5.64 45.70 20.68 1.18 1.00 24.45 0.58 1.09 1.00 0.62 0.26 0.31 0.21 0.81 0.38 1.18 0.02

7.5-9.0 8.25 8.00 1.85 54.11 7.11 47.01 24.47 1.08 1.00 26.53 0.56 1.09 1.00 0.60 0.25 0.35 0.23 0.92 0.39 1.00 0.02

9.0-10.5 9.75 6.40 1.85 56.89 8.58 48.31 27.72 1.01 1.00 27.90 0.54 1.09 1.00 0.59 0.25 0.41 0.27 1.07 0.38 0.88 0.01

10.5-12.0 11.25 6.40 1.85 59.66 10.05 49.62 24.75 0.94 1.00 23.35 0.52 1.09 1.00 0.58 0.24 0.30 0.19 0.78 0.38 1.36 0.02

12.0-13.5 12.75 6.40 1.85 62.44 11.52 50.92 26.00 0.89 1.00 23.17 0.49 1.09 1.00 0.58 0.24 0.30 0.19 0.81 0.37 1.34 0.02

13.5-15.0 14.25 6.40 1.85 65.21 12.99 52.23 30.63 0.85 1.00 25.92 0.47 1.09 1.00 0.57 0.23 0.31 0.19 0.84 0.37 1.05 0.02

15.0-16.5 15.75 6.40 1.85 67.99 14.46 53.53 33.25 0.81 1.00 26.86 0.45 1.09 1.00 0.57 0.22 0.35 0.22 0.97 0.36 1.00 0.02

16.5-18.0 17.25 7.33 1.85 70.76 15.93 54.84 36.13 0.77 1.00 27.97 0.43 1.09 1.00 0.56 0.21 0.36 0.22 1.02 0.35 0.50 0.01

18.-19.5 18.75 7.00 1.85 73.54 17.40 56.14 39.00 0.74 1.00 29.03 0.40 1.09 1.00 0.55 0.21 0.45 0.27 1.31 - - -

19.5-21 20.25 7.00 1.85 76.31 18.87 57.45 44.63 0.72 1.00 32.04 0.38 1.09 1.00 0.55 0.20 (N l ) 60 Greater than 30 hence

soil is Non - Liquefiable

TOTAL D 0.23

rd is calculated at the center of depth range below top of embankment Units : Tons & Meters

Liquefaction Analysis (Example)


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Slide 63

S.No. Details Depth of Liquefaction

Soil Settlement

Functional Evaluation Motion (0.1g)

1. North Embankment (Rail & Road) - -

2. South Rail Embankment - -

3. South Road Embankment - -

Safety Evaluation Motion (0.6g)

4. North Embankment (Rail & Road) 18.75 0.26

5. South Rail Embankment 14.25 0.19

6. South Road Embankment 14.25 0.18

Liquefaction Analysis (Example) …


Slide 64

Embankment Analysis


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Slide 65

Slope failure due to

Inertial loading, and/or

Softening of material strength or liquefaction

Fault displacement under the foundation

Not being addressed here

Issues for Embankments


Slide 66

Pseudo-static slope stability analysis

Factor of safety concept

Permanent deformation analysis as per Newmark’s sliding block approach

Two Methods


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Slide 67

Complex ground shaking replaced by a single constant unidirectional pseudo-static acceleration

Ensure adequate factor of safety against sliding

How to choose seismic coefficient and factor of

safety?

Pseudo-Static Analysis


Slide 68

Newmark’s Sliding Block Model

Developed originally for evaluation of seismic slope stability

Motivated by concerns about realistic ground motions that are much higher than the traditional design based on pseudo-static stability analysis

If FOSslid < 1.0

What will happen? Will the structure collapse?

Not, if the permanent deformation is within an

acceptable limit


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Slide 69

Time (s)

Figure: Development of permanent displacement for actual earthquake ground motion

W

T N

θ

Newmark’s Sliding Block Model …


Slide 70

Deformations accumulate when the rigid body acceleration exceeds the yield acceleration.

Newmark’s Sliding Block Model …


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Slide 71

Embankment Analysis (Example)


Slide 72

Assuming No Liquefaction

Static load (only self weight): FOS=1.99

Pseudo-static: Yield coeff (FOS=1.0) is 0.275g

FOS Approach by:

Terzaghi (1950): Yield coeff. should be >0.20g

Mercuson (1981): Yield coeff. > 0.2g-0.3g

Hynes and Franklin (1984): Yield coeff of 0.1g

will give permanent deformation less than 1m.

Embankment Analysis (Example) …


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Slide 73

Permanent Deformation by Newmark’s Sliding Block Concept

Makdisi and Seed (1978) approach: 5 -15 mm

Ambraseys and Menu (1988): 39 mm

Yegian et al. (1991): 30mm

Permanent deformation of about 40mm quite acceptable.



Slide 74

Liquefied soil layers may not transmit significant amount of shear waves.

Will the embankment be stable under its own weight?

Liquefied soil layers will loose considerable amount of strength.

Liquefaction of Sub-Soil


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Slide 75

Residual strength of liquefiable soil strata considered as per Seed and Harder (1990)

FOS against self weight:

1.39 for North Embankment

1.18 for South Rail Embankment

1.31 for South Road Embankment

Embankment will be stable due to its own weight after foundation soils have liquefied



Slide 76

Figure: Relationship between Corrected ‘Clean Sand’ Blow Count (N1)60

and Undrained Residual Strength (Sr) (Seed and Harder, 1990)

Liquefaction of Sub-Soil …


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Slide 77

Conservative Assumptions:

Liquefaction occurs early during shaking

Base of embankment still sustains PGA of

0.60g

Deformations computed for 0.60g but with residual strength of liquefiable soils



Slide 78

Details North Bank

Embankment

South Bank Embankment

Rail Road

FOS for Static Case (No earthquake)

1.39 1.18 1.31

Yield Acceleration (for FOS = 1.0)

0.1g 0.048g 0.08g

Permanent Displacement Considering PGA = 0.6g (in mm)

Ambraseys and Menu (1998) 353 1310 497

Yegian et al. (1991) 317 1190 502

Makdisi and Seed (1978) 50-400 200-1000 80-700


Analysis Using Residual Strength


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Slide 79

Deformation of 500mm: acceptable.

Deformation of 1300mm: on the higher side; but can be handled as an emergency measure in a relatively short time

These deformations are for maximum embankment height and with conservative assumptions

Remedial measures not recommended.



Slide 80 Sudhir K. Jain Course on Geotech Earthq Engg / March 2013

Documents

General Principles and Design Criteria