Liquefation

CONTENTS

1.1 Introduction 1

1.2 Soil Liquefaction 1

1.3 Failure Mechanism causing Liquefaction 4

1.4 Factors Affecting Liquefaction 6

1.5 Evaluation of Liquefaction Potential by cyclic shear stress 8

1.6 Evaluation of Liquefaction Potential by Standard 10

Penetration Resistance

1.7 Measures to reduce Liquefaction of soils 14

1.8 Conclusion 15

References

1.1 INTRODUCTION:-

General :-

During the recent Bhuj earthquake on 26 January 2001, a number of medium to high rise

residential buildings collapsed in Ahmedabad city, which is located about 300 km away

from the epicenter. The city is founded over thick recent unconsolidated sediments. The

severe damages in this location are attributed to the response of such unconsolidated

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sediments to violent shaking. This catastrophic earthquake has provided a serious

reminder that liquefaction of sandy soils and sands with non-plastic fines as a result of

earthquake ground shaking poses a major threat to the safety of civil engineering

structures. Investigations to evaluate the liquefaction potential of soil deposits during

earthquakes have been the subject of attention in recent years.

Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by

earthquake shaking or other rapid loading. Liquefaction and related phenomena have

been responsible for tremendous amounts of damage in historical earthquakes around the

world.

Liquefaction occurs in saturated soils, that is, soils in which the space between individual

particles is completely filled with water. This water exerts a pressure on the soil particles

that influences how tightly the particles themselves are pressed together. Prior to an

earthquake, the water pressure is relatively low. However, earthquake shaking can cause

the water pressure to increase to the point where the soil particles can readily move with

respect to each other.

1.2 Soil Liquefaction:-

During heavy ground shaking by earthquakes, liquefaction occurs when the pressure

exerted by the water present in saturated soil becomes so great that the soil particles

become ‘suspended’ in the water. A soil deposit that is liquefied behaves like the better-

known phenomena: quicksand. The most commonly used terms in liquefaction include

a) Saturated soils: soils in which the space (voids) between the soil particles is

completely filled with water.

b) Pore water pressure: pressure exerted on particles of soil by the water in the

voids. Most of the time this pressure is relatively low (hydrostatic) and results

in an equilibrium condition of effective stress state. However, there are some

circumstances in which rapidly increased stresses can cause the pore water

pressure to increase.

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In more technical terms, liquefaction is imminent when the porewater pressure (u) equals

the total overburden stress (VO). This creates an effective stress state equal to zero

VO' = [VO – u] = 0

Due to the forces exerted by gravity, soil particles naturally rest upon each other and,

depending on the properties of the soil, form sort of grid that is relatively stable (or can

be made so by compaction or other construction practices). During liquefaction the water

pressures become high enough to counteract the gravitational pull on the soil particles

and effectively ‘float’, or suspend, the particles. The soil particles can then move freely

with respect to each other. Since the soil is no longer behaving as an inactive grid of

particles, the strength and stiffness of a liquefied soil is significantly decreased, often

resulting in a variety of structural failures. (Plate 1 shows overturned apartment buildings

in Niigata, Japan due to liquefaction in 1964. Plate 2 shows an example of lateral spread

failure due an earthquake in Kobe, Japan in 1995.)

Typically when liquefaction is discussed due to a seismic event, addressing “cyclic

liquefaction” is important, this occurs when repeated cycles of shearing generate an

accumulation of pore water pressures. However, if the soil is very loose sand, “flow

liquefaction” can occur from first time loading during site development. Also, “quasi

liquefaction” describes a state of partial liquefaction of a soil deposit that does not

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Plate 1: Nigata, Japan, 1964 Plate 2: Kobe, Japan, 1995

propagate fully throughout the site; however the subsurface liquefaction response still

negatively affects structures at the surface.

If liquefaction occurs beneath a surface that has hardened as a result of compaction,

weathering, or some other process; ‘sand boiling’ can occur. The water pressures build

below the surface to the point that the water breaks through the solid surface much like a

bubble in boiling water.

On the US West Coast, these sand boils are normally

about one to three feet in diameter (0.3 to 1 meter), plate

3 shows such a phenomena. In the New Madrid Seismic

Zone, the level of sand liquefaction was so extensive that

the sand boils in this region are called “sand blows” since

they generally are 10 to 100 feet diameter (3 to 30

meters), plate 4.

Figures 1 and

2 show a typical view of soil

grains in an unexcited

saturated

deposit. The blue column on the right indicates the

magnitude of pore water pressure present. The arrows in

Figure 2 indicate the forces created by the interactions of

the soil grains. Figure 3 shows elevated water pressure

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Plate 4: New Madrid Seismic Zone

Plate3: Olympia, Washington, 2001

Figure 1

created by additional

loading (as from a seismic

event. The increased water

pressure acts to ‘float’ the grains and thereby decreases the

interaction between grains,

thus causing the

characteristic properties of

liquefaction.

1.3 Failure Mechanisms causing Liquefaction:-

The term liquefaction has actually been used to describe a number of related phenomena.

Because the phenomena can have similar effects, it can be difficult to distinguish between

them. The mechanisms causing them, however, are different. These phenomena can be

divided into two main categories: flow liquefaction and cyclic mobility.

Flow Liquefaction:-

Flow liquefaction is a phenomenon in which the static equilibrium is destroyed by static

or dynamic loads in a soil deposit with low residual strength. Residual strength is the

strength of a liquefied soil. New buildings on a slope that exert additional forces on the

soil beneath the foundations can apply static loading, for example. Earthquakes, blasting,

and pile driving are all example of dynamic loads that could trigger flow liquefaction.

Once triggered, the strength of a soil susceptible to flow liquefaction is no longer

sufficient to withstand the static stresses that were acting on the soil before the

disturbance.

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Figure 2

Figure 3

Cyclic Mobility:-

Cyclic

mobility is a

liquefaction

phenomenon,

triggered by

cyclic

loading, occurring in soil deposits with static shear

stresses

lower than the soil

strength.

Deformations due to cyclic mobility develop

incrementally

because of static and

dynamic stresses that exist during an earthquake. Lateral spreading, a common result of

cyclic mobility, can occur on gently sloping and on flat ground close to rivers and lakes.

The 1976 Guatemala earthquake caused lateral spreading along the Motagua River.

Observe the cracks parallel to the river in plate 5.

On level ground, the high pore water pressure caused by

liquefaction can cause pore water to flow rapidly to the

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Strain softening behaviour Strain hardening behaviour

Flow liquefaction Cyclic softening

Monotonic/cyclic trigger Size and duration of cyclic loading

Gravitational stresses > undrained shear strength

Shear stress reversal No shear stress reversal

Contained deformation

Uncontained deformation

Cyclic liquefaction

Cyclic mobility

Potential for progressive failure

Large deformations

Small deformations

Deformation can continue after the trigger event

Deformations essentially stop after cyclic loading

Flow chart for liquefaction (Robertson, 1994)

Plate 6: El Centro earthquake

Material characterization

Plate 5: Motagua River

ground surface. This flow can occur both during and after an earthquake. If the flowing

pore water rises quickly enough, it can carry sand particles through cracks up to the

surface, where they are deposited in the form of sand volcanoes or sand boils. These

features can often be observed at sites that have been affected by liquefaction, such as in

the field along Hwy 98 during the 1979 El Centro earthquake shown in plate 6.

1.4 Factors Affecting Liquefaction:-

1. Soil type:-

Liquefaction occurs in cohesionless soils as they lose their strength completely under

vibration due to the development of pore pressures which in turn reduce the effective

stress to zero. Liquefaction does not occur in case of cohesive soils. Only highly sensitive

clays may loose their strength substantially under vibration.

2. Grain Size and Its Distribution:-

Fine and uniform sands are more prone to liquefaction than coarser ones. Since the

permeability of coarse sand is greater than fine sand, the pore pressure developed during

vibration can dissipate faster.

3. Initial Relative Density:-

It is one of the most important factors controlling liquefaction. Both pore pressures and

settlement are considerably reduced during vibrations with increase in initial relative

density and hence chances of liquefaction and excessive settlement reduce with increased

relative density.

4. Vibration Characteristics:-

Out of the four parameters of dynamic load namely (i) frequency ;( ii) amplitude ;( iii)

acceleration; and (iv) velocity; frequency and acceleration are more important. Frequency

of the dynamic load plays vital role only if it is close to the natural frequency of the

system. Further the liquefaction depends on the type of the dynamic load i.e. whether it is

a transient load or the load causing steady vibrations.

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Whole stratum gets liquefied at the same time under transient loading, while it may

proceed from top to lower layers under steady state vibrations (Florin and Ivanov, 1961).

For a given acceleration, liquefaction occurs only after a certain number of cycles

imparted to the deposit. Further, horizontal vibrations have more severe effect than

vertical vibrations. Multi directional shaking is more severe than one directional loading

(Seed et al., 1977), as the pore water pressure build up is much faster and the stress ratio

required is about 10 percent less than that required for unidirectional shaking.

5. Location of Drainage and Extent of Deposit:-

Sands are more pervious than fine-grained soil. However, if an impervious deposit has

large dimensions, the drainage path increases and the deposit may behave as undrained,

thereby, increasing the chances of liquefaction of such a deposit. The drainage path is

reduced by the introduction of drains made out of highly pervious material.

6. Surcharge Load:-

If the surcharge load, i.e., the initial effective stress is large, then transfer of stress from

soil grains to pore water will require higher intensity vibrations or vibrations for longer

duration. If the initial stress condition is not isotropic as in field, then stress condition

causing liquefaction depends upon Ko (coefficient of earth pressure at rest) and for Ko >5,

the stress condition required to cause liquefaction increases by at least 50%.

7. Method of Soil Formation:-

Sands unlike clays do not exhibit a characteristics structure. But recent investigations

show that liquefaction characteristics of saturated sands under cyclic loading are

significantly influenced by method of sample preparation and by soil structure.

8. Period under Sustained Load:-

Age of sand deposit may influence its liquefaction characteristics. A 75% increase in

liquefaction resistance has been reported on liquefaction of an undisturbed sand

compared to freshly prepared sample which may due to some form of cementation or

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welding at contact points of sand particles and associated with secondary compression of

soil.

9. Previous Strain History:-

Studies on liquefaction characteristics of freshly deposited sand and of similar deposit

previously subjected to some strain history reveal, that although the prior strain history

caused no significant change in the density of the sand, it increased the stress that causes

liquefaction by factor of 1.5.

10. Trapped Air:-

If air is trapped in saturated soil and pore pressure develops, a part of it is dissipated due

to the compression of air. Hence, trapped air helps to reduce the possibility to

liquefaction.

1.5 Evaluation of Liquefaction Potential by Cyclic Shear Stress:-

Evaluation of the potential for liquefaction to occur is accomplished by comparing

equivalent measures of earthquake loading and liquefaction resistance. The most

common approach for characterization of earthquake loading is through the use of cyclic

shear stresses. By normalizing the cyclic shear stress amplitude by the initial effective

vertical stress, a cyclic stress ratio (CSR) can represent the level of loading induced at

different depths in a soil profile by an earthquake. There are different procedures for

evaluating the cyclic shear stresses - site response analyses may be performed or a

"simplified" approach may be used to estimate CSR as a function of peak ground surface

acceleration amplitude.

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Fig 4: CSR versus N or qc

Liquefaction resistance is most commonly characterized on the basis of observed field

performance. Detailed investigation of actual earthquake case histories has allowed

determination of the combinations of insitu properties (usually SPT or CPT resistance)

and CSR for each case history. By plotting the CSR- (N1) 60 (or CSR-qc) pairs for cases

in which liquefaction was and was not been observed, a curve that bounds the conditions

at which liquefaction has historically been observed can be drawn. This curve, when

interpreted as the maximum CSR for which liquefaction of a soil with a given penetration

resistance can resist liquefaction, can be thought of as a curve of cyclic resistance ratio

(CRR). Then, the potential for liquefaction can be evaluated by comparing the earthquake

loading (CSR) with the liquefaction resistance (CRR) - this is usually expressed as a

factor of safety against liquefaction,

FS = CRR / CSR

A factor of safety greater than one indicates that the liquefaction resistance exceeds the

earthquake loading, and therefore liquefaction would not be expected.

CSR, is Estimated by SEED and IDRISS (1971) based on the maximum ground surface

acceleration (amax) at the site

CSR= τav/σ′vo = 0.65(MWF) (amax/g) (σvo / σ′vo)*rd

Where:

τav = average cyclic shear stress

MWF = Magnitude Weighting Factor = (M)2.56 /173

M = earthquake magnitude, commonly M= 7.5

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amax = maximum horizontal acceleration at ground surface

g = acceleration due gravity = 9.81m/s2

σvo = total vertical overburden stress

σ′vo = effective vertical overburden stress

z = depth in meters (for z>25m)

rd = stress reduction factor, typically (1-0.015z)

CRR can be evaluated by Laboratory and field tests such as:

1. Cyclic Triaxial test

2. Hollow cylindrical torsion test

3. cyclic simple shear test

4. Standard penetration test

5. Cone penetration test (CPT)

6. Piezo Vibrocone test

7. Siesmic cone penetration test(SCPT)

But most commonly SPT and CPT test are conducted, as they are popular.

1.6 Evaluation of Liquefaction Potential By Using Standard Penetration

Resistance:-

The standard penetration test is most commonly used insitu test in a borehole to have

fairly good estimation of relative density of cohesionless soil. Since liquefaction

primarily depends on the initial relative density of saturated sand, many researchers have

made the attempt to develop correlations between liquefaction potential and standard

penetration resistance. IS: 2131-1981 gives the standard penetration test. SPT values (N)

obtained in the field for sand have to be corrected for accounting the effect of overburden

pressure as below:

N1 = CN * N

Where, N1 = Corrected value of standard penetration resistance

CN = Correction factor

The correlation between N1 values and relative density of granular soils suggested

by Terzaghi and Peck.

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After the occurrence of Niigata earthquake, Kishida (1966), Kuizumi (1966), and

Ohasaki (1966) studied the areas of Niigata where the liquefaction had not occurred and

developed criteria for differentiating between liquefaction and nonliquefaction conditions

in that city, based on N-values of the sand deposits (Seed, 1979). The results of these

studies for Niigata are shown in the Fig 5. Ohasaki (1970) gave a useful rule of thumb

that says liquefaction is not a problem if the blow count from a standard penetration test

exceeds twice the depth in meters.

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Standard penetration value-N

Fig 5: Relationship between the possibility of liquefaction and N values at various depths. (After Kishida, 1969).

1.7 MEASURES TO REDUCE LIQUEFACTION OF SOILS:-

General:-

There are several ways in which risk and severity of damage as a result of soil

liquefaction can be reduced. The first and most obvious is, to avoid planning

development on liquefaction susceptible soils. Besides in-situ testing, vulnerable sites

can also be identified by researching any prior events at the site. Maps showing sites of

prior liquefaction can be located from many government and research entities.

If it necessary to construct on liquefaction

susceptible soils, one can modify the design of a

structure in several ways to make the structure

more resistant damage potential from liquefaction.

A structure that incorporates ductility, has

supports that are adjustable to accommodate

differential settlement, possesses the ability to

accommodate large deformations, and has a

foundation design that can span ‘soft’ spots, can all decrease the amount of damage

incurred in the case of a liquefaction event.

Avoid Liquefaction Susceptible Soils:-

The first possibility is to avoid construction on liquefaction susceptible soils. There are

various criteria to determine the liquefaction susceptibility of a soil. By characterizing the

soil at a particular building site according to these criteria one can decide if the site is

susceptible to liquefaction and therefore unsuitable for the desired structure.

Build Liquefaction Resistant Structures:-

If it is necessary to construct on liquefaction susceptible soil because of space

restrictions, favorable location, or other reasons, it may be possible to make the structure

liquefaction resistant by designing the foundation elements to resist the effects of

liquefaction.

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Example of foundation design that spans over a soft spot

Improve the Soil:-

The third option involves mitigation of the liquefaction hazards by improving the

strength, density, and/or drainage characteristics of the soil. This can be done using a

variety of soil improvement techniques.

Soil improvement techniques:-

The main goal of most soil improvement techniques used for reducing liquefaction

hazards is to avoid large increases in pore water pressure during earthquake shaking. This

can be achieved by densification of the soil and/or improvement of its drainage capacity.

Vibroflotation:-

Vibroflotation involves the use of a vibrating probe that

can penetrate granular soil to depths of over 100 feet. The

vibrations of the probe cause the grain structure to

collapse thereby densifying the soil surrounding the

probe. To treat an area of potentially liquefiable soil, the

vibroflot is raised and lowered in a grid pattern. Vibro

Replacement is a combination of vibroflotation with a

gravel backfill resulting in stone columns, which not only

increases the amount of densification, but also provides a

degree of reinforcement and a potentially effective means of drainage.

Dynamic Compaction:-

Densification by dynamic compaction is performed by

dropping a heavy weight of steel or concrete in a grid

pattern from heights of 30 to 100 ft. It provides an

economical way of improving soil for mitigation of

liquefaction hazards. Local liquefaction can be initiated

beneath the drop point making it easier for the sand

grains to densify. When the excess pore water pressure from the dynamic loading

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dissipates, additional densification occurs. As illustrated in the photograph, however, the

process is somewhat invasive; the surface of the soil may require shallow compaction

with possible addition of granular fill following dynamic compaction.

Stone Columns:-

Stone columns are columns of gravel constructed in the ground. Stone columns can be

constructed by the vibroflotation method. They can also be installed in other ways, for

example, with help of a steel casing and a drop hammer as in the Franki Method. In this

approach the steel casing is driven in to the soil and gravel is filled in from the top and

tamped with a drop hammer as the steel casing is successively withdrawn.

Compaction Piles:-

Installing compaction piles is a very effective way of improving soil. Compaction piles

are usually made of prestressed concrete or timber. Installation of compaction piles both

densifies and reinforces the soil. The piles are generally installed in a grid pattern and are

generally driven to depth of up to 60 ft.

Compaction Grouting:-

Compaction grouting is a technique whereby a slow-

flowing water/sand/cement mix is injected under pressure

into a granular soil. The grout forms a bulb that displaces

and hence densifies, the surrounding soil. Compaction

grouting is a good option if the foundation of an existing

building requires improvement, since it is possible to inject the

grout from the side or at an inclined angle to reach beneath the

building.

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1.8 Conclusions:-

i. Because liquefaction only occurs in saturated soil, its effects are most commonly

observed in low-lying areas near bodies of water such as rivers, lakes, bays, and

oceans.

ii. Cyclic shear stress to initiate liquefaction was higher than the cyclic shear stress

induced by the earthquake.

iii. Sands were considered to be the only type of soil susceptible to liquefaction, but

liquefaction was also observed in gravel and silt.

iv. Soil of medium to fine texture that is clay, silty clay, loam, and gravelly soils with

well to moderate drainage has no liquefaction vulnerability.

v. Soil of medium coarse texture that is very fine sandy loam, sandy loam with well to

moderate drainage has 50% liquefaction vulnerability.

vi. Soil of coarse texture that is sand, and loamy sand with well to moderate drainage

has 70% liquefaction vulnerability and imperfect drainage has 90% liquefaction

vulnerability.

vii. The SPT- and the CPT-based liquefaction assessment charts are the preferred means

of evaluating liquefaction potential .

viii. They are most reliable because they are supported by large databases on the

occurrence of liquefaction .

ix. The SPT test provides soil samples for identification of soil type and many

empirical design procedures are based on the SPT, N.

x. The CPT provides the best picture of soil stratification and is the most reliable

penetration test. Many design procedures are also based on CPT data .

xi. If the CPT is run with a seismic cone, the shear wave velocities can be measured at

the same time. The shear moduli can be readily obtained from the velocity data and

can be used as input into dynamic and static analyses.

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REFERENCES

1. Kramer Steven., Text Book of Geotechnical Earthquake

Engineering.

2. Dr. Swami Saran., Text Book of Soil Dynamics And Machine Foundation.

3. Hans F.Winterkorn and Hsai-Yang Fang., ‘Foundation Engineering

Handbook’.

4. T.Lunne, P.K.Robertson and J.J.M.Powell., ‘Cone Penetration Testing in

Geotechnical Practice’.

5. T.G.Sitharam, L.GovindaRaju and A. Sridharan (2004).,‘Dynamic properties

and liquefaction potential of soils’, Special Section:Geotechnics and

Earthquake Hazards, Current Science, Vol.87,No.10,25 November 2004.

6. Alisha Kaplan (2004), ‘Soil Liquefaction’ Undergraduate Research, Mid-

America Earthquake Center and Georgia Institute of Technology, May 2004.

7. www.google.com .

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