effects of liquefaction on structures

Effects of Liquefaction on Effects of Liquefaction on Structures Structures

&& Its Remedial Measures Its Remedial Measures

ByBy

Tushar Ghosh Tushar Ghosh Final YearFinal Year

K.I.T.S, RamtekK.I.T.S, Ramtek

Index:Index:

IntroductionIntroduction Liquefaction-Related Phenomenon Liquefaction-Related Phenomenon Effects of LiquefactionEffects of Liquefaction Remedial MeasuresRemedial Measures ConclusionConclusion ReferenceReference

Introduction:Introduction:

During an Earthquake, large scale During an Earthquake, large scale devastation occurs due to failure of devastation occurs due to failure of buildings, dams and other structures. buildings, dams and other structures. There are various factors resulting to There are various factors resulting to this failure. In this paper we study one this failure. In this paper we study one such Geotechnical factor causing large such Geotechnical factor causing large scale damages. We study a particular scale damages. We study a particular phenomenon called phenomenon called LiquefactionLiquefaction. . Liquefaction is caused due to Liquefaction is caused due to Earthquake. Earthquake.

Introduction:Introduction:

This phenomenon was very little known This phenomenon was very little known until it drew the attention of Geotechnical until it drew the attention of Geotechnical Engineers in 1964 when a devastating Engineers in 1964 when a devastating earthquake occurred in Alaska followed by earthquake occurred in Alaska followed by Niigata earthquake in Japan which caused Niigata earthquake in Japan which caused huge scale damages due to Liquefaction huge scale damages due to Liquefaction including slope failures, bridge and building including slope failures, bridge and building foundation failures.foundation failures.

Liquefaction is a phenomenon by which Liquefaction is a phenomenon by which loose saturated sand becomes liquid when loose saturated sand becomes liquid when rapid loading occurs under undrained rapid loading occurs under undrained conditions.conditions.

Index:Index:


Liquefaction-Related PhenomenonLiquefaction-Related Phenomenon

Solids

Pore Water

During Earthquake, the saturated sand is vibrated as a result of this,

it tends to densify. As the particles tend to come close to each other, the excess pore water pressure increases and hence effective stress decreases.

The Mohr-Coulomb strength equation is The Mohr-Coulomb strength equation is given by,given by,

ττ = c + = c + σσ’tan’tanΦΦWhere,Where,

ττ is the shear strength is the shear strength c is cohesionc is cohesion σσ’ is effective normal stress’ is effective normal stress ΦΦ is the angle of internal friction is the angle of internal friction

As the sand is cohesionless, c = 0As the sand is cohesionless, c = 0But, But, σσ’ = ’ = σσ – u, where – u, where σσ is total normal stress that depends on is total normal stress that depends on unit weight and u is pore water pressure.unit weight and u is pore water pressure.

In case of loose sand, σ itself is very small and since In case of loose sand, σ itself is very small and since there is a possibility of the excess pore water pressure there is a possibility of the excess pore water pressure developed during Earthquake being equal the σ, the effective developed during Earthquake being equal the σ, the effective stress σ’becomes zero. As a result of this the shear strength, stress σ’becomes zero. As a result of this the shear strength, τ = 0 and the sand becomes a liquid.τ = 0 and the sand becomes a liquid.

Index:Index:


Effects of LiquefactionEffects of Liquefaction

The effects of Liquefaction is The effects of Liquefaction is divided into 3-parts:divided into 3-parts:

i.i. Alteration of Ground MotionAlteration of Ground Motion

ii.ii. Development of Sand BoilsDevelopment of Sand Boils

iii.iii. SettlementSettlement


Nonliquified

Nonliquified

Liquefied

Pile

Potential Effects of Subsurface Liquefaction on Pile.

The development of The development of positive excess pore water positive excess pore water pressures causes soil pressures causes soil stiffness to decrease stiffness to decrease during an Earthquake during an Earthquake resulting into large resulting into large displacement. These displacement. These displacement may affect displacement may affect the buried structures, the buried structures, utilities and structures utilities and structures supported on pile supported on pile foundations that extend foundations that extend through liquefied soils.through liquefied soils.


Before EarthquakeBefore Earthquake After EarthquakeAfter Earthquake

G.LSand Boils

The surface soils are often broken into blocks separated by fissures that can open and close during the Earthquake.

ii.ii. Development of Sand BoilsDevelopment of Sand Boils

Liquefaction is often accompanied by Liquefaction is often accompanied by development of sand boils. During development of sand boils. During and following Earthquake shaking, and following Earthquake shaking, seismically induced excess pressure seismically induced excess pressure are dissipated predominantly by the are dissipated predominantly by the upward flow of pore water. The upward flow of pore water. The upward pore water flow carries the upward pore water flow carries the solid particles and ejects at the solid particles and ejects at the ground surface to form sand boils.ground surface to form sand boils.

iii.iii. SettlementSettlement

The tendency of sand to densify when The tendency of sand to densify when subjected to earthquake shaking is well subjected to earthquake shaking is well documented. Subsurface densification documented. Subsurface densification is manifested at ground surface in the is manifested at ground surface in the form of settlement. This type of form of settlement. This type of settlement causes distress to settlement causes distress to structures supported on shallow structures supported on shallow foundations and damage to utilities foundations and damage to utilities that support the pile supported that support the pile supported structures.structures.

Index:Index:


Remedial MeasuresRemedial Measures

There are basically three possibilities to There are basically three possibilities to reduce Liquefaction hazards when reduce Liquefaction hazards when designing and constructing new buildings designing and constructing new buildings or other structures as bridges, tunnels or other structures as bridges, tunnels and roads.and roads.

These are as follows:These are as follows:

1)1) Avoid liquefaction susceptible soilAvoid liquefaction susceptible soil

2)2) Build liquefaction resistant structuresBuild liquefaction resistant structures

3)3) Improvement of SoilImprovement of Soil

Avoid Liquefaction Susceptible Avoid Liquefaction Susceptible SoilsSoils

The criteria by which liquefaction The criteria by which liquefaction susceptibility of the soil is judged susceptibility of the soil is judged include:include:

1)1) Historic CriteriaHistoric Criteria

2)2) Geologic CriteriaGeologic Criteria

3)3) Compositional CriteriaCompositional Criteria

4)4) State CriteriaState Criteria

Liquefaction Resistant Liquefaction Resistant Structures:Structures:

There are basically two aspects to There are basically two aspects to construct liquefaction resistant construct liquefaction resistant structure. These are:structure. These are:

1)1)Shallow foundations aspectsShallow foundations aspects

2)2)Deep foundation aspectsDeep foundation aspects

1)1) Shallow Foundation AspectsShallow Foundation Aspects

A stiff foundation mat (below) is a good type of A stiff foundation mat (below) is a good type of shallow foundation, which can locally transfer shallow foundation, which can locally transfer loads from locally liquefied zones to adjacent loads from locally liquefied zones to adjacent stronger ground.stronger ground.

2)2) Deep Foundation AspectsDeep Foundation Aspects

Liquefaction can cause Liquefaction can cause large lateral loads on pile large lateral loads on pile foundations. Piles driven foundations. Piles driven through a weak, potentially through a weak, potentially liquefiable, soil layer to a liquefiable, soil layer to a stronger layer not only have stronger layer not only have to carry vertical loads from to carry vertical loads from the superstructure, but the superstructure, but must also be able to resist must also be able to resist horizontal loads and horizontal loads and bending moments induced bending moments induced by lateral movements if the by lateral movements if the weak layer liquefies. weak layer liquefies. Sufficient resistance can be Sufficient resistance can be achieved by using piles of achieved by using piles of larger dimensions and/or larger dimensions and/or more reinforcement.more reinforcement.

3)3) Improvement of SoilImprovement of Soil

The main goal of most soil improvement The main goal of most soil improvement techniques used for reducing techniques used for reducing liquefaction hazards is to avoid large liquefaction hazards is to avoid large increases in pore water pressure during increases in pore water pressure during earthquakes. This can be achieved in earthquakes. This can be achieved in the following ways:the following ways:

I.I. VibrofloatationVibrofloatationII.II. Dynamic CompactionDynamic CompactionIII.III. Compaction PilesCompaction PilesIV.IV. Compaction GroutingCompaction Grouting

I.I. Vibrofloatation Vibrofloatation

Vibrofloatation involves the Vibrofloatation involves the use of a vibrating probe use of a vibrating probe that can penetrate that can penetrate granular soil to depths of granular soil to depths of over 100 feet. The over 100 feet. The vibrations of the probe vibrations of the probe cause the grain structure cause the grain structure to collapse thereby to collapse thereby densifying the soil densifying the soil surrounding the probe. To surrounding the probe. To treat an area of potentially treat an area of potentially liquefiable soil, the liquefiable soil, the vibrofloat is raised and vibrofloat is raised and lowered in a grid pattern.lowered in a grid pattern.

II.II. Dynamic CompactionDynamic Compaction

Densification by Densification by dynamic compaction is dynamic compaction is performed by dropping performed by dropping heavy weight of steel heavy weight of steel or concrete in a grid or concrete in a grid pattern from heights of pattern from heights of 30 to 100 ft. it provides 30 to 100 ft. it provides an economical way of an economical way of improving soil for improving soil for mitigation of mitigation of liquefaction hazards.liquefaction hazards.

III.III. Compaction PilesCompaction Piles

Installing compaction piles is a very Installing compaction piles is a very effective way of improving soil. effective way of improving soil. Compaction piles are usually made of Compaction piles are usually made of prestressed concrete or timber. prestressed concrete or timber. Installation of compaction piles both Installation of compaction piles both densifies and reinforces the soil. The densifies and reinforces the soil. The piles are generally installed in a grid piles are generally installed in a grid pattern and are generally driven to pattern and are generally driven to depth of up to 60ft.depth of up to 60ft.

IV.IV. Compaction GroutingCompaction Grouting

Compaction grouting is a Compaction grouting is a technique whereby a slow technique whereby a slow flowing water/sand/cement flowing water/sand/cement mix is injected under mix is injected under pressure into granular soil. pressure into granular soil. The grout forms a bulb The grout forms a bulb that displaces and hence that displaces and hence densifies the surrounding densifies the surrounding soil. It is a good option if soil. It is a good option if the foundation of an the foundation of an existing building requires existing building requires improvement since it is improvement since it is possible to inject the grout possible to inject the grout from the side or at an from the side or at an inclined angle to reach inclined angle to reach beneath the building.beneath the building.

Index:Index:


Conclusion:Conclusion:

Thus, alteration of ground motion, Thus, alteration of ground motion, development of sand boils and settlement are the development of sand boils and settlement are the effects of liquefaction which can be reduced by effects of liquefaction which can be reduced by avoiding liquefaction susceptible soils, building avoiding liquefaction susceptible soils, building liquefaction resistant structures and by liquefaction resistant structures and by densification of soil.densification of soil.

Liquefaction resistant structure include Liquefaction resistant structure include shallow and deep foundation aspects while soil shallow and deep foundation aspects while soil improvement techniques include vibroflotation, improvement techniques include vibroflotation, dynamic compaction if soil, installation of dynamic compaction if soil, installation of compaction piles, compaction grouting.compaction piles, compaction grouting.

Thus in these ways the liquefaction related Thus in these ways the liquefaction related hazards can be reduced to a great extent.hazards can be reduced to a great extent.

Index:Index:


References:References:

[1] Kramer S.L, ’Geotechnical Earthquake [1] Kramer S.L, ’Geotechnical Earthquake Engineering’; Pearson Education (Singapore) Pte. Engineering’; Pearson Education (Singapore) Pte. Ltd., New DelhiLtd., New Delhi

[2]Gopal Ranjan and Rao, A.S.R, ’Basic & Applied [2]Gopal Ranjan and Rao, A.S.R, ’Basic & Applied Soil Mechanics’; Willey Eastern Ltd, New Delhi, Soil Mechanics’; Willey Eastern Ltd, New Delhi, 1991.1991.

[3]www.ce.washington.edu[3]www.ce.washington.edu

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effects of liquefaction on structures

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