Effects Of Liquefaction On Structures

Effects of Liquefaction on Structures & Its Remedial Measures By Tushar Ghosh Final Year

K.I.T.S, Ramtek

Index: Introduction  Liquefaction-Related Phenomenon  Effects of Liquefaction  Remedial Measures  Conclusion  Reference

Index: Introduction  Liquefaction-Related Phenomenon  Effects of Liquefaction  Remedial Measures  Conclusion  Reference

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

Introduction: This phenomenon was very little known until it drew the attention of Geotechnical Engineers in 1964 when a devastating earthquake occurred in Alaska followed by Niigata earthquake in Japan which caused huge scale damages due to Liquefaction including slope failures, bridge and building foundation failures. Liquefaction is a phenomenon by which loose saturated sand becomes liquid when rapid loading occurs under undrained conditions.

Index: Introduction  Liquefaction-Related Phenomenon  Effects of Liquefaction  Remedial Measures  Conclusion  Reference

Liquefaction-Related Phenomenon During Earthquake, Solids the saturated Pore sand is Wate r vibrated as a result of it tends to densify. As the particles tend to this, come close to each other, the excess pore water pressure increases and hence effective stress decreases.

The Mohr-Coulomb strength equation is given by, τ = c + σ’tanΦ Where, τ is the shear strength c is cohesion σ’ is effective normal stress Φ is the angle of internal friction As the sand is cohesionless, c = 0 But, σ’ = σ – u, where σ is total normal stress that depends on unit weight and u is pore water pressure. In case of loose sand, σ itself is very small and since there is a possibility of the excess pore water pressure developed during Earthquake being equal the σ, the effective stress σ’becomes zero. As a result of this the shear strength, τ = 0 and the sand becomes a liquid.

Index: Introduction  Liquefaction-Related Phenomenon  Effects of Liquefaction  Remedial Measures  Conclusion  Reference

Effects of Liquefaction The effects of Liquefaction is divided into 3-parts: i. Alteration of Ground Motion ii. Development of Sand Boils iii. Settlement

i.

Alteration of Ground Motion

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

Pile

Nonliquified Liquefied Nonliquified

Potential Effects of Subsurface Liquefaction on Pile.

i.

Alteration of Ground Motion

Before Earthquake

After Earthquake Sand Boils

G.L

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

ii.

Development of Sand Boils

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

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

Index: Introduction  Liquefaction-Related Phenomenon  Effects of Liquefaction  Remedial Measures  Conclusion  Reference

Remedial Measures

1) 2) 3)

There are basically three possibilities to reduce Liquefaction hazards when designing and constructing new buildings or other structures as bridges, tunnels and roads. These are as follows: Avoid liquefaction susceptible soil Build liquefaction resistant structures Improvement of Soil

Avoid Liquefaction Susceptible Soils The criteria by which liquefaction susceptibility of the soil is judged include: 1) Historic Criteria 2) Geologic Criteria 3) Compositional Criteria 4) State Criteria

Liquefaction Resistant Structures: There are basically two aspects to construct liquefaction resistant structure. These are: 1) Shallow foundations aspects 2) Deep foundation aspects

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

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

3) Improvement of Soil The main goal of most soil improvement techniques used for reducing liquefaction hazards is to avoid large increases in pore water pressure during earthquakes. This can be achieved in the following ways: I. Vibrofloatation II. Dynamic Compaction III. Compaction Piles IV. Compaction Grouting

I.

Vibrofloatation

Vibrofloatation 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 vibrofloat is raised and lowered in a grid pattern.

II.

Dynamic Compaction

Densification by dynamic compaction is performed by dropping 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.

III.

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 60ft.

IV.

Compaction Grouting

Compaction grouting is a technique whereby a slow flowing water/sand/cement mix is injected under pressure into granular soil. The grout forms a bulb that displaces and hence densifies the surrounding soil. It 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.

Index: Introduction  Liquefaction-Related Phenomenon  Effects of Liquefaction  Remedial Measures  Conclusion  Reference

Conclusion: Thus, alteration of ground motion, development of sand boils and settlement are the effects of liquefaction which can be reduced by avoiding liquefaction susceptible soils, building liquefaction resistant structures and by densification of soil. Liquefaction resistant structure include shallow and deep foundation aspects while soil improvement techniques include vibroflotation, dynamic compaction if soil, installation of compaction piles, compaction grouting. Thus in these ways the liquefaction related hazards can be reduced to a great extent.

Index: Introduction  Liquefaction-Related Phenomenon  Effects of Liquefaction  Remedial Measures  Conclusion  Reference

References: [1] Kramer S.L, ’Geotechnical Earthquake Engineering’; Pearson Education (Singapore) Pte. Ltd., New Delhi [2]Gopal Ranjan and Rao, A.S.R, ’Basic & Applied Soil Mechanics’; Willey Eastern Ltd, New Delhi, 1991. [3]www.ce.washington.edu

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