Seismic Response of Pile Foundations

Lecture-39

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A pile foundation is a deep foundation with depth more than the width of the foundation.

A pile foundation will overcome problems of soft surface soils by transferring load to stronger, deeper stratum, thereby reducing settlements.

In general, pile foundations are more expensive than shallow foundations.

Based on the way the resistance is derived, piles are classified as friction piles and end bearing piles. Some piles have both friction and end bearing resistance but for most of the piles, one of these components is important than the other.

Pile Foundations

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3

Various applications of pile foundations

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Influencing parameters

Pile foundations can have –

Different static loads

Vertical

Horizontal (2)

Moment (2)

Different dynamic loads

Vertical

Horizontal (2)

Moment (2)

Pile foundations can have –

Different group configurations

Different pile lengths

Different pile cap dimensions

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End Bearing Piles

ROCK

SOFT SOILPILES

In case of end bearing piles, maximum resistance is derived from the firm soil at the bottom of the pile.

The load of the structure is transmitted through the pile into this firm soil or

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SOFT SOILPILES

Friction Piles

In case of friction piles, maximum resistance is derived from the skin friction between the pile and the soil in to which it is driven.

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Combinations of vertical, horizontal and moment loading may be applied at the soil surface from the overlying structure.

For the majority of foundations the loads applied to the piles are primarily vertical.

For piles in jetties, foundations for bridge piers, tall chimneys, and offshore piled foundations the lateral resistance is an important consideration.

The analysis of piles subjected to lateral and moment loading is more complex than simple vertical loading because of the soil-structure interaction.

Loads applied to PilesH

Static Bearing Capacity of Piles

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QfL

Qb

Qu

Pile base or tip

Pile shaft

fbu QQQ

VERTICAL LOAD BEARING CAPACITY OF A SINGLE VERICAL PILE• Bearing capacity of a pile depends upon

1. Type, size and length of the pile.2. Type of soil3. The method of installation

• If a static vertical load (ultimate load) of Qu is applied at the top, a part of the load is transmitted to the soil along the length of the pile –ultimate friction load or skin load, Qf

• and the balance is transmitted to the pile base called the base or point load Qb

ie,

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fbu QQQ

Static Bearing Capacity of Piles

Cohesionless soilsThe end bearing capacity is calculated by analogy with the bearing capacity of shallow footings and given by

where fb = Nq(σ’z)b= base resistance

Nq = the bearing capacity coefficient, a function of φ’

(σ’z)b=vertical effective stress at base

Ab=cross sectional area of base

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fbu

j

iiiiizif

j

iiiiixf

QQQ

KQ

Q

1

''

1

''

LengthPerimetertan

LengthPerimetertan

bbqbb AzNAfQ 'b

Static Bearing Capacity of Piles

Cohesive soils

α = adhesion factor= average undrained shear strength of clay along the

shaft cb = undrained shear strength at the base level

Nc= bearing capacity factor

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usf

bcbb

fbu

cAQ

ANcQ

QQQ

..

uc

Static Bearing Capacity of Piles

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Most people believe that in earthquake prone areas end-bearing piles should perform better than friction piles due to their end restraints. However, a significant number of cases of damage to end-bearing piles and pile-supported structures have been observed in most major earthquakes.

Structural failure by the formation of plastic hinges in piles passing through liquefiable soils has been observed in many of the recent strong earthquakes.

Bending moments or shear forces that are experienced by the piles exceed those predicted by design methods. All current design codes apparently provide a high margin of safety, yet occurrences of pile failure due to liquefaction are abundant.

Actual moments or shear forces experienced by the pile are many times those predicted.

Seismic Response of Piles

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The seismic loading induces large displacements or strains in the soil. The shear modulus of the soil degrades and damping (material) increases with increasing strain. The stiffness of piles should be determined for these strain effects.

The stiffness of the pile group is estimated from that of the single piles byusing group interaction factors. The contribution of the pile cap, if any, is alsoincluded. The response of the single pile or pile groups may then be determined using principles of structural dynamics.

The design of pile foundations subjected to earthquakes requires a reliable method of calculating the effects of earthquake shaking and post-liquefaction displacements on pile foundations.

Keys to good design include reliable estimates of environmental loads, realistic assessments of pile head fixity, and a mathematical model which can adequately account for all significant factors that affect the response of the pile-soil-structure system to ground shaking and/or lateral spreading in a given situation

Seismic Response of Piles

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For pile foundations built in seismic areas, the demands to sustain load and deformation during an earthquake will probably be the most severe in their design life

There are two sources of loading of the pile by the earthquake: “inertial” loading of the pile head caused by the lateral forces imposed on the superstructure, and “kinematic” loading along the length of the pile caused by the lateral soil movements developed during the earthquake, assuming zero inertia at the superstructure.

The foundation damages in past earthquakes showed that the effect of ground displacement should be taken into consideration and the design of foundation beam and pile cap were also important in the seismic design.

In case of fixed head connection using conventional structural method, the large bending moment acts on the pile head due to the inertial force of building and ground displacement during earthquake. This bending moment is important for the design of not only pile but also foundation beam and pile cap.

Seismic Response of Piles

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Pile failure during earthquakes

Hanshin Expressway, Kobe 1995

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Define the amplitude of ground displacement, including accounting for pile pinning effects.

Define the ground displacement profile

Conduct soil-pile interaction analysis

Assess the pile performance

Designing the Pile Foundation Against the Kinematic Ground Displacement

Loading

mass

soil layer

seismic waves S, P

seismic waves R, L

Soil-Pile-Structure System

Free-Field

base rock acceleration

acceleration of structure

free-field acceleration

effective foundation acceleration

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Response of the foundation ignoring the inertial forces from the structure

Determine the oscillatory motion of the raft (effective foundation of the structure)

Determine the bending moments and shear forces of the piles caused by the seismic waves

Kinematic Pile Response Analysis

seismic waves S, P

seismic waves R, L

Kinematic Soil-Structure Interaction

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Considers the dynamic motion of structure and loads

Determines the structure’s

• internal dynamic forces• differential displacements• “floor response spectra” • any additional pile motion and stresses

Inertial Pile Response Analysis

Inertial Soil- structure Interaction

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Methods to analyze soil-structure interaction

• Boundary element type methods– can handle a single pile and pile groups– takes into account kinematic and inertial loadings– works well for elastic isotropic conditions– cannot handle the nonlinearities of soil

• Winkler models– nonlinear springs and dashpots– can handle a single pile and pile groups as well as

kinematic and inertial loadings

In most of the earthquakes, damages to piles have occurred when soil deposits have lost their strength and appeared to flow as fluids.

In this phenomenon, the strength of soil is reduced, often drastically, to the point where it is unable to support structures or remain stable.

As it occurs in saturated soils, liquefaction is most commonly observed near rivers, bays and other bodies of water.

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Liquefaction induced failures

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Liquefaction can cause large lateral loads on pile foundations. Hence if the piles are driven through weaker soils which could liquefy, they should be designed to carry the additional lateral loads and lateral moments induced by the liquefaction of this layer.

Since piles usually extend to deeper depths and the zone of liquefaction is limited to the depth where the shear stresses induced by the earthquake exceed the shear strength of the soil, the possibility liquefaction of the complete strata into which the piles are driven is remote. However, the liquefied zone would cause differential stresses in piles along with the additional lateral loads and moments and the portion of pile in liquefied layer undergoes extensive deformations, which changes the overall pile configuration.

Sufficient resistance can be achieved by piles of larger dimensions and/or more reinforcement. It is important that the piles are connected to the cap in a ductile manner that allows some rotation to occur without a failure of the connection. If the pile connections fail, the cap cannot resist overturning moments from the superstructure by developing vertical loads in the piles.

Piles in liquefied soils

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Stiff Layer

Pile failure during liquefaction

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 piles of larger dimensions and/or more reinforcement.

Fill

Liquefied Layer

Locations of high bending moments

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Fill

Liquefied Layer

Stiff Layer

Movement

Cap failure

Pile failure during liquefaction

It is important that the piles are connected to the cap in a ductile manner that allows some rotation to occur without a failure of the connection. If the pile connections fail, the cap cannot resist overturning moments from the superstructure by developing vertical loads in the piles, as shown in figure.

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NONLIQUEFIELDCRUST

FREE-FIELD SOILDEFLECTION

NONLIQUEFIELDSAND

LOOSELIQUEFIELDSAND

DEFORMED SHAPE OF PILE

PILE CONFIGURATION DUE TO LIQUEFACTION

Piles in liquefied soils

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Niigata Earthquake, 1964

Pile failure of Niigata Family Court House building

Photo courtesy, Hamada (1991)

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As the seismic waves arrive in the soil surrounding the pile, the soil layers will tend to deform. This seismically deforming soil will try to move the piles and the embedded pile-cap with it. Subsequently, depending upon the rigidity of the superstructure and the pile-cap, the superstructure may also move with the foundation. The pile may thus experience two distinct phases of initial soil-structure interaction.

1)Before the superstructure starts oscillating, the piles may be forced to follow the soil motion, depending on the flexural rigidity (EI) of the pile. Here the soil and pile may take part in kinematic interplay and the motion of the pile may differ substantially from the free field motion. This may induce bending moments in the pile.

1)As the superstructure starts to oscillate, inertial forces are generated. These inertia forces are transferred as lateral forces and overturning moments to the pile via the pile-cap. The pile-cap transfers the moments as varying axial loads and bending moments in the piles. Thus the piles may experience additional axial and lateral loads, which cause additional bending moments in the pile.

Piles in liquefied soils

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In loose saturated sandy soil, as the shaking continues, pore pressure will build up and the soil will start to liquefy. Under these conditions, an end-bearing pile passing through liquefiable soil will experience distinct changes in its stress state.

• The pile will start to lose its shaft resistance in the liquefied layer and shed axial loads downwards to mobilize additional base resistance. If the base capacity is exceeded, settlement failure will occur.

• The liquefied soil will begin to lose its stiffness so that the pile acts as an unsupported column. Piles that have a high slenderness ratio will then beprone to axial instability, and buckling failure may occur in the pile, enhanced by the actions of lateral disturbing forces and also by the deterioration of bending stiffness due to the onset of plastic yielding.

In sloping ground, even if the pile survives the above load conditions, it may experience additional drag load due to the lateral spreading of soil. Under these conditions, the pile may behave as a beam-column.

Piles in liquefied soils

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Piles in liquefied soils

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1. Decrease or loss in vertical pile load capacity from both skin and end bearing resistances, depending upon the zone of liquefaction along the pile depth.

2. Decrease in lateral pile load capacity and increase in lateral deflection: Especially if the liquefaction occurs near the surface and extends only to a limited portion of the total pile depth.

3. Increase in pile settlement: Any decrease in skin or end bearing resistance may result in some pile settlement. However, if the soil below an end bearing pile liquefies, objectional settlement and possible damage to the structure may also occur.

4. Increase in pile load from downdrag forces caused by the settlements of the liquefied or partially liquefied soils along the pile length

Effects of liquefaction in piles

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Complicated combinations for probabilistic estimate of pile displacement

Bridgeconfigurations

Pile group configurations

Static loading conditions

Dynamic loading conditions

Multiple ground motion levels

Multiple bridge configurations

Multiple pile group configurations

Multiple static load

states – 5 loads for each

Multiple dynamic load cases – 5 loads for each

Dynamic response

x y z yx

Multiple response measures (EDPs)

Ground motions

Multiple time histories

Ground motion hazards

For 5 hazard levels, 5 bridge configurations, 5 pile groups, 4 initial load levels, 3 hazard levels, and 100 simulations with 40 input motions, we need 30,000,000 calculations for calculating pile displacements

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Day, R.W. (2001) Geotechnical Earthquake Engineering Handbook, McGraw-

Hill.

Dowrick, D. J. (1987). Earthquake Resistant Design, John Wiley & Sons.

Das, B. M. (1993). Principles of Soil Dynamics, Brooks/Cole

http://iisee.kenken.go.jp/staff/tamura/work/design/06design.html (Accessed

on 14 April 2012)

http://www.nibs.org/client/assets/files/bssc/Topic14-FoundationDesignNotes.

pdf

(Accessed on 14 April 2012)

http://xa.yimg.com/kq/groups/21948400/2051664753/name/foundation+desi

gn+principles.pdf

(Accessed on 14 April 2012)

ReferencesReferences