PART 2

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1 INTRODUCTION

Earthquakes are naturally occurring broad-banded vibratory ground motions that are due to a number of

causes, including tectonic ground motions, volcanism, landslides, rock bursts, and manmade explosions, the

most important of which are caused by the fracture and sliding of rock along tectonic faults within the Earth’s

crust. For most earthquakes, shaking and ground failure are the dominant and most widespread agents of

damage. Ground failure due to weakening of the soil results in liquefaction and lateral spreading.

Earlier, pile foundations of ordinary bridges have often been designed for axial and lateral load due to

static loading. In the mid-1990, Ductility based design was implemented for the superstructure and

substructure components of bridges, following which major research was conducted by investigating the

bridge failures. A major cause of damage to bridges in the past earthquakes has been liquefaction and

associated lateral spreading of the foundation soils. The impact of liquefiable soil on the design of bridge

pile foundations was researched to safeguard the existing bridges against ground failure.

Ductile design of piles in laterally spreading soil provides a unified approach in seismic design that is

rational and leads to an improved representation of the system response to earthquakes. In ductility based

design, piles behaving in elastically would make the entire foundation system more flexible, which in turn

could lead to an increase in earthquake energy dissipation and a potential reduction in the adverse impact

of the earthquake on the bridge structure. Analysis of the inelastic behaviour of the pile allows engineer to

control the amount of plastic deformation of the pile structure to avoid significant pile damage. Design of the

transverse reinforcement and indication of pile damage can be met using a ductile displacement based design

approach for bridge pile foundations.

The allowable deflection of the bridge was satisfied by designing elastic bridge pile foundations in

liquefiable soil. But, in order to perform adequately during a maximum credible design earthquake, the

dynamic loading due to earthquakes was superimposed onto the working loads already acting on the pile.

Research in recent years has led to significant advances in our understanding of the mechanics of the soil-

pile structure interaction in liquefying and laterally spreading ground.

The performance based bridge design philosophy has replaced the conventional load/stress based factor

of safety approach. For satisfying serviceability criterion, the displacements undergone by the pile must be

minimum. The seismic design guidelines also describes an acceptable level of damage in relation to Ultimate

Limit State. The performance based bridge design practice emphasizes in modelling the soil behaviour in

liquefiable soil and studying the loading behaviour of the laterally spreading soil on the piles.

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2 PILE FOUNDATIONS

Piles are structural members that are made of steel, concrete, and/or timber. They are used to build pile

foundations, which are deep and which cost more than shallow foundations. Despite the cost, the use of piles

often is necessary to ensure structural safety.

2.1 Necessity of Pile Foundations

The following list identifies some of the conditions that require pile foundations (Vesic, 1977).

i. When the upper soil layer(s) is (are) highly compressible and too weak to support the load transmitted

by the superstructure, piles are used to transmit the load to underlying bedrocks or a stronger soil

layer. (figure 2.1a)

ii. When bedrock is not encountered at a reasonable depth below the ground surface, piles are used to

transmit the structural load to the soil gradually. The resistance to the applied structural load is derived

mainly from the frictional resistance developed at the soil-pile interface (figure 2.1b).

iii. When subjected to horizontal forces (see figure 2.1c), pile foundations resist by bending while still

supporting the vertical load transmitted by the superstructure. This type of situation is generally

encountered in the design and construction of earth-retaining structures and foundations of tall

structures that are subject to high wind and/or earthquake forces.

iv. In many cases, expansive and collapsible soils (such as loess) may be present at the site of a proposed

structure. These soils may extend to a great depth below the ground surface. Expansive soils swell

and shrink as the moisture content increases and decreases, and the swelling pressure of such soils can

be considerable. If shallow foundations are used in such circumstances, the structure may suffer

considerable damage. However, pile foundations may be considered as an alternative when pies are

extended beyond the active zone, which swells and shrinks (figure 2.1d).

v. Foundations of some structures, such as transmission towers, offshore platforms, and basement mats

below the water table, are subjected to uplifting forces. Piles are sometimes used for these foundations

to resist the uplifting force (figure 2.1e).

vi.

Bridge abutments and piers are usually constructed over pile foundations to avoid the possible loss of

bearing capacity that a shallow foundation might suffer because of soil erosion at the ground surface

(figure 2.1f)

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2.2 Types of Piles and Their Structural Characteristics

Different types of piles are used in construction work, depending on the type of load to be carried, the subsoil

conditions, and the location of the water table. Pile foundations are classified based on material of pile

construction, type of soil, and load transmitting characteristic of piles.

Figure 2.1 Conditions for use of pile foundations

Based on the material used, piles can be divided into the following categories: (a) steel piles (b) concrete

piles, (c) wooden /timber piles, and (d) composite piles. Depending on their lengths and the mechanisms of

load transfer to the soil piles are classified as: (a) end bearing piles, (b) friction piles, and (c) compaction

piles.

2.2.1 End bearing piles

Load is transferred through weaker soils to a competent bearing stratum using pile tip resistance. If soil-

boring records establish the presence of bedrocks or rocklike material at a site within a reasonable depth, piles

can be extended to the rock surface. (Figure 2.2a). In this case, the ultimate capacity of the piles depends

entirely on the load bearing capacity of the underlying material; thus the piles are called end/point bearing

piles. In most of these cases, the necessary length of the pile can be fairly well established. If, instead to bedrock,

a fairly compact and hard stratum of soil is encountered at a reasonable depth, piles can be extended a few

meters into the hard stratum (figure 2.2b). The required pile length maybe estimated accurately if proper

subsoil exploration records are available.

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2.2.2 Friction piles

When no layer of rock or rocklike material is present at a reasonable depth at a site, point bearing piles become

very long and uneconomical. For this type of subsoil condition, piles are driven through the softer material

to specified depths. These piles are called friction piles because most of the resistance is derived from skin

friction.

However, the term friction pile, although used often in the literature, is a misnomer: in clayey soils, the

resistance to applied load is also caused by adhesion. They are also known as floating piles. The length of

friction of piles depends on the shear strength of the soil, the applied load and the pile size. (figure 2.3)

Figure 2.2(a) Figure 2.2(b) Figure 2.3 Figure 2.4

2.2.3 Compaction piles

Under certain circumstances, piles are driven into loose granular soils to achieve proper densification of soil

close to the ground surface. These piles are called compaction piles. The length of compaction piles depends

on factors such as (a) relative density of the soil before compaction, (b) desired relative density of the soil

after compaction, and (c) required depth of compaction. These piles are generally short; however, some field

tests are necessary to determine a reasonable length. (figure2.4)

2.3 Axial capacity of a single pile

The axial load carrying capacity of a single pile can be written as

Qu = Q b + Qs(2.1)

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0

where Qu - axial capacity of the pile, Q b - base resistance at the pile tip, Qs - Shaft friction of the pile .The

base resistance at the pile tip can be calculated as follows

Q b = A b b(Nq-1) (2.2)

where A b - Base area of the pile , b - Effective overburden pressure at the pile tip level , Nq - Bearing

capacity factor that depends on the angle of shearing resistance of the soil as given by Berezantsev et al.

(1961).The shaft capacity can be obtained by estimating the shear stress generated along the shaft as follows

s = K s’vtancv (2.3)

where K s - Earth pressure coefficient, ’v - Effective vertical stress at a given elevation, cv - Friction angle

between the pile material and soil. The shaft capacity due to skin friction is obtained by integrating the shear

stress over the surface area of the pile using the below equation

Qs = 2r ∫Lτ s (2.4)

where r is the pile radius and L is the length of the pile

Table 2.1: Values of earth pressure coefficient K s and pile soil friction angle (Broms, 1966)

Pile Material cv K s

Low relative density High relative density

Steel 20° 0.5 1.0

Concrete 0.75ψ’ 1.0 2.0

Wood 0.66ψ’ 1.5 4.0

The general equation to find axial pile capacity isn’t valid in practice. Both end bearing and shaft resistance

increases nonlinearly with depth; i.e., the increased axial capacity of the pile below a critical depth of

penetration, Dc is rather limited. CPT tests are performed to estimate the in situ pile base capacity and shaft

friction. When a single pile is loaded to failure, then the limiting values of the end bearing and shaft resistance

are fully mobilized. However, when the pile is subjected to working loads, the proportion of end bearing

compared to shaft resistance that is mobilized may vary. The actual end bearing and shaft resistances mobilizedwill depend on the displacements suffered by the pile under the working loads applied.

.

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3 BRIDGE PILE FOUNDATIONS

Nowadays bridges are designed to perform adequately during a maximum credible design earthquake. The

dynamic loading due to earthquakes is superimposed onto the working loads already acting on the pile The

performance based bridge design philosophy has replaced the conventional load/stress based factor of safety

approach. For satisfying serviceability criterion, the displacements undergone by the pile must be minimum.

The seismic design guidelines also describes an acceptable level of damage in relation to Ultimate Limit

State.

3.1 Types of Bridge Foundations Piles

3.1.1 Large-diameter shafts

These foundations consist of one or more large-diameter piles, usually in the range of 1.2 to 3.6 m embedded

in the soils to sufficient depths to reach firm soil strata or rock where a high degree of fixity can be achieved,thus allowing the forces and moments imposed on the shafts to be safely transferred to the embedment soils

within allowable soil-bearing pressure limits and/or allowable foundation displacement limits.

3.1.2. Slender-pile groups

Slender piles refer to those piles having a diameter or cross-sectional dimensions less than 0.6 m. These piles

are usually installed in a group and provided with a rigid cap to form the foundation of a bridge pier. In real

situations, the vertical resistance offered by the pile is usually achieved by a combination of end bearing and

side friction. Resistance to lateral loads is achieved by a combination of soil passive pressure on the pile cap,

soil resistance around the piles, and flexural resistance of the piles. The uplift capacity of a pile is generally

governed by the soil friction or cohesion acting on the perimeter of the pile.

Figure 3.1: Large-diameter shaft Figure 3.2: Slender pile group

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4 GROUND FAILURE

Strong earthquake shaking can produce a dynamic response of soils that is so energetic that the stress waves

exceed the strength of the soil. In such cases, ground failure characterized by permanent soil deformations

may occur. Ground failure may be caused by weakening of the soil or by temporary exceedance of the strength

of the soil by transient inertial stresses. The former can result in phenomena such as liquefaction and lateral

spreading, the latter in inertial failures of slopes and retaining wall backfills.

4.1 Liquefaction

The term Liquefaction has been widely used to describe a range of phenomena in which the strength

and stiffness of a soil deposit are reduced due to the generation of pore water pressure. It occurs most

commonly in loose saturated sands, although it has been observed in gravels and non-plastic silts. The effects

of liquefaction can range from massive landslides to displacements measured in tens of meters to relatively

small slumps or spreads with small displacements. Many bridges, particularly, those that cross water bodies,

are located in areas with geologic and hydrologic conditions that tend to produce liquefaction.

Seed (1979) describes liquefaction as a “….process of reduction of shear strength for low plastic loose

cohesion less soil during which pore-water pressure builds up due to the application of static or cyclic stresses”

The soil looses contact between its grains and upward flow of water takes place. If the magnitude of pore-

water pressure generated equals the total vertical stress, the effective stress becomes zero and the soil is said

to have liquefied. The possibility of its occurrence depends on the initial void ratio or relative density of sand

and the confining pressure

Although bridge failures are most commonly associated with lateral spreading, it is not the only potentially

damaging failure mechanism. Subsidence and increased lateral pressures can also have severe consequences.

All of the following general conditions are necessary for liquefaction to occur:

i. The presence of groundwater, resulting in a saturated or nearly saturated soil.

ii. Predominantly cohesion less soil that has the right gradation and composition. Liquefaction has

occurred in soils ranging from low plasticity silts to gravel. Clean or silty sands and non-plastic silts

are most susceptible to liquefaction.

iii. A sustained ground motion that is large enough and acting over a long enough period of time to

develop excess pore-water pressure, equal to the effective overburden stress, thereby significantly

reducing effective stress and soil strength.

iv. The state of the soil is characterized by a density that is low enough for the soil to exhibit contractive

behavior when sheared undrained under the initial effective overburden stress.

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4.1.1 Mechanism of Liquefaction

The mechanisms that produce liquefaction related phenomena can be divided into 2 categories. The first, flow

liquefaction, can occur when the shear stresses required for static equilibrium of a soil mass are greater than

the shear strength of the soil in its liquefied state. While not common, flow liquefaction can produce

tremendous instabilities know as flow failures. In such cases, the earthquake serves to trigger liquefaction,

but the large deformations that result are actually driven by the pre-existing static stresses. The second

phenomenon, cyclic mobility occurs when the initial static stresses are less than the strength of the liquefied

soil. The effects of cyclic mobility lead to deformations that develop incrementally during the period of

earthquake shaking, and are commonly called lateral spreading. Lateral spreading can occur on very gentle

slopes in the vicinity of free surfaces such as riverbanks and beneath and adjacent to embankments. Lateral

spreading occurs much more frequently than flow failure and can cause significant distress to bridges and

their foundations.

4.1.2 Liquefaction Susceptibility

The first step in an evaluation of liquefaction hazards is the determination whether the soil is susceptible to

liquefaction. If the soils at a particular site are not susceptible to liquefaction, liquefaction hazards don’t exist

and the liquefaction hazard evaluation can be terminated. If the soil is susceptible, however the issues of

initiation and effects of liquefaction must be considered.

Liquefaction occurs readily in loose, clean, uniformly graded saturated soils. Therefore, geologic

processes that sort soils into uniform grain size distributions and deposit them in loose states produce soil

deposits with high liquefaction susceptibility. As a result, fluvial deposits, and colluvial and aoelian deposits

when saturated, are likely to be susceptible to liquefaction. Liquefaction also occurs n alluvial beach and

estuarine deposits, but not as frequently as in those previously listed, because bridges are commonly

constructed in such geological environments, liquefaction is a frequent and important consideration in the

design Liquefaction susceptibility also depends on the stress and density characteristics of the soil. Very

dense soils, even if they have the other characteristics listed in the previous paragraph, will not generate high

pore water pressures during earthquake shaking and hence are not susceptible to liquefaction. The minimum

density at which soils are not susceptible to liquefaction increases with increasing effective confining

pressure. This characteristic indicates that, for a soil deposit of constant density, the deeper soils are more

susceptible to liquefaction than the shallower soils

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4.1.3 Initiation of Liquefaction

The fact that a soil deposit is susceptible to liquefaction does not mean that liquefaction will occur in a given

earthquake. Liquefaction must be triggered by some disturbance, such as earthquake shaking with sufficient

strength to exceed the liquefaction resistance of the soil. Even a liquefaction-susceptible soil will have some

liquefaction resistance. Evaluating the potential for the occurrence of liquefaction (liquefaction potential)

involves comparison of the loading imposed by the anticipated earthquake with the liquefaction resistance

of the soil. Liquefaction potential is most commonly evaluated using the cyclic stress approach in which both

earthquake loading and liquefaction resistance are expressed in terms of cyclic stresses, thereby allowing

direct and consistent comparison.

4.1.4 Liquefaction-Induced Failure Mechanisms

Liquefaction of loose, saturated, cohesion less soils can produce several different types of ground failure

depending on site conditions. These failure mechanisms include lateral spreading, loss of bearing capacity

and settlement, ground oscillations, and flow failure (Youd, 1992). Any of these mechanisms can potentially

cause damage to bridges due to the ground and foundation movements that occur.

4.1.4.1 Lateral spreading

The bridge pile foundation undergoes settlement and /or rotation when the soil layers are liquefied. When the

soil below the pile base becomes liquefied, then there is a decrease in the base capacity and the pile endures

excessive settlements. On the other hand, if the depth to which the soil liquefies is rather limited, say in arelatively small magnitude earthquake, then the soil surrounding the shaft may liquefy and a loss of shaft

friction is expected. This can cause an increase in the baseload of the pile, which can lead to an increased

settlement. In addition, the pile foundation is also subjected lateral spreading of the ground. Lateral spreading

is the finite, lateral movement of gently to steeply sloping, saturated soil deposits caused by earthquake-

induced liquefaction.

Damage to many bridges due to earthquake-induced liquefaction has resulted from lateral spreading of gently

sloping ground towards river channels. Lateral spreading consists of the displacement of ground down gentleslopes (i.e. - typically having inclinations less than 3 degrees according to Youd, 1992) or towards an incised

channel, as a result of liquefaction of underlying soils. The displacements are usually incremental, occurring

at periods during the earthquake when the strength of the liquefied material is less

than needed to resist the lateral forces acting on the overlying non-liquefied soil (Kramer, 1996). The

overlying soil is usually broken up in blocks which displace downslope or towards the incised channel, on

top of the liquefied soil.General characteristics of lateral spreading that are manifested in the ground, as

described by Youd (1993), are “… extensional deformations at the head of the feature, shear deformations

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along the margins, and compressed ground at the toe.” Displacements can range from a few centimetres to

several meters.

Since bridges are typically located at the toe of a lateral spread, they are commonly subjected to

compression. Damage to the bridge is generally caused by differential lateral ground displacement. The type

and magnitude of damage depend on the foundation, superstructure, substructure, and connection

characteristics of the bridge.

4.1.4.2 Loss of bearing capacity and settlement

Loss of bearing capacity results from the loss of soil strength associated with the increase in pore water

pressures and softening of the soil occurring during partial or full liquefaction. The reduction in bearing

capacity can result in excessive settlements/movements of a bridge pier or abutment whose foundation bearing pressure exceeds the reduced capacity. Excessive movements can also occur in the absence of a

catastrophic or sudden ground failure, as a result of the cyclic loading of the foundation which causes it to

gradually penetrate into the weakened soil. In addition, settlements can be induced due to the densification

which occurs when excess pore water pressures dissipate in partially or fully liquefied soils. Similar to a loss

of bearing capacity is the loss of axial and lateral support for deep foundations extending through liquefiable

soil. This loss of support can cause excessive deformations and stresses in piles or drilled shafts resulting in

damage.

4.1.4.3 Ground oscillation

Ground oscillation is a phenomenon that occurs on relatively level ground where lateral spreading does not

occur. In this phenomenon, broken blocks of nonliquefied soil oscillate back and forth and up and down on

top of an underlying liquefied layer during an earthquake. (Youd, 1992). A bridge supported by the surficial

layer can experience severe deformations when substructure columns or walls supported by shallow

foundations on the blocks undergo differential movements.

4.1.4.4 Flow failure

Flow failure is the rapid movement of liquefied soil and overlying layers down more steeply inclined slopes

(i.e. - typically greater than 3 degrees according to Youd, 1992). According to Youd (1992), “these failures

commonly displace large masses of soil tens of meters and, in a few instances, large masses of soil have

traveled tens of kilometres down long slopes at velocities ranging up to tens of kilometres per hour.” The

large displacements result from the residual strength of the liquefied soil being less than necessary to resist

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The static gravitational forces acting on overlying nonliquefied soils during and after earthquake shaking

(Kramer, 1996). Although such failures have primarily been observed to occur in offshore seabed or tailings

dams, they may be possible at a bridge site given sufficient ground slope and the proper subsurface

conditions. This type of failure could cause severe damage to a bridge supported on, or even through, the

liquefiable soil.

4.2 Implications for Bridges

Although all of the failure mechanisms mentioned above are possible at a bridge site given the proper

conditions, lateral spreading and bearing capacity failure are probably more common. Lateral spreading has

often caused damage to bridges and bridge foundations in earthquakes. Lateral spreading generally involves

the lateral movement of soil at and below the ground surface, often in the form of relatively intact surficial

blocks riding on a mass of softened and weakened soil. The lateral soil movement can induce large bending

moments in pile foundations. The damage produced by lateral spreading is closely related to the magnitude

of the lateral soil displacements. Because cyclic mobility, the fundamental phenomenon that produces lateral

spreading, is so complex, analytical procedures for prediction of lateral spreading displacements have not yet

reached the point at which they can be used for design. As a result, currently accepted procedures for

prediction of lateral spreading displacements are empirically based.

The previous observations illustrate the type of damage that may be experienced as a result of liquefaction-

induced ground failures such as lateral spread and ground subsidence. Based on the case studies, observations

and/or impacts from liquefaction include the following. (Raunch, 1997)

1. Lateral ground displacements have been extremely damaging to bridge foundations andabutments.

2. Movement of foundation elements may create large shear forces and bending moments at connections

and compressional forces in the superstructure.

3. Compressional forces generated by lateral ground displacement generally cause one of the following

reactions:

a. The superstructure may act as a strut, bracing the tops of abutments and piers and holding them

relatively in place while the bases of these elements shift stream ward with the spreading

ground

b. The deck may buckle laterally or vertically, causing severe damage to the superstructure

c. The connections between the foundation and the superstructure may fail, allowing piers and

abutments to shift or tilt toward the river with little restraint.

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4. Subsidence and increased lateral earth pressures can also lead to deleterious consequences for bridge

foundations. Waterfront retaining structures, especially in areas of reclaimed land, can experience

large settlements and lateral earth pressures adjacent to bridge foundations. These movements lead to

the rotation and translation of bridge abutments and increased lateral forces on pile foundations

5.

A number of failure modes may occur in pile foundations, depending on the conditions of fixity, pilereinforcement and ductility. Generally, if concrete piles were well embedded in the pile caps, shear

or flexural cracks occurred at pile heads, often leading to failure; if steel pipe piles were fixed tightly

in the pile caps, failure was at the connection or pile cap; or if the pile heads were loosely connected

to the pile caps, they either rotated or were detached.

5 CHARACTERIZATION OF EARTHQUAKE LOADING

The level of pore water pressure generated by an earthquake is related to the amplitude and duration of

earthquake-induced shear stresses. Such shear stresses can be predicted in a site response analysis using either

the equivalent linear method or nonlinear methods. Alternatively, they can be estimated using a simplified

approach that does not require site response analyses.

5.1 Earthquake Induced Pile Loads

In the analysis of the behaviour of piles in liquefied soils, it is useful to distinguish between two different

phases in the soil-pile interaction process

1. A cyclic phase in the course of the intense ground shaking. The soil will impose a load on the pile

due to its transient movement, whether liquefaction occurs or not.

2. A permanent deformation phase following the occurrence of liquefaction. This may comprise lateral

spreading or flow failure (where a free face is present), and/or vertical settlement. The permanent

horizontal deformation of the ground around the pile imposes a load on the pile.

The total earthquake-induced loads on the pile shall comprise of

1. Inertial loads imposed by the superstructure to the pile head. This is a function of frequency of the

superstructure and the input motion and varies as the stiffness of the soil changes.

2. Kinematic forces acting along the embedded length of the pile due to the movement of the soil.

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5.2 Analysis of Bridge Piles

Depending upon the foundation type and its soil-support condition, the procedures currently being used in

evaluating SFSI effects on bridges can broadly be classified into two main methods, namely, elastodynamic

method and the empirical p – y .The two methods are described in detail below

5.2.1 Elastodynamic Method

This method is based on the well-established elastodynamic theory of wave propagation in a linear elastic,

viscoelastic, or constant-hysteresis-damped elastic half-space soil medium. The fundamental element of this

method is the constitutive relation between an applied harmonic point load and the corresponding dynamic

response displacements within the medium called the dynamic Green’s functions. Since these functions apply

only to a linear elastic, viscoelastic, or constant-hysteresis damped elastic medium, they are valid only for

linear SFSI problems. Since application of the elastodynamic method of analysis uses only mass, stiffness,

and damping properties of an SFSI system, this method is suitable only for global system response analysis

applications. However, by adopting the same equivalent linearization procedure as that used in the seismic

analysis of free field soil response, the method has been extended to one that can accommodate global soil

nonlinearities, i.e., those nonlinearities induced in the free-field soil medium by the free-field seismic waves

However, the validity of applying this method to large-diameter shaft foundations depends on the diameter

of the shafts and on the amplitude of the imposed loadings. When the shaft diameter is large so that the load

amplitudes produce only small local soil nonlinearities, the method is reasonably valid.

5.2.2 Empirical p – y Method

This method is adopted for the evaluation of seismic response of slender pile – foundation by solving a soil-

supported pile foundation subjected to applied loadings at the pile head. The p – y relation is generally

developed on the basis of an empirical curve that reflects the nonlinear resistance of the local soil surrounding

the pile at a specified depth As used, it characterizes the lateral soil resistance per unit length of pile, p, as a

function of the lateral displacement, y and depends mainly on soil material strength parameters, e.g., frictionangle, f, for sands and cohesion, c, for clays. It involves not just the lateral p – y curves but also the axial t – z

and Q – d curves for characterizing the soil resistances. the axial resistance of soils to piles per unit length of

pile, t, is treated as a nonlinear function of the corresponding axial displacement, z , resulting in the so-called

axial t – z curve, and treating the axial resistance of the soils at the pile tip, Q, as a nonlinear function of the

pile tip axial displacement, d, resulting in the so-called Q – d curve. Again, the construction of the t – z and Q –

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d curves for a soil-supported pile is based on empirical curvilinear forms and the soil strength parameters as

functions of depth. By utilizing the set of p – y, t – z , and Q – d curves developed for a pile foundation, the

response of the pile subjected to general three dimensional (3-D) loadings applied at the pile head can be

solved using the model of a 3-D beam supported on discrete sets of nonlinear lateral p – y, axial t – z, and axial

Q –

d springs. The soil – pile systems developed in this manner are then coupled with the remaining bridgestructure to form the complete SFSI system for use in a seismic- demand analysis. The validity and

applicability of this method are based on model calibrations and correlations with field experimental

results This method is not suitable for seismic-demand evaluations since it. does not incorporate soil mass,

stiffness, and damping characteristics

6 CASE STUDIES

Foundation damage associated with liquefaction-induced lateral spreading has probably been the singlegreatest cause of extreme distress and collapse of bridges .Liquefaction and lateral spreading have affected

many bridges in past earthquakes, inducing damage that ranged from negligible, to moderate, to collapse.

The problem is especially critical for bridges with simple spans. Examples of collapse include the Showa

Bridge (Hamada and O’Rourke 1992) where excessive deformation of the piers caused unseating of simply-

supported spans. The Yachiyo Bridge also suffered large displacements during the Niigata earthquake

(Hamada 1992). The Landing Road Bridge suffered moderate reparable damage as a result of as much as 2

m of lateral spreading of a non liquefiable crust layer over liquefied sand (Berrill et al. 2001)

6.1 Case I-Showa Bridge, Japan (Hamada 1992; Bhattacharya et al. 2004)

The 12m span, 307 m long Showa Bridge failed during the 1964 Niigata earthquake of 7.5 magnitude. Five

simply supported steel girders each with 28 m span, fell into the river. The piers were constructed by driving

steel pipe piles. The damaged pile was recovered and analysed during the post-earthquake investigation. The

pile with a diameter of 60.9 m and a thickness of 0.18 cm was bent towards the right bank of the river at

appoint 7 to 8 m below the river bed. The liquefied layer on the left bank of the river was estimated to be about

10 m thick, slid towards the centre of the river by about 5 m. Studies by Bhattacharya et al. (2004) suggested

that the failures of piles of this particular case might have resulted from pile buckling and estimated buckling

load was 1095 kN.Yoshida et al . (2007) concluded that that the possibility of the bridge collapsing due to

inertial loading or liquefaction induced soil flow is rather low and argued that it was most likely due to the

increased displacement of the ground owing to the liquefaction wherein the pile deformation can occur

more easily. The bridge has suffered total collapse and complete loss of serviceability following the

earthquake.

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Figure 6.1: A view of the collapsed Showa Bridge during the 1964 Niigata earthquake

6.2 Case II –

Yachiyo Bridge, Japan (Hamada 1992)

The abutment and piers of the Yachiyo Bridge was damaged during the Niigata earthquake. The foundations

of the abutment and the two piers next to the abutment were sitting on the reinforced concrete piles with a

diameter of 300 mm and a length of about 10 m.The damaged pile was extracted and examined after the

earthquake. It was reported that the pile was severely destroyed at a depth of about 8 m from the top of the

pile, 500 m downstream, both abutments of the bridge moved up to 50 cm towards the channel, but the bridge

didn’t collapse. Post-earthquake investigation of 60 cm diameter precast concrete piles revealed horizontal

cracks spaced continuously along the full length of the piles which had caused large bending moments. The

permanent ground displacement on the banks of the river was found to be 4-6 m towards the river

Investigations by Hamada (1992) concluded that the foundations of the piers were pushed towards the river

due to the large ground displacement

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Figure 6.2: Cracked Precast Reinforced Concrete piles from the Yachiyo Bridge during the 1964 Niigata

earthquake (Fukuoka 1966)

6.3 Case III- Landing Road Bridge, New Zealand ((Berrill et.al. 1997)

The Landing Road Bridge is located over the Whakatane River, New Zealand. The 18.3 m long 13 span

bridge is supported by 8 precast pretensioned 406mm square raked piles of length 10 m.The piles were driven

into the dense sands underlying the layer that liquefied in 1987.The abutments are supported by 5 piles on

the river side and 3 piles on the approach side without any approach slabs. The average axial load on each of

the piles was determined to be 310 kN.

Figure 6.3: Schematic diagram of Landing Road Bridge (Berrill et al. 2001)

Figure 6.4: Potential Collapse Mechanism for Landing Load Bridge (Keenan, 1996)

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The bridge was subjected to lateral spreading during the Edgecumbe Earthquake of 1987 and cracks were

developed in the ground and sand bowls were observed. The bridge superstructure was restrained against

movement caused by lateral spreading but the piles underwent displacement without formation of plastic

hinges.

6.4 Case IV- South Brighton Bridge, New Zealand ,Darfield earthquake 2010 and Christchurch

earthquake 2011

Both the approach embankments of the South Brighton Bridge, which were built over wetlands, developed

severe cracking and settlements in both the Darfield and Christchurch earthquakes. Lateral spreading resulted

in back-rotation of the abutment by approximately 4°, with evidence of plastic hinging in the abutment piles

and cracking of the abutment. This damage was exasperated following the Christchurch earthquake, with

additional lateral spreading further damaging the piles and abutment, and increasing the rotation by 3°. The

rigid beam-deck practically prevented any displacement in the longitudinal direction of the bridge, which

resulted in deck-pinning and consequent back rotation of the abutments about the beam abutment joint. This

back rotation of the abutments in turn caused damage at the top of the abutment. The large lateral

displacements at the pile tops, in conjunction with the rotational constraints imposed by the rigid pile-

abutment connection, caused bending of the piles that resulted in tensile cracking on the river side

Figure 6.5: South Brighton Bridge showing typical spreading-induced deformation/damage mechanism

for short-span (stiff deck) bridge representing the side view of the bridge

and concrete crushing/spalling on the compressed land side of the piles. Minor flexural cracking developed in

the central piers as a result of transverse inertial movement of the superstructure .A large distress in the

foundation soils surrounding the piles with ground cracks, fissures, and slumping toward the river were

indicative of permanent spreading displacement of the foundation soils toward the river. There wasn ’t any

serious damage to the bridge superstructure, though some relative movement was apparent in the dislocated

bearings. Following the temporary repair of the approaches and infilling of the offsets between the approaches

and the deck, the bridge was back in service practically immediately after each earthquake event.

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Figure 6.6 (a): The bending cracks at the top of the pile (river side) and 6.6 (b): Back rotation of the

abutment due to deck (girder)-pinning

FIGURE 6.7: Schematic illustration of the characteristic spreading-induced deformation (damage)

mechanism of short-span bridges involving deck pinning with consequent back rotation of

abutments, damage to abutment piles, and slumping of the approaches.

7 REMEDIATION USING GROUND IMPROVEMENT

In order to mitigate the potential for damage to a bridge due to one or several of these mechanisms,

improvement measures must be implemented to counter the development of failure and to limit movements.

Ground improvement methods that might be considered for this purpose are discussed below

7.1 Applicability of liquefaction mitigation to different bridge types

The applicability of using different ground improvement methods for remediating liquefaction effects at

existing highway bridges are depends on the space and geometry limitations of the bridge site affecting

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accessibility and working space for construction equipment, subsurface conditions affecting the effectiveness

of a particular method in producing the required improvement, potential for construction- induced movements

and vibrations of the bridge caused by the remediation method along with the likelihood of resulting damage,

the desired post-treatment performance of the bridge, the potential environmental effects of improvement

implementation, and the cost of the improvement method relative to other methods

7.2 Soil Improvement categories and methods

When existing subsurface conditions introduce significant seismic hazards that adversely affect safety or

impact construction costs, improved performance may be achieved through a program of soil improvement.

There are a variety of ground treatment methods available for improving the properties of liquefiable soils

in order to reduce the potential for earthquake-induced liquefaction and associated damage. A variety of

techniques are available for soil improvement and may be divided into four main categories: densification,

drainage, reinforcement, and grouting/mixing. Each soil improvement technique has advantages and

disadvantages that influence the cost and effectiveness under different circumstances. Descriptions of the

principles behind the improvement mechanism associated with each category are provided in Table 2.1

(Cooke and Mitchell, 1999)

7.2.1Densification techniques

The densification techniques are most efficient in improving the mechanical properties of sands and gravels

Vibratory densification of large volumes of soil in underdeveloped areas can be accomplished most

economically by dynamic compaction. Vibrations from probes that penetrate below the ground surface have

also proved to be effective for densification. Vibroflotation, for example, is accomplished by lowering a

vibrating probe into the ground (with the aid of water jets, in some cases). By vibrating the probe as it is

pulled back toward the surface, a column of densified soil surrounding the vibroflot is produced. Blast

densification of cohesion less soils is usually accomplished by detonating multiple explosive charges spaced

vertically at different elevations. Compaction grouting involves the injection of very low slump (usually less

than 25 mm) cement grout into the soil under high pressure. The grout forms an intact bulb or column that

densifies the surrounding soil by displacement.

7.2.2 Drainage techniques

Excessive soil settlements and foundation movements can often be eliminated by lowering the groundwater

table by using dewatering technique. The build-up of high pore water pressures in liquefiable soils can also

be suppressed using drainage techniques, although drainage alone is rarely relied upon for mitigation of

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liquefaction hazards. Stone columns provide means for rapid drainage by horizontal flow, but also improve

the soil by densification (during installation) and reinforcement.

7.2.3 Reinforcement techniques

The strength and stiffness of some soil deposits can be improved by installing discrete inclusions that

reinforce the soil. Stone columns reinforce the soil in which they are installed. Granular soils can also be

improved by the installation of compaction piles. Drilled shafts or drilled piers have been used to stabilize

many slopes, although the difficulty in drilling through loose granular soils limits their usefulness for slopes

with liquefiable soils. Soil nails, tiebacks, micro piles, and root piles have also been used.

TABLE 7.1: Categories of Ground Improvement Methods for Liquefaction Mitigation at Existing Bridges

Improvement

MechanismPrinciple

Potential Improvement

Methods

DensificationSoil particles moved into tighter configurationincreasing density

Compaction grouting

Vibro-systems

Drainage

High permeability drainage elements installed todecrease drainage distance in soil mass limiting

development, and providing faster dissipation, of

excess pore water pressures.

Gravel and sand drains

Vibro-replacement

Reinforcementand

containment

Soil mass reinforced with stiff elements used to

provide additional shear resistance. Whenelements are overlapped and arranged to form

enclosed areas, containment also provided.

Mixed-in-place columns

and walls ,Jet groutingVibro-replacement

Root piles

Cementation

Soil particles bound together by filling voidswith cementing material

Particulate grouting

Chemical grouting

Jet grouting

7.2.4 Grouting/mixing techniques

Grouting involves injection of cementitious materials into the voids or fractures in the soil to strengthen thesoil structure. In mixing, the cementitious materials are mechanically or hydraulically mixed into the soil,

completely destroying the initial particle structure.

Permeation grouting involves the injection of low-viscosity grouts into the voids of the soil without disturbing

the particle structure. Both particulate grouts (aqueous suspensions of cement, fly ash, bentonite, micro fine

cement, etc.) and chemical grouts (silica and lignin gels, or phenolic and acrylic resins) may be used for

coarse and fine grained sands respectively.

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8 CONCLUSIONS

Damage to piles due to liquefaction has occurred in previous earthquakes resulting in damage to many bridges

and buildings. Previous research has examined the characteristics and failure mechanisms of piles in

liquefiable soils by documenting case histories, conducting experimental tests and developing analytical

models. The key conclusions include the following; the cyclic phase and lateral spreading are two distinct

phases in the seismic response of piles in liquefiable soil. For both cases the pile behaviour depends on the

stiffness of the pile relative to the liquefied soil. Relatively flexible piles move with the ground; whereas

relatively stiff piles resist the ground movement. In the cyclic phase, large cyclic ground displacements and

inertial loads occur. Piles suffered damage at the pile head and at the interface between liquefied and

nonliquefied soil layers. In the lateral spreading phase the soil has lost stiffness and large unilateral ground

displacements occur. Inertial loads are small during this phase, and damage is again concentrated at the

interface between liquefied and non-liquefied soil layers. During lateral spreading stiff piles can attract large

loads from overlying non liquefied layers, these forces are much larger than drag forces from the liquefied

soil. The nature of lateral spreading causes different lateral ground displacements to be applied to different

piles connected at the same pile cap. This causes different lateral loads on the piles, resulting in distinct

damage features depending on the location of the pile in the group. Pile head fixity has an important role;

fixed head piles suffered damage at the pile head, pinned head piles did not. The stiffness of the pile cap also

affects the pile behaviour.

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REFERENCES

1. Vesic, Aleksandar Sedmak.,(1977), Design Of Pile Foundations. Washington, Transportation

Research Board, National Research Council

2. Cubrinovski, M. and Ishihara, K. (2004). “Simplified method for analysis of piles undergoing lateral

spreading in liquefied soils.” Journal of Soils and Foundations 44:5, 119-133

3. Hamada, M. and O’Rourke, T. (1992). “Case studies of liquefaction and lifeline performance during

past earthquakes”, Japanese Case Studies Technical Report , National Center for Earthquake

Engineering Research, Volume 1,Buffalo, New York

4. Berrill, J.B. et al (1997), “Lateral-Spreading loads on a piled bridge foundation”, Journal for Seismic

behavior of ground and Geotechnical Structures., 512-542

5. Cubrinovski, M.; Ishihara. K. (2005), “Simplified Method for Analysis of Piles Undergoing Lateral

Spreading in Liquefied Soils” , Japanese Geotechnical Society Publication, 401-420

6. Keenan, Richard P., (1996),”Foundation loads due to lateral spreading at the Landing Road Bridge,

Whak atane”, Journal of Geotechnical Engineering , 39-47.

7. Kramer, Steven.L (1996), Geotechnical Earthquake Engineering , Prentice Hall

8. Seed, H.B., (1979), “ Soil Liquefaction and cyclic mobility evaluation for level ground during

earthquakes” , Journal of Geotechnical Engineering Division, 105, 201-255.

9. Zhang, J., Huo, Y., Brandenberg, S., and Kashighandi, P. (2008). "Effects o f structural

characterizations on fragility functions of bridges subject to seismic shaking and lateral spreading."

Earthquake Engineering and Engineering Vibration, 369-382.

10. Beaty, M. (2012). "Liquefaction Effects on Piled Bridge Abutments: Centrifuge Tests and

Numerical Analyses." Journal of Geotechnical and Geoenvironmental Engineering , 433.

11. Madabhushi et al., (2010), Design of Pile Foundations in Liquefiable Soils, Imperial College Press,

102-208

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