Module 2 - Pile Group Effect [Compatibility Mode]

5th Apr 2013 Dr. S.Nallayarasu, Department of Ocean Engineering

IIT Madras Chennai-36.

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PILE GROUP EFFECTS



2

Pile group arrangements for Onshore Structures

Pile Cap



3

Pile group arrangements for 4 legged platform



4




5




6

STRESS DISTRIBUTION AROUND THE PILE



7

EFFECT ON VERTICAL AXIAL CAPACITY EFFECT ON LATERAL CAPACITY EFFECT ON LOAD DISPLACEMENT CURVES

T-Z EFFECTS Q-Z EFFECTS P-Y EFFECTS

PILE GROUP EFFECTS

Considerable research work on the effect of pile spacing on the axial andlateral capacity has been carried out. API RP 2A suggests that the pileswhen installed in a group in several geometry, when the distancebetween the piles are closure than 8D where D is the diameter of thepiles, the pile group effect shall be evaluated.

Following effects shall be evaluated



8

GROUP EFFECT ON VERTICAL AXIAL CAPACITY

The piles in offshore platforms such as jackets, piles are spaced at less than 8 D since the larger spacing will be difficult to arrange connect to jacket legs. It is a general practice to limit the spacing to 3D and evaluate the effects on the axial capacity.

Hence normally no degradation in axial capacity is required if the pile spacing is greater than 3D. If the spacing is less than 3D, then the efficiency of pile group can be evaluated using the following approach.

Axial Capacity of a pile in a group = Axial Capacity of individual pile x

whereUltimate load capacity of groupsum of ultimate load capacities of

individual piles

=



9

METHOD BY LABARRE FORMULA

1 11 / 90

n m m nmn

Where m=number of rowsn=number of piles in a row= tan-1( D/S), in degreesD=pile diameterS=center-to-center spacing

of piles



10

METHOD BY TERAZAGHI AND PECKIn order to obtain a more realistic estimate of the ultimate load capacity of a group, the following empirical relationship is suggested:

2 2 2 21

1 1 1

u BP n P P

WherePu=ultimate load capacity of groupP1=ultimate load capacity of single pilen=number of piles in groupPB=ultimate load capacity of block

2 21

2 21 1

B

n PP

Where

=group efficiency

The above equation may be re expressed as follows



11

Effect on Axial Load and Displacement

The effect of axial load on adjacent pile is taken into consideration in the following form

Reduced axial capacity (Static)

Increased deflection (t-Z)

Increased deflection can be calculated using method proposed by Randolf based on empirical formula as a Z multiplier.

The increased deflection is calculated as

z=z zmWhere

(1+v) is called zm multiplier.



12

INTERACTION DUE TO AXIAL LOAD



13

EFFECT ON AXIAL LOAD - DEFLECTION (t-Z) CURVE

Groups effects in axial load for load displacement behaviour can be incorporated as increased deflection due to load from adjacent pile by a multiplier called z multiplier

1m vz Where v depends on spacing and diameter of piles.

Dr. S.Nallayarasu, Department of Ocean Engineering


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S < 3D S > 3D

TWO PILE GROUP AND FAILURE ZONE

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15

THREE PILE GROUP AND FAILURE ZONE

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16

22S D

D

2

2

SD

TWO PILE GROUP EFFICIENCY FOR AXIAL LOAD

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CALCULATION OF AZIAL INTRACTION FACTOR FOR (t-Z)

Length of pile = L

Poissons ratio of soil =

Modulus of soil = soil

4

..

p pR

soil

E IK

E L

Modulus of steel = p

Ratio of soil modulus at L/2 to the soil modulus L

at L/2

at L

soil

soil

EE

Relative stiffness

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18

1 11 (1 ) .1

1 (1 ). .v

1m vz

Interaction Factor

LD

CALCULATION OF AZIAL INTRACTION FACTOR FOR (t-Z)

2.5(1 )m L

ln 2. mD

2

ln 2 mD

zm multiplier

Where

Relative length and Spacing

Interaction Factor Displacement of pile due load on the adjacent pileDisplacement of pile due to its loadv

SD



19

Effect of Spacing ratio on v

v

S/DD/S

L/D = 10

= 0.5KR



20

v


S/D D/S

L/D = 25

= 0.5

KR



21


v

L/D = 50

= 0.5

S/D

D/S

KR



22

v


S/D

D/S

L/D = 100

= 0.5

KR



23

EFFECT Es on Pile settlement

v

L/D = 50

= 0.5

h/L =

S/DD/S



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T-Z Curve with T Multiplier for End Bearing

displacement

Cap

acity



25

t-Z Curve with t Multiplier for Skin friction



26

t-Z Curve with z Multiplier for Skin friction

zm

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27

t-Z Curve with z Multiplier for End Bearing

zm

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28

Effect on Lateral Load Capacity and Displacement

The effect of Lateral Capacity on adjacent pile is taken into consideration in the following form

Reduced Lateral capacity (Static)

Increased Lateral deflection (p-y)

Increased deflection can be calculated using method proposed by Poulos based on empirical formula as a y multiplier.

The increased deflection is calculated as

y=y ymWhere

(1+h) is called ym multiplier.



29

GROUP EFFECT ON LATERAL LOAD CAPACITY

1

k

i j hij ij

y y P P

Lateral Deflection of a pile in a group

Where= horizontal displacement of single pile due to unit applied horizontal load

yi = horizontal displacement of pile iPj = horizontal load on pile jhij = y multiplier factorPi = horizontal load on pile i

y



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Lateral deflection of a pile in a group due to loads on other piles plus load on that pile can be expressed as



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y Multipliers p-y curveym

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32

Establishment of Esoil value for layered soil

The above chart can be developed by any software with Pile Soil Interaction facility

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Establishment of Esoil value for layered soil

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4. Esoil value can be back calculated using for the lateral load of L, IpF, Phand h

4

..

p pR

soil

E IK

E L

.p F h

soilh

I PE

L

1. Initial value of Esoil can be obtained from vertical load deflection curve

2. Calculate the KR value using

3. Obtain IpF from the chart for the given value of L/d and KR

5. Recalculate the value of KR 4..

p pR

soil

E IK

E L

6. Read chart to obtain ah the value of KR and

Using chart to obtain h

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35

Use of ChartsAdditional displacement caused by adjacent pile

Displacement of pile caused by its own loadingh=

Additional rotation caused by adjacent pile

Rotation of pile caused by its own loading=

hp, p = values of h and for a free-head pile subjected to horizontal load

hm, m = values of h and for a free-head pile subjected to moment

hF = value of h for fixed headed pileIhF = Influence factor for fixed headed pile

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hF

KR

Influence Factor

Interaction factor h for Fixed Headed Pile

hF hF

S/D D/SS/D

D/S

= 0.5 = 0 = 90

= 0.5 = 0 = 90



36

Interaction factor h for Fixed Headed Pile

hFhF

S/DD/S

S/DD/S

= 0.5 = 0 = 90

= 0.5 = 0 = 90



37

Interaction factor hp for Free Headed Piles

h

S/D D/SS/D

D/S

= 0.5 = 0 = 90

= 0.5 = 0 = 90



38

hp hp

Interaction factor hp for Free Headed Piles

S/DD/S

S/DD/S

= 0.5 = 0 = 90

= 0.5 = 0 = 90



39

hp hp

Interaction factor hm and m for Free Headed Piles

S/DD/S

S/DD/S

= 0.5 = 0 = 90 = 0.5

= 0 = 90



40

hm&m

hm&m

Interaction factor hm and m for Free Headed Piles

S/DD/S

S/DD/S

= 0.5 = 0 = 90

= 0.5 = 0 = 90



41

hm&m

hm&m

Interaction factor m for Free Headed Piles

S/DD/S

S/DD/S

= 0.5 = 0 = 90

= 0.5 = 0 = 90



42

mm

Interaction factor m for Free Headed Piles

S/D D/S S/D D/S

= 0.5 = 0 = 90

= 0.5 = 0 = 90



43

mm



44

CALCULATION OF PILE GROUP EFFECT BY RANDOLF & WORTH METHOD

Young's Modulus of pile EP 2 105 MPa

Diameter/thickness of pile D 2134 mm tp 50 mm

Length of pile below mud-level Lp 85 m

Pile spacing S 4.38 m

Poisson's ratio 0.5

Modulus ratio 0.5 (Linear modulus)

is the ratio of EsL/2/ EsL (a measure of the in-homogenity of the soil profile along the pile shaft = 1 for a constant modulus profile = 0.5 for a linear distribution of modulus with depthwhere EsL is the soil modulus at the pile tip & EsL/2 is the soil modulus at depth of L/2

AP4

D2 D 2 tp 2 Pile area AP 0.33m2

Moment of Inertia of pile IP64

D4 D 2 tp 4 IP 0.18m4

LpD

SD

1

Parameters

39.83 2.05 0.49

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A) AXIAL DEFORMATION OF PILE BY RANDOLF

m 2.5 1 ( ) LpVarious Factors m 53.13m

ln 2 mD

3.91

ln 2 m

2

D m

7.88

Interaction Factor for axial v

1S

D

S 1 ( )

1

1

1 1 ( )

v 0.463

zm 1 v zm 1.46

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46

B) LATERAL DEFORMATION OF PILE BY POULOS

Soil Modulus calculation by pile settlement with SACS

Initial value of Esoil obtained from vertical load displacementrelationship

Esoil 605712kN

m2

Lateral pile load from Inplace analysis Ph 1.7 MN

Pile Lateral displacement from SACS single pile analysis h 9.923 cm

Initial vaslue of KR KREP IP

Esoil Lp4

KR 1.12 106

Influence factor obtained from chart IPF 18

EsoilIPF Ph

h LpRevised value of Esoil Esoil 3.63 MPa

KREP IP

Esoil Lp4

Revised value of KR KR 2 104

Departure angle is the angle between the line joining the pile centers and the direction of loadingrefer figure above

Departure angle 45 deg

With , (L/d), KR, and (S/d) values, Interaction factor is read from the graph h 0.458

ym 1 h ym 1.46

5th Apr 2013

Module 2 - Pile Group Effect [Compatibility Mode]

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Transcript of Module 2 - Pile Group Effect [Compatibility Mode]