development of an approximate nonlinear analysis of piled raft foundations [presentation] (song,...

7/27/2019 Development Of An Approximate Nonlinear Analysis Of Piled Raft Foundations [Presentation] (Song, 2008).ppt

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Development of an Approximate

Nonlinear Analysis of Piled RaftFoundations

2008.04.02

Myung Jun Song

Ph.D. Candidate

Geotechnical & Geoenvironmental Engineering LabSeoul National University


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Contents

Introduction Approximate Nonlinear Analysis

Modeling

Pile-Soil Interaction

Pile-Soil-Pile Interaction

Raft-Soil-Pile interaction

Evaluations

Comparison with 3D FEM analysis

Conclusions

Further Study

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Design Philosophies of Piled Rafts

Conventional Pile Design Method

Disregard of the capacity of raft Increase the number of piles or length of piles

Very small allowable settlement

Piled Raft Design Method

Design for fully utilization of pile capacity Settlement Reducing Pile

Design for the calculation of settlement

Consideration of Complex Soil-StructureInteraction(Pile-Raft-Soil)

Consideration of the optimal location of piles to

decrease the differential settlement and bendingmoment of raft

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Design Philosophies of Piled Rafts

Curve 0 : raft only(settlement excessive)

Curve 1 : raft with piles designed forconventional safety factor

Curve 2 : raft with piles designed for lower

safety factor

Curve 3 : raft with piles designed for fullyutilization of capacity

Loadsettlement curves for piled rafts (Poulos, 1997)

Increasing number of piles

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Application of Piled Raft Foundations to Civil Structure

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Pile-Soil Interaction

Kondner(1963)

wp

qpbu

kpb

qpba

qpsu(i)

qpsa(i)

kps(i)

pba

p

pb

p

pb

q

w

K

wq

1

)()(

)(1

ipsa

p

ips

p

ips

q

w

K

wq

wp

wp : settlementqpb : unit end bearing

kpb : initial stiffness of toe(Randolph & Worth, 1978)qpba = qpbu /Rf : an asymptote of qpbqpbu : ultimate unit end bearingRf : reduction factor

qps(i) : unit skin friction at element ikps(i) : initial stiffness of skin at element iqpsa(i) = qpsu(i)/Rf : an asymptote of qps(i)qpsu(i) : ultimate unit skin friction at element i

End bearing Skin friction

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Apparent stiffness reduction due to ground settlement generated by

pile settlement(Randolph & Worth, 1979)

r

pw s

w

mpm

s

ps

s rrrrr

Grrw

,ln

ms rrrw ,0

psp rww

)( psp rww

ws(r)ws(rp)

wslipwp=ws(rp)+wslip

No interface slip Interface slip

pfeslippps wRwwrw )(

p

ps

p

slipp

few

rw

w

wwR

)(

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i j

jir,

iwp iiwp ,

,,p s i jw i j w r

)(irw ps

iwslip

)( jrw ps

jwslip

Superposition of settlement with the effects of the adjacent

piles settlement makes apparent stiffness reduction in the grouppile.

The slip of pile does not affect to adjacent piles settlement.

)()())((),(),()( , jisslippsppp rwiwirwjiwiiwiw

)())((

),(),()( , jisfe

ps

ppp rwR

irwjiwiiwiw

ni

mp

i

mp

i

mp

fe

pppp

r

nrnrn

rrr

rrr

R

niwiwiwiw

,

2,1,

)(ln)()(

)2(ln)2()2()1(ln)1()1(1

),()2,()1,()(

)(

)2(

)1(

)(ln

)()2(ln)2(

)1(ln)1(

)(ln)(

)2(ln

)2()1(ln)1(

)(ln)(

)2(ln)2(

)1(ln

)1(

)(

)2(

)1(

,2,1,

,22,21,2

,12,11,1

n

r

nr

R

nr

r

rr

r

rr

r

nrnr

r

r

R

r

r

rr

r

nrnr

r

rr

r

r

R

r

nw

w

w

nn

m

fe

p

n

mp

n

mp

n

mp

m

fe

pmp

n

mp

mp

m

fe

p

p

p

p

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Raft-Soil-Pile Interaction

Apparent stiffness reduction of pile by raft

Relative settlement for the calculation of pile reaction

)()()( iwiwiw spnet

)1(sw

)2(s

w

)3(sw

)4(sw

)0(sw

0

1

2

3

4

wp

0)0( netw

)1(netw

)2(netw

)3(netw

)4(netw

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Apparent stiffness reduction of soil spring in raft by piles

ws,raftnode i node i

pile j

ws,pile

+

ws,piled raft(i)= ws,raft (i)+ ws,pile(i)

node i

)(

)()(

,

,iw

iQik

rafts

rafts

)()(

)(

)(

)()(

,,,

,iwiw

iQ

iw

iQik

pilesraftsraftpileds

raftpileds

)()()(

)()( ,

,,

,

, ikiwiw

iwik rafts

pilesrafts

rafts

raftpileds

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Stiffness hardening of piles by raft would be considered by

increasing the effective stress and unit skin friction

),(),(),( ''' zizizi vviv

k

),( ziv

q

pile i

zs

q

w

psuq

psuq

qpsu : ultimate unit skin friction

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Raft-Soil-Pile Interaction reduced scale test

Set-up of Test Piles Comparison of Load -Settlement Curves

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C i i h 3D FEM A l i14/24


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Comparison with 3D FEM Analysis(PLAXIS 3D Foundation)

Example models for the evaluation of developed analysis program

0

2107

3650

4712

5576

6322

6989

7598

8162

8689

9186

9657

10107

1

2

3

4

5

6

7

8

10

11

9

z(m) Es(kPa)

12

= 18kN/m3

c = 0

= 30

Er= 30,000,000 kPa

Br= 6m

Lr= 6m

tr = 1.2m

Ep = 30,000,000 kPa

Dp = 0.5m

Lp = 10m

np = 33

spacing = 2m

Single Raft Single PilePiled Raft

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Comparison with 3D FEM Analysis

3D FEM mesh model for piled raft analysis

Soil model raft and pile model

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Analysis results of raft foundation without piles

0

20

40

60

80

100

0.0 1.0 2.0 3.0 4.0 5.0

Load(MN)

Settlement(mm)

Plaxis 3D

Present Study

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Analysis results of single pile

0

20

40

60

80

100

0 200 400 600 800 1000

Load(kN)

Settlement(mm

)

Plaxis 3D

Present stud y

0

20

40

60

80

100

0 200 400 600 800 1000

Load(kN)

Settlement(mm

)

Plaxis 3D

Present stud y

0

20

40

60

80

100

0 200 400 600 800 1000

Load(kN)

Settlement(mm

)

Plaxis 3D

Present stud y

Toe Skin Total

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Pile behaviors from the piled raft foundation analysis

Plaxis 3D This study

0

40

80

120

160

200

0 200 400 600 800 1000

Load(kN)

Settlement(mm)

Center

Edge

Corner

Single

0

40

80

120

160

200

0 200 400 600 800 1000

Load(kN)

Settlement(mm)

Center

EdgeCorner

Single

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Raft behaviors from the piled raft foundation analysis

0

20

40

60

80

100

0 1 2 3 4 5

Load(MN)

Settlement(mm)

Raft only (Plaxis3D)

Piled raft (Plaxis3D)

Raft only (present study)

Piled raft (present s tudy)

/

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Total behavior of piled raft

0

20

40

60

80

100

0 2 4 6 8 10

Load(MN)

Settlement(mm)

Plaxis3D

Present study

Piled raft coefficient

0

20

40

60

80

100

0 20 40 60 80 100

LoadonPiles(%)

Settlement(mm)

Plaxis 3D

Present Study

/

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Comparison with other approximate programs

Comparison with linear elastic analysis programs

0

20

40

60

80

100

0 2 4 6 8 10

Load(MN)

Settlement(mm)

Plaxis3D

Pile+RFEAR8.1

Present study

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Conclusions

An approximate method has been developed for the practical design

of piled raft foundations, which analyze non-linear behaviors and soil-structure interaction effects efficiently in multi-layered soils.

This method considers the apparent stiffness reduction in piles by theadjacent piles and raft and apparent stiffness reduction in raft by thepiles and generates very similar results to 3D FEM analysis.

The effect of stiffness hardening of pile is under estimated. So, itmakes conservative results in comparison with 3D FEM analysis.

The results of comparisons with 3D FEM analysis also show the

sufficient applicability to practical analysis and design of piled raftfoundations.

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Further Study

Intelligent soil spring

Linear spring for raft-soil interaction was applied in this study. Intelligent soil spring is needed to model true raft-soil interaction

and depend on the followings; the relative stiffness of the raft and soil

the plan size and shape of the foundation

the distribution of applied loading on the raft

Intelligent soil spring will be determined by the iterative process ofsoil spring reaction and ground settlement analysis.

Variable shape of raft foundation

Field test

Proto type field test Centrifuge test

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Thank you

development of an approximate nonlinear analysis of piled raft foundations [presentation] (song,...

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