Seminar Piled Raft Foundation

Development of an Approximate Development of an Approximate Nonlinear Analysis of Piled Raft Nonlinear Analysis of Piled Raft FoundationsFoundations

2008.04.022008.04.02

Myung Jun SongMyung Jun Song

Ph.D. CandidatePh.D. Candidate

Geotechnical & Geoenvironmental Engineering LabGeotechnical & Geoenvironmental Engineering Lab

Seoul National UniversitySeoul National University

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

2/24

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-Structure

Interaction(Pile-Raft-Soil) Consideration of the optimal location of

piles to decrease the differential settlement and bending moment of raft

3/24

Design Philosophies of Piled Rafts

Curve 0 : raft only(settlement excessive)

Curve 1 : raft with piles designed for conventional safety factor

Curve 2 : raft with piles designed for lower safety factor

Curve 3 : raft with piles designed for fully utilization of capacity

Load –settlement curves for piled rafts (Poulos, 1997)

Increasing number of piles

4/24

Application of Piled Raft Foundations to Civil Structure

5/24

Modeling Summary6/24

Linear elastic spring for raft-soil interaction

Nonlinear behavior of pile

Applicable to multi-layered ground

Description of apparent stiffness reduction phenomena

Description of stiffness hardening phenomena

q : Distributed Load

Q : Point Load

Raft

Soil Spring

w

Qs

Shaft Resistance

Qpsi

wnet

Qpb

wnet

Base Resistance

Iteration

for

CompatibilityQp: Pile Load

Qp: Pile Load

Raft analysis on the spring Pile group behavior analysis

Pile-Soil Interaction

Kondner(1963)

wp

qpbu

kpb

qpba

qpsu(i)

qpsa(i)

kps(i)

pba

p

pb

ppb

q

w

K

wq

1

)()(

)( 1

ipsa

p

ips

pips

q

w

K

wq

wp

wp : settlement qpb : unit end bearing kpb : initial stiffness of toe(Randolph & Worth, 1978)qpba = qpbu /Rf : an asymptote of qpb

qpbu : 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

7/24

Pile-Soil-Pile Interaction

Apparent stiffness reduction due to ground settlement generated by pile settlement(Randolph & Worth, 1979)

r

pw sw

mpm

s

pss rrr

r

r

G

rrw

,ln

ms rrrw ,0

psp rww

)( psp rww

ws(r)ws(rp)

wslip

wp=ws(rp)+wslip

No interface slip Interface slip

pfeslippps wRwwrw )(

p

ps

p

slippfe w

rw

w

wwR

)(

8/24

Pile-Soil-Pile Interaction

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 pile’s settlement makes apparent stiffness reduction in the group pile.

The slip of pile does not affect to adjacent pile’s settlement.

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

)())((

),(),()( , jisfe

psppp rw

R

irwjiwiiwiw

ni

mp

i

mp

i

mp

fe

pppp

r

nrnrn

r

rr

r

rr

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

9/24

Raft-Soil-Pile Interaction Apparent stiffness reduction of pile by raft

Relative settlement for the calculation of pile reaction

)()()( iwiwiw spnet

)1(sw

)2(sw

)3(sw

)4(sw

)0(sw

0

1

2

3

4

wp

0)0( netw

)1(netw

)2(netw

)3(netw

)4(netw

10/24

Raft-Soil-Pile Interaction

Apparent stiffness reduction of soil spring in raft by piles

ws,raft

node i node i

pile j

ws,pile

+

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

node i

)(

)()(

,, iw

iQik

raftsrafts

)()(

)(

)(

)()(

,,,, iwiw

iQ

iw

iQik

pilesraftsraftpiledsraftpileds

)()()(

)()( ,

,,

,, ik

iwiw

iwik rafts

pilesrafts

raftsraftpileds

11/24

Raft-Soil-Pile Interaction

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

zsq

w

psuq

psuq

qpsu : ultimate unit skin friction

12/24

Raft-Soil-Pile Interaction – reduced scale test

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

13/24

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

Example models for the evaluation of developed analysis program

02107

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

γ= 18kN/m3

c = 0φ = 30°

Er = 30,000,000 kPaBr = 6mLr = 6mtr = 1.2m

Ep = 30,000,000 kPaDp = 0.5mLp = 10m

np = 3×3spacing = 2m

Single Raft Single PilePiled Raft

14/24

Comparison with 3D FEM Analysis

3D FEM mesh model for piled raft analysis

Soil model raft and pile model

15/24


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)

Set

tlem

ent(

mm

)Plaxis 3D

Present Study

16/24


Analysis results of single pile

0

20

40

60

80

100

0 200 400 600 800 1000

Load(kN)

Set

tlem

ent(

mm

)

Plaxis 3D

Present study

0

20

40

60

80

100

0 200 400 600 800 1000

Load(kN)

Set

tlem

ent(

mm

)

Plaxis 3D

Present study

0

20

40

60

80

100

0 200 400 600 800 1000

Load(kN)

Set

tlem

ent(

mm

)

Plaxis 3D

Present study

Toe Skin Total

17/24


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)

Set

tlem

ent(

mm

)

CenterEdge

CornerSingle

0

40

80

120

160

200

0 200 400 600 800 1000

Load(kN)

Set

tlem

ent(

mm

)

CenterEdgeCornerSingle

18/24


Raft behaviors from the piled raft foundation analysis

0

20

40

60

80

100

0 1 2 3 4 5

Load(MN)

Set

tlem

ent(

mm

)Raft only (Plaxis3D)Piled raft (Plaxis3D)

Raft only (present study)Piled raft (present study)

19/24


Total behavior of piled raft

0

20

40

60

80

100

0 2 4 6 8 10

Load(MN)

Set

tlem

ent(

mm

)

Plaxis3D

Present study

Piled raft coefficient

0

20

40

60

80

100

0 20 40 60 80 100 L

oad

on P

iles(

%)

Settlement(mm)

Plaxis 3D

Present Study

20/24

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)S

ettle

men

t(m

m)

Plaxis3D

Pile+RFEAR8.1

Present study

21/24

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 the adjacent piles and raft and apparent stiffness reduction in raft by the piles and generates very similar results to 3D FEM analysis.

The effect of stiffness hardening of pile is under estimated. So, it makes 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 raft foundations.

22/24

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 of soil spring reaction and ground settlement analysis.

Variable shape of raft foundation Field test

Proto type field test Centrifuge test

23/24

Thank youThank you

24/24

Seminar Piled Raft Foundation

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