Seminar Piled Raft Foundation
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Transcript of 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
Comparison with 3D FEM Analysis
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
Comparison with 3D FEM Analysis
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
Comparison with 3D FEM Analysis
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
Comparison with 3D FEM Analysis
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
Comparison with 3D FEM Analysis
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