Post on 16-Mar-2018
transcript
10/04/2014
1
© Imperial College London Page 1
Recent developments in axial design
of driven offshore piles
DGF Copenhagen
April 1st 2014
Richard Jardine
© Imperial College London Page 2
Themes:
• Changed API, ISO axial capacity recommendations
for sands, including ICP-05
• Research background leading to new methods
• Applications, case histories, some surprising results
• New research: ageing, cyclic and lateral loading
• Next set of issues: clay methods & improving load-
displacement predictions
10/04/2014
2
© Imperial College London Page 3
New rules for static axial capacity in silica
sand: 2011 API recommendations
• Before: had τ = K σ´vo tanδ for shaft, qb = Nq σ´vo for base. Limits to τ, qb and values for δ, Nq depend on D50 and Dr
• Now: modified to β = K tanδ, no loose sand case
• Recognises: API main text method’s significant bias and poor reliability:
Qcalculated /Qmeasured ratios = Qc/Qm subject to CoV ~ 65%
• Recommend: completely different ‘CPT’ based methods, including ICP. Note need for different SI & specialist staff
© Imperial College London Page 4
The axial capacity prize:
Feedback from one UK
wind-farm design team
Critical economies through
ICP-05 or UWA-05 sand
axial capacity methods
Also applied in onshore civil
engineering: Williams et al 1997
Derived from field ‘ICP’ research
Lehane et al 1993
Chow 1997
Jardine et al 2005
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© Imperial College London Page 5
Research background:
Critique of conventional approach &
how new ‘CPT’ methods were derived
© Imperial College London Page 6
MTD and ICP methods
First proposed in 1996,
extended in 2005 to cover:
Group action
Pile shape
Seismic effects
‘Special and problem’ soils
Factors of safety
Ring shear test methodology
Ageing
Cyclic loading
Databases: mini-to-mega piles
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© Imperial College London Page 7
Shaft capacities from 81 tests in sand; Jardine et al (2005)
0 10 20 30 40 50 60 70 80 90 100
Relative density, Dr (%)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Qc /Q
m
DK
EU
EU EUEU
H
H
BD
GDK
EU EU EUEU
H
SP
SP
SP
Pile ty pe & test direction
Steel, closed-ended, tension
Steel, closed-ended, compression
Concrete, closed-ended, tension
Concrete, closed-ended, compression
Steel, open-ended, tension
Steel, open-ended, compression
Concrete, open-ended, tension
0 20 40 60 80 100
L/D
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Qc /Q
m
DK
EU
EU EUEU
H
H
BD
DK
HOEU EU EUEU
H
SP
SP
SP
0 10 20 30 40 50 60 70 80 90 100
Relative density, Dr (%)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Qc /Q
m
DKEU
EU
EU
EUH
HBD
DK
EU
EU
EU
EUEUEU
DK
HSP
SP
SP
0 10 20 30 40 50 60 70 80 90 100
L/D
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Qc /Q
m
DKEU
EU
EU
EUH
HBDEU
EU
EU
EUEUEU
DK
HSP
SP
SP
API: skewed for relative density, pile length and tension loading
API API
ICP ICP
Pile failures at
Hound Point and
Sungai Perak Bridge
Williams et al (1997)
ICP shows no skewing (tension solid) Average pile age = 25 days
© Imperial College London Page 8
Critical review of ‘conventional theory’
• What controls shear (τ) and normal stress σ΄rf at failure?
• How does σ΄rf vary with σ΄v0 and local Dr?
• What controls the friction angle δ?
• Any missing key variables?
• Is tension loading different to compression?
• Does ‘shallow foundation’ Nq apply to end-bearing?
• Do any limits apply to τ and end bearing pressure qb?
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© Imperial College London Page 9
Background IC
research with
instrumented piles
Closed-end, 102mm
OD; up to 20m long
SSTs measure local
σr and τ
Bond, Jardine and
Dalton (1991)
Intensive testing at 2
sand and 4 clay sites
0
1.0
2.0
3.0
4.0
Dis
tan
ce fro
m p
ile t
ip, h
(m
)
surface stress transducer
pore pressure probe
axial load cell
leadinginstrument cluster, h/R=8
followinginstrument cluster, h/R=27
trailinginstrument cluster, h/R=50
lagginginstrument cluster, h/R=72
ICP Configuration for Labenne tests, SW France
Definition of stresses and tip parameter - h
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Geotechnical profile: Labenne, after Lehane (1992)
Loose dune sand, including thin organic layer
Labenne end bearing
Measured base resistance qb and CPT qc
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Local s′r during
penetration at
Labenne
K not constant but
varies with:
Sand state - qc
Pile tip depth (h/R)
Effect of h/R
Combined effects of qc and h/R on σ´r
ICP tests at Labenne: Lehane et al (1993)
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Page 15
Denser North Sea Marine sand Dunkerque France,
Chow (1997)
Borehole log
Very dense, light brown, uniform, fine to
medium, subrounded SAND with occasional shell fragments (Hydraulic fill)
GWL
Dense with shell fragments
(Flandrian Sand)
Organic layer
Dense, green-brown and grey-brown,uniform fine to medium, subrounded
SAND with some shell fragments(Flandrian Sand)
Becoming very dense
Dep
th (
m)
CPT q (MPa) CPT f (kPa)C C
0 10 20 30 40 0 100 200 300 4000
2
4
6
8
10
12
14
16
18
20
22
24
© Imperial College London Page 16
Influence of CPT qc and pile tip position on σ΄r
Dunkerque, after Chow (1997)
API
For fixed depth σ΄r falls with h/R
σ΄r varies with qc
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© Imperial College London Page 17
Possible
causes for h/R
influence on
local stresses
along pile
length;
Chow (1997)
After Chow (1997)
Heave
Whip
Extreme
driving
cycles
Relaxation as
tip stress
concentration
moves away
‘Friction fatigue’?
Page 18
Load cycles
degrade shaft
capacity
Can recover with time
Not true ‘fatigue’
See recent keynotes
on cyclic design:
Jardine et al (2012)
Andersen et al (2013)
-0.2 0 0.2 0.4 0.6 0.8 1
Qaverage / Qmax static
0.2
0.4
0.6
0.8
1
Qcyclic
/ Q
max s
tatic
First failureCyclic failure after previous cyclic or static failureAged pile, no previous failureAged pile, after previous failure
13
31
345
24
27221
>200
12
41
10
20
50
100
200
400
1
Nf
>1000
2069
Datapoint number = N f> indicates unfailed by cycling
Field tests on 457mm OD,
19m long steel pipe piles, Dunkerque
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© Imperial College London Page 19
Delft University photoelastic particulate test rig
Tip installation stress focus and bearing failure
© Imperial College London Page 20
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Dunkerque: loading response & ICP effective
stress paths, similar patterns to Labenne
Base qb≈ qc not related
linearly to σ΄v0
σ΄r varies under load
Tension ≠ compression
Δσ΄rd = 2G δr/R
Dr affects response
through G
δcv not affected by Dr
δcv angles: sand-on-steel interface ring-shear tests
Ho et al (2010) Steel interface
Crushed sand
Intact sand
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10/04/2014
Interface shear δcv: silt-to-fine gravel,
Interfaces with roughness of industrial piles
Direct & ring shear: Barmpopoulos et al (2009), Ho et al (2010)
Direct shear trend
Ring shear
© Imperial College London Page 24
‘Full’ ICP design principles: closed ended piles
• Radial shaft stresses σ΄r = A qc (σ΄v0)a (h/R)b
• Loading to failure alters σ΄r by factor that varies with 1/R
• Differences in tension and compression responses
• At failure τf /σ΄r = tan δcv with δcv from interface lab tests
• Base capacity qb linked to CPT qc – diameter dependent
• No upper limits to τf or qb - care needed in variable profiles
How to deal with open ended piles?
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© Imperial College London Page 25
Generalisation of ICP
shaft expressions to
open ended piles
Choices considered by
Chow (1997)
B2: R* = (R2o – R2
i)0.5
© Imperial College London Page 26
Assessment by Chow (1997)
• Five hypotheses checked
with instrumented open-
ended (325mm)
Dunkerque pile
• B2 choice considered
most practical
• Re-checked against full
scale data base, adopted
for MTD-96
• Later UWA-05 follow
alternative A2 route
-200 -150 -100 -50 0 50 100 150 200
Peak shear stress (kPa)
0
2
4
6
8
10
12
Dep
th (
m)
0
2
4
6
8
10
12
Pile CST'89aC'89aPrediction
-200 -150 -100 -50 0 50 100 150 200
Peak shear stress (kPa)
0
2
4
6
8
10
12
Dep
th (
m)
0
2
4
6
8
10
12
Pile CST'89aC'89aPrediction
A2 – Scalar reduction based on IFR
B2 – h/R term revised with R*
defined by solid area of pipe pile
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© Imperial College London Page 27
‘Full’ ICP: checks for possible wall thickness ratio bias
Shaft capacity of open piles in sand: IC data base
0 10 20 30 40 50 60 70 80 90 100
D/t
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5Q
c/Q
m
Pile type & test direction
Steel, open-ended, tension
Steel, open-ended, compression
Concrete, open-ended, tension
© Imperial College London Page 28
Pile plugs and open-end bearing: diameter dependence
Plug qb falls with
diameter D
ICP qb/qc = f(D)
IFR rises
sharply with D
Because of
interface shear
scale effect
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© Imperial College London Page 29
Debate over new API/ISO recommendations
• Agreement: ‘CPT based’ methods offer great potential
• Debate over how to scale up from closed ended
model piles to full scale
• API (2011) cites four approaches: ICP, Fugro, NGI
and UWA
• What are the differences? Practical evidence that the
‘full’ ICP and other methods work?
© Imperial College London Page 30
2011 API commentary methods: NGI-05
‘Sliding triangle’ approach to capture h/R effects, z = depth
τ = z/ztip Patmospheric FD Fs Ft Fl Fm
z/ztip term not normalised by D or affected by Length L
F factors depend on: Dr; σ΄v0 ; loading sense; pile type
Base qb does not vary with D, τ unaffected by L/D
Terms fitted from NGI data base, checked against 28 high
quality load tests
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© Imperial College London Page 31
2011 API commentary methods
Fugro-05, Kolk et al (2005)
ICP shaft expressions, coefficients revised to fit 37 steel pipe piles (some non-silica sand, some repeat tests) τ varies with L/D
Interface dilation neglected
Different expressions for tension and compression
δ = 29o for all cases
Base capacity not affected by D
© Imperial College London Page 32
API commentary methods
UWA-05; Lehane et al (2005)
Re-working of ICP, adopting Chow Set A2 approach
σ΄v0 term dropped and (h/D) term models ‘friction fatigue’
model, with modified exponenent
σ΄rc = a qc (Ar)b (h/D)c
‘Effective area’ Ar depends on Incremental Filling Ratio
(IFR) to define stresses near tip τ varies with L/D
‘Full’ and more conservative ‘offshore’ variants
Choices for a, b, c checked against large database
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© Imperial College London Page 33
UWA-05 & ICP-05 end bearing for plugged piles vary with D
UWA qb/qc depends on Final IFR, which varies with D
Open ended piles in sand, taking D/t = 30
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 0.5 1 1.5 2 2.5 3
Outside diameter, m
qb
/qc
ICP-05
UWA-05
© Imperial College London Page 34
UWA data base study: Lehane et al (2005)
• 74 high quality tests after ≈ 30 days, silica sands, CPT profiles
• New CPT methods greatly reduce bias and scatter (CoVs)
• Overall: Mean Qc/Qm ± CoV
API-93 0.81 ± 0.67
NGI-05 1.11 ± 0.37
Fugro-04 1.11 ± 0.38
(full) ICP-05 0.95 ± 0.30
(full) UWA-05 0.97 ± 0.27 (measured IFRs)
• Similar data-base results to Jardine et al (2005)
New studies in hand: IC & ZJU, UWA and new NGI JIP
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© Imperial College London Page 35
Applications:
all with ‘full’ ICP approach
© Imperial College London Page 36
Sungai Perak, Western Malaysia; Williams et al 1997
Pile tests
1&2 3 4
Balanced cantilever bridge on 1.5m OD driven steel piles
API design 33m penetration had to be doubled after tests
Medium-dense
gravelly sands
Average results
API: Qc/Qm= 1.99
ICP: Qc/Qm= 1.10
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© Imperial College London Page 37
760mm OD heavily
instrumented steel
tubulars
Average results:
API: Qc/Qm= 0.58
ICP: Qc/Qm= 0.97
ICP predictions (thick
lines) fit tests at 3 L/D
values, no skew or bias
with L/D
Dense North Sea sand
EURIPIDES - Holland
Kolk et al (2005)
Page 38
Oil and gas: all N. Sea Shell’s installations since 1996
Now widespread wind energy applications
Piled tripods for Borkum West II
German N. Sea Merritt et al 2012
Overy 2007
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© Imperial College London Page 39
North Sea track record since 1996
• 13 installations of ICP designed foundations reported
Overy (2007)
• Encouraging correlations with driving SRDs and stress
wave matches in sand, clay and mixed profiles
• Substantial ± variations from conventional API, depending
on soil profile, pile details etc
• Engineering potential – facilitated new low-cost marginal
field options: Sayer & Overy (2007)
• Borkum West II wind-turbine tripods: see Merritt et al 2012
© Imperial College London Page 40
If conventional API is so unreliable, why are
offshore failures rarely reported?
• Almost no offshore static testing, limited driving monitoring
• Problems revealed by tests performed for ‘near-shore’ projects: Hound Point, Sungai Perak, Jamuna Bridge etc
• Systematic conservative bias in some conditions – such as very dense marine sands
• Unrecognised positive factors: shaft ageing characteristics in sand
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Page 41
Dunkerque
programme:
Dense marine sand
Eight steel pipe piles
457mm OD, 19m
Static & cyclic loading
9 days to 1 year after
driving
Jardine et al 2006
Jardine & Standing
2012
1st time tension tests at Dunkerque
235 days
81 days
9 days
Creep important at Q > 1MN
ICP capacity after ≈9 days
EoD shaft ≈ 0.63 ICP
Low driving base capacity
Ageing disrupted by pre-testing
Pile age after driving: a missing parameter
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© Imperial College London Page 43
New and ongoing research
Pile installation and stress system it creates
Ageing in laboratory and field
Cyclic axial loading
Layering and base capacity
Lateral loading response
Extending the field database
© Imperial College London Page 44
Long term calibration chamber tests in Grenoble
with new 36mm OD mini-ICP: Jardine et al (2009)
• 1.2 m ID, 1.5 m deep chamber
• On pile stress measurements
• Multiple soil stress cells installed in sand mass
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Experiments on NE34 Fontainebleu sand:
Yang et al (2010)
CPT cone resistance, qc
Critical depth
Shear zone
Crushing zone
Shear zone developed around the pile shaft,
Yang et al (2010)
Plan view Side aspect
Zone 1 material 0.5 to 1.5mm adheres to pile shaft
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Schematic
development of
Zones 1 to 3
Related to stress
regime in:
Crushing area
beneath tip
Degradation over
shaft length
1
2
3
Microscope images: progressive grain crushing
(a) Fresh sand (b) Zone 1 sand
(c) Zone 2 sand (d) Zone 3 sand
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Stresses in soil
mass during
penetration
(and at rest)
σ´r normalised
by CPT qc, %
Similar plots for
σ´θ and σ´z
Jardine et al (2013)
0.25
0.50
0.75
0.75
1.0
0.50
1.5
2.0
3.04.06.0
14
0.50
0.25
0 5 10 15 20-30
-20
-10
0
10
20
30
40
50
r / R
h /
R
0
4.0
8.0
12
16
20
0.500.75
1.0
1.5
2.0
2.0
1.5
3.0
1.0
1.0
4.06.0
1012
0.75
16
0 5 10-10
-5
0
5
10
r / Rh /
R
0
4.0
8.0
12
16
20
30
Local stress paths at Leading pile instrument
One cycle towards end of installation
0 50 100 150 200 250 300-150
-100
-50
0
50
100
150
o
2nd
P.T.
end point
She
ar
str
ess (
kP
a)
Radial stress (kPa)
start point
1st P.T.
o
peak load
(c)Peak load
Start of push
Unloading
End Point
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© Imperial College London Page 51
Interim conclusions from field & laboratory
• Driving analyses highly variable. Base capacity well below static estimates
• Shaft capacities build over time from low EoD values to far exceed ‘full’ ICP or UWA Qs estimates
• Installation stress regime promotes ageing, as may interface ‘crust’ and physiochemical effects
• Base resistance very sensitive to local variations – take lower bound CPT profile for qb design
• Limit qc to 100 MPa in North Sea sands, beware tip buckling
• Address cyclic loading in design & consider ICP clay method
© Imperial College London Page 52
ICP effective-stress clay approach:
• τ = σ΄rf tan δ, analogous to sand: σ΄rf/σ΄v0 = f (YSR, St, h/R*)
• Good predictions for ICP data base, reduces CoV and bias
• Applied since 1996, particularly in North Sea
• Needs different SI approach. Key issues, including low IP clays, debated at OSIG 2007
• Micro-fabric in shear zones is crucial, as in landslides. Altered by driving, promotes progressive failure
• Ring shear tests to measure δ; qb related directly to CPT qc
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© Imperial College London Page 53
Microscope thin section
through clay around piles at
Pentre; Chow (1997)
Residual shearing mode,
unusually low δ′ for given Ip
More common with plastic clays
Pile shaft
Principal displacement shears
Reidel shears
© Imperial College London Page 54
Interface friction angles for piles driven in clay
Measure δmax, δmin in ring shear interface tests, can be surprising!
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Load-displacement behaviour
Axial, lateral and moment monitoring of Magnus and Hutton
TLP foundations: errors of 400% in conventional T-z, P-y
predictive approaches
Far better fit with (Class A) non-linear “small strain” FE
predictions; Jardine and Potts (1988), (1993)
Central role in new DONG-led PISA lateral loading JIP
Widely used in onshore Civil Engineering: many conferences
and case histories
© Imperial College London Page 56
Advanced soil testing & non-linear modelling:
Six TC-29/101 conferences since 1994
Lyon 2003 Atlanta 2008 Seoul 2011
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200 x 100mm samples
Accurate cyclic loading
Longer term creep tests
Interactions with cycling
Higher resolution strain gauges
Multi-axial BE systems
Advanced stress-path
triaxial equipment
Jardine 2013
Page 58
Or, IC Resonant column HCA
Static mode
σ΄1 , σ΄2 , σ΄3 and αcontrol
Dynamic mode
Torsional resonant column
38/71 mm hollow cylinder
71/101 mm hollow cylinder
Static loading ram
torque system
and hydraulic
pressures
Oscillator (RC)
Specimen sizes
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© Imperial College London Page 59
Predictive tools
• FE code ICFEP, fully coupled includes range of possible
elastic-plastic soil models, can model progressive failure
• Simulate ‘small-strain’ behaviour by tangent stiffness
functions between (Y1) elastic and outer (Y3) yield surfaces
G/p΄ = f(εD)
K/p΄ = g(εvol)
• Fitted from lab tests, applied in 100s of projects
• Illustrate with simulations of tension tests on 19m long,
456mm OD Dunkerque steel piles driven in dense sand
Page 60
Dunkerque anisotropic stiffness profiles: lab and field
Anisotropic Y1 stiffness profiles: Dunkerque
0 100 200 300 400 500 600 700
Elastic stiffness, MPa
0
5
10
15
20
25
Dep
th, m
Legend:Eu from TXC testsE`v from TXC tests E`h from TX testsGvh from TX BE testsGhh from TX BE testsGvh from field seism. CPT tests
Elastic stiffness MPa
Depth
m
Field seismic & lab Gvh measurements agree within ≈ 10%
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Page 61 0.001 0.01 0.1 1
es, %
0
200
400
600
800
1000
1200
1400
G/p
'
Legend:Curve used for FE analysisTC test curve OCR=1TE test curve OCR=1TS test curve for OCR=1
Dense sand non-linear secant shear stiffness data
OCR=1, other tests at different OCRs
Non-linear ICFEP predictions for tension test
19m, 457mm OD, steel pipe pile at Dunkerque
0 5 10 15 20 25 30 35
Pile cap displacement, (mm)
0
500
1000
1500
2000
2500
Pil
e re
sist
an
ce, Q
(M
N)
Legend:
predicted - ICFEP
observed
Shaft σ′rc from ICP-05
CPT approach
Estimates for other
soil σ′ components
Non-linear stiffness
and interface shear
from lab tests
Jardine et al 2005b
Pile h
ead load, Q
(M
N)
Pile head displacements, δ (mm)
Good for capacity &
working load stiffness
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© Imperial College London Page 63
Conclusions
• Need for improved capacity methods highlighted
• Background instrumented pile & data base research reviewed
• New API/ISO ‘sand’ methods and practical application discussed
• Case histories demonstrate ‘full’ ICP is fit for purpose
• New factors highlighted, including strong time effects, base capacity in variable strata & cyclic loading
© Imperial College London Page 64
Conclusions
• Recent research outlined and interim conclusions noted
• Focus on stress regime and soil fabric around shaft
• Greatest practical impact with sands
• Key aspects of ICP clay effective stress approach outlined
• Way to improve pile-soil deformation analysis reviewed and illustrated
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© Imperial College London Page 65
Acknowledgments
Sponsors & partners: BP,
BRE, IFP, EPSRC, Exxon
HSE, Shell, INPG 3S-R
group, Total and others
Current and former co-
workers: Andrew Bond,
Fiona Chow, Reiko
Kuwano, Barry Lehane,
Siya Rimoy, Jamie
Standing, Zhongxuan
Yang, Bitang Zhu and
many others Pierre Foray
1949-2014