Post on 01-Feb-2018
Addis Ababa, September 2010
Prof. Dr.-Ing. Martin AchmusInstitute of Soil Mechanics, Foundation Engineering and Waterpower Engineering
Design of axially loaded piles
Design of axially loaded piles Addis Abbaba, September 20102
Presentation structure:
• Onshore design procedure
• Offshore design methodsAPI α- and β-methodsCPT-based methods for sands
• Comparison for North Sea conditions
• Consideration of cyclic loading effects
Design of axially loaded piles
Design of axially loaded piles Addis Abbaba, September 2010
Tripod and Jacket Foundations for Offshore Wind Energy Converters
• large base area, with piles in the edges• for water depth larger than 25m• most relevant loading (design-driving): axial (tension/compression)• pile diameters 1.5 to 2.5m, connected to the structure by grouted pile sleeves• questions: 1) ultimate pile capacity
2) effect of cyclic loading
Design of axially loaded piles Addis Abbaba, September 2010
Axial capacity of piles
kbksk RRR ,1,1,1 +=Tension:
Skin friction:
kbbkb qAR ,,1 ⋅=∑ ∑ ⋅⋅==i i
iiksiksks lqURR ,,,,1,1
Compression:ksk RR ,1,1 =
Base resistance:
According to DIN 1054 (2005-01)
Onshore: • Tabulated experience values• Pile load tests
Offshore: • API approach• CPT-based methods (NGI – 2005, ICP – 2005,
UWA – 2005, Fugro – 2005)
Methods to derive skin friction and base resistance
Index 1 for ULS
Design of axially loaded piles Addis Abbaba, September 2010
Design proofs according to DIN 1054
Ultimate limit state (ULS) design
Based on load tests: (Compression pile)
(Tension pile)
Based on experience:
Serviceability limit state (SLS) design
dd RE ,1,1 ≤
QkQGkGd EEE γγ ⋅+⋅= ,,,1
Pckd RR γ,1,1 =
Ptkd RR γ,1,1 =
Pkd RR γ,1,1 =
kdkd RREE ,2,2,2,2 =≤= Index 2 for SLS
LC 1 LC 2 LC 3
γG 1.35 1.25 1.1
γQ 1.5 1.3 1.1
γP 1.4 1.4 1.4
γPc 1.2 1.2 1.2
γPt 1.3 1.3 1.3
Partial safety factors, dependent on load cases
Design of axially loaded piles Addis Abbaba, September 2010
Open steel pipe piles
Compression piles:
If plugging occurs, base resistance acts along the whole cross section; if not, inner skin friction is to be considered
Tension piles:
Capacity due to inner skin friction is limited by the weight of the soil plug
Tensile skin friction is smaller than compressive skin friction
Design of axially loaded piles Addis Abbaba, September 2010
API α-method for cohesive soils
Base resistance:
cu(z) [kPa]= undrained shear strength in depth z
(z)cq ub,k ⋅= 9
Skin friction:
cu(z) [kPa] = undrained shear strength in depth zα [-] = dimensionless factor
Ψ [-]= c(z)/ σ‘v(z)
(z)c(z)=αq us,k ⋅
0.15.0
0.15.0250
5.0
>⋅=
≤⋅=−
−
ψforψα
ψforψα,
Design of axially loaded piles Addis Abbaba, September 2010
Skin friction:
k [-] = earth pressure coefficientk = 1.0 for closed-ended/plugged pilesk = 0.8 for open-ended piles
δ [°] = wall friction angle between pile and soilσ‘v [kPa]= γ‘ z = effective vertical stressβ= k tan(δ) [-] = dimensionless coefficient according to table 1
β can be increased by 25% for closed-ended orplugged piles
maxtan s,k,'vvs,k qσβδ(z)σ'k(z)q ≤⋅=⋅⋅=
D
z
qs,k
qb,k
qs,k(z)
E
Base resistance:
Nq [-] = dimensionless factor according to table 1σ‘v = γ‘ z = effective vertical stress
maxb,k,v,bqb,k qσ'Nq ≤=
API β-method for non-cohesive soils
Design of axially loaded piles Addis Abbaba, September 2010
API β-method for non-cohesive soils
Design of axially loaded piles Addis Abbaba, September 2010
Proof of serviceabilityt-z, Q-z-curves according to API RP 2A 2007
Clay
Design of axially loaded piles Addis Abbaba, September 2010
API β-method: critics
• API gives conservative results for short piles and also for long piles in dense sand
• for long piles in loose or medium dense sand results can be unsafe
0
0.5
1.0
1.5
2.0
2.5
0 10 20 30 40 50
RA
PI/ R
Mes
swer
t 3.41
offengeschlossen
Toolan et al. 1990
Embedded length in m
RA
PI/ R
mea
sure
d
openclosed-ended
Design of axially loaded piles Addis Abbaba, September 2010
EffectsInstallation effects during driving
Partial or total plugging affects radial stresses
Incremental filling ratio
During driving skin friction fatigue occurs; the farer away from pile tip, the larger the friction degradation
Design of axially loaded piles Addis Abbaba, September 2010
EffectsInteraction skin friction – base resistance
Rs
qs,Druck
EDruck
Rb
(Witzel 2004)
Design of axially loaded piles Addis Abbaba, September 2010
CPT-based methods (API RP 2A 2007)
Ar = 1 – (Di / D)² effektives FlächenverhältnisD = PfahldurchmesserDi = innerer PfahldurchmesserL = EinbindetiefePa = atmosphärischer Druckqc = Drucksondierungsspitzendruck qs = f (z) = Mantelreibung
σ‘v0 = effektive Vertikalspannungδcv = Wandreibungswinkela, b,c, d, e, u, v = empirische Beiwerte,
abhängig vom CPT-Basierten Verfahren
New methods for prediction of skin friction and base resistanceICP-05, Fugro-05 and UWA-05:
MethodeArt der
Belastung
Parameter
a b c d e u ν
ICP-05Druck 0,1 0,2 0,4 1 0 0,023
Zug 0,1 0,2 0,4 1 0 0,016
UWA-05Druck 0 0,3 0,5 1 0 0,030 2
Zug 0 0,3 0,5 1 0 0,022 2
Fugro-05Druck 0,05 0,45 0,9 0 1 0,043
Zug 0,15 0,42 0,85 0 0 0,025
NGI-05:
(DIN EN ISO 19902)
Design of axially loaded piles Addis Abbaba, September 2010
CPT-based methods: Base resistance
Design of axially loaded piles Addis Abbaba, September 201016
Back-calculation of pile test results for open-ended piles in sand
Parametric study on the effect of relative density and pile slendernessAchmus & Müller, 2010
Design of axially loaded piles Addis Abbaba, September 2010
Effect of cyclic loading
Large amplitudes can lead to severe friction degradation. Influence factors:
• Number of cyclic load amplitudes N• Static load E0• Cyclic load Ezykl• Soil type
Schwelllast
Ezykl
Last-zyklen N
Einw
irkun
g
1 2 3
E0
Ezykl
Wechsellast
0
Cyclic load
Incremental collapse
Number of cycles N
Sedation
Shakedown
Pla
stic
stra
in ε
pl
Design of axially loaded piles Addis Abbaba, September 2010
Effect of cyclic loading
Verpresspfähle(Nauroy et al. 1985)
Verpresspfähle(Deane et al. 1988)
versagtnicht versagt
Legende:
versagtnicht versagt
Kleine Modellpfähle (Poulos und Lee 1989)
Pfahlprobebelastungen
fast versagt bei N = 2000nicht versagt bei N = 2000
Modellpfähle
N = 101
N = 102
N = 103
Normierte mittlere Belastung E / Ro k
Nor
mie
rte z
yklis
che
Bean
spru
chun
g E
/ R
zykl
k
0 0,2 0,4 0,6 0,8 1,00
0,2
0,4
0,6
0,8
1,0
instabil
stabil
N = 10
N = 50
N = 100
N = 600 Mittag und Richter 2005Zyklenanzahl N Faktor κ
1 0,50
10 0,45
100 0,40
1 000 0,35
10 000 0,30
100 000 0,25
1 000 000 0,20
⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎠⎞
⎜⎝⎛−⋅⋅≤
201REREzykl κ
(Mittag und Richter 2005)
Design of axially loaded piles Addis Abbaba, September 201019
Design of axially loaded piles Addis Abbaba, September 2010
Gründungen – Buckets
Bucket-Prototyp, Fredrikshavn
Design of axially loaded piles Addis Abbaba, September 2010
Bucketgründungen
Entwicklung aus dem „Suction caisson“Einbringung mittels Unterdruck („suction“)Unterdruck ist begrenzt durch hydraulischenGrundbruch begrenzte Eindringtiefe
Einwirkungen beim EindringvorgangBucketgründung (schematisch)
Design of axially loaded piles Addis Abbaba, September 2010
Hydraulischer Gradient am Austritt derStrömung < 1 (entspricht hydraulischerGundbruchsicherheit von rd. 1)Strömungsnetzberechnung
146,1
68,01
'
+−
=Δ
DH
Hukritγ
kritzul uu Δ=Δη1
Zulässiger Unterdruck
Design of axially loaded piles Addis Abbaba, September 2010
Realisierbare Eindringtiefe
Eindringwiderstand
Einpresskraft
zNzKzK
qss
ii
aa
')1('tan)1('tan)1(
γασγδατγδατ
−=−=+=
sm
H
zii
H
zaaEin tDdzDdzDR σππτπτ ++= ∫∫
== 00
Ei
Ein GDuF +Δ=4
2π
Erreichbare Eindringtiefen für Buckets in mitteldichtem Sand
Design of axially loaded piles Addis Abbaba, September 2010
Vergleich Buckets – Monopiles
Tragfähigkeit von Buckets ist geringer als von MonopilesMonopod-Buckets eher für moderate Wassertiefen geeignet
Design of axially loaded piles Addis Abbaba, September 2010
Three-degrees-of-freedom loading device in Oxford
Three-dimensionalYield surface
Design of axially loaded piles Addis Abbaba, September 2010
Achtung: Gefahr des Ausbeulens
Design of axially loaded piles Addis Abbaba, September 2010
Danke für Ihre Aufmerksamkeit !