Design of axially loaded piles - IGBE - AGTZE · PDF fileDesign of axially loaded piles. 2....

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


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


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 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


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


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,


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


API β-method for non-cohesive soils


Proof of serviceabilityt-z, Q-z-curves according to API RP 2A 2007

Clay


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


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


EffectsInteraction skin friction – base resistance

Rs

qs,Druck

EDruck

Rb

(Witzel 2004)


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)


CPT-based methods: Base resistance


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


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


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)


Gründungen – Buckets

Bucket-Prototyp, Fredrikshavn


Bucketgründungen

Entwicklung aus dem „Suction caisson“Einbringung mittels Unterdruck („suction“)Unterdruck ist begrenzt durch hydraulischenGrundbruch begrenzte Eindringtiefe

Einwirkungen beim EindringvorgangBucketgründung (schematisch)


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


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


Vergleich Buckets – Monopiles

Tragfähigkeit von Buckets ist geringer als von MonopilesMonopod-Buckets eher für moderate Wassertiefen geeignet


Three-degrees-of-freedom loading device in Oxford

Three-dimensionalYield surface


Achtung: Gefahr des Ausbeulens


Danke für Ihre Aufmerksamkeit !

Design of axially loaded piles - IGBE - AGTZE · PDF fileDesign of axially loaded piles. 2....

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