Monotonic and Cyclic p-y Curves for Clay based on Soil ...
Transcript of Monotonic and Cyclic p-y Curves for Clay based on Soil ...
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Monotonic and Cyclic p-y Curves for Clay based on Soil Performance Observed in Laboratory Element TestsYouhu Zhang Knut H Andersen, Rasmus T. Klinkvort, Hans Petter Jostad, Nallathamby Sivasithamparam, Noel P. Boylan, Thomas Langford
Presentation at Geotechnical Input to Well Integrity Assessment Workshop , BP Helios Plaza building, Houston, Texas, 29 April 2016
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Outline
• General
• The monotonic p-y model and validation
• Extension to cyclic p-y model
• Numerical procedure for calculation overall pile response
• Example case and validation
• Conclusions
Slide 2
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General: p-y springs
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http://www.findapile.com/p-y-curves/definition
Piles under lateral loading are typically designed by the beam-column approach with the lateral support represented by “p-y springs”
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p-y springs for cyclic loading: current practice
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• Developed from limited field tests with specific soil and loading conditions
• No account of site specific soil response under cyclic loading
• Either cyclic/static, with no possibility to account for storm loading make-up
• Should cyclic p-y curves applied along the whole pile?
• Implied fully smooth pile-soil interface
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p/p u
displacement, y/yc
yc 3yc
0.72(X/Xr)
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Matlock (1970) monotonic
Matlock (1970) cyclic for X≥Xr
Matlock (1970) cyclic for X<Xr
'modified Matlock' cyclic for X<Xr
8yc
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The monotonic p-y model: basic idea
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γ
τ/su
y/D
p/pu
(y/D, p/pu)(γ, τ/su) Scaling
p/pu = τ/su
y/D = ξγ
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Development of monotonic model
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10D
20D
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τ/su
γ(%)
γpf (%) = 2, 4, 6, 8, 12, 16, 20
A comprehensive parametric study:• A wide range of stress-strain response• Different pile interface roughness
NGI-ADP soil model
A simple linear relation between scaling coefficient ξ and pile-soil interface factor α is found: ξ= 1.35 + 0.25α
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Retrospective prediction
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The p-y curves calculated from Plaxisanalyses are back-predicted by the p-y model, showing excellent match for all parametric ranges considered.
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p/s u
y/D
γpf (%) = 2, 4, 6, 8, 12, 16, 20
Gmax/su = 500α = 0.5
Plaxis
model prediction
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p/p u
y/D
Jeanjean(2009)
Gamma = 0.10
Series5
Gmax/su = 550γp
f = 10%α = 1ε50 = 0.5% (γM=2 = 0.75%)b = 0.33
Jeanjean (2009)
Currently proposed
API, εc = 0.5%
(b)
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τ/su
γ(%)
Power law fit
NGI ADP fit
Test data
NGI ADP fitGmax/su = 550γp
f = 10%Power law fitγM=2 = γ50 = 0.75%b = 0.33
(a)
Comparison with Jeanjean (2009)
For the same soil examined by Jeanjean (2009), the current model does match well.
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Validation of monotonic model
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Dep
th [m
]
Lateral displacement [m]
Plaxis
p-y springs
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Dep
th [m
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Cross-sectional shear force [kN]
Plaxis
p-y springs
A 84” pile embedded 40 m in a clay profile:• su
DSS = 5.1 + 2.4z• Gmax/su
DSS = 500, and γpf = 4%
• Fully rough pile-soil interface• Plaxis: NGI-ADP nonlinear
stress-strain response
The p-y spring model offers an excellent prediction of overall pile response!
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Extension to cyclic p-y response
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τcy/su, Npcy/pu, Nequivalent to
with pcy/pu = τcy/su
Randolph and Houlsby (1984)
The postulation
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How to obtain cyclic p-y curves?
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τcy
τcy
time
τcy
time
τcy
timeTest 1
Test 2
Test 3
Test 1
Test 2
Test 3
No. of cycles
τ cy/s
u
Stress strain cross-section for Neq=100
γ (%)
τ cy/s
u
Cyclic DSS tests
Neq=100
Scale stress-strain curve to obtain p-y curve corresponding to Neq
Test results
Cyclic contour diagram
The extension to cyclic p-y curves is verified at pile slice level against finite element analyses using undrained cyclic accumulation soil model UDCAM
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Numerical procedure to analyse overall pile response
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Lateral load history
Spring 1
Spring 2
Spring 3
Spring n
y
y
y
y
y
y
pNeq_1
Neq_2
Neq_3
Neq_n
Perform accumulation for each of the springs, and calculate Neq
Construct p-y curves for each spring based on respective Neq
No. of cyclesτ c
y/su
Global beam-column model
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Validation of the numerical procedure Slide 12
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Dep
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Lateral displacement (m)
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Dep
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Cross-sectional shear force (kN)
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Dep
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Cross-sectional bending moment (kNm)
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Neq
FEA
NGI-PILE
Parcel 1, 2, 3
End of Parcel 3, predicted by Program
Parcel No. Load, kN No. of cycles 1 2000 20 2 3000 10 3 4000 5
A 84” pile embedded 40 m in a clay profile:• su
DSS = 3.4 + 1.6z• Drammen clay, OCR =1• Cyclic pile head lateral load parcels• Symmetric load cycles• Fully rough soil-pile interface• Lateral load applied 2D above mudline
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Further developmentsSlide 13
• Develop procedures for 1-way cyclic loading with non-zero average
component
• Models for sand
• Models for t-z response
• Ultimate goal: a practical tool for designing offshore piles under cyclic lateral
and axial loading
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ConclusionsSlide 14
• A model for constructing site-specific monotonic p-y curves for piles in clay is
introduced
• The monotonic p-y model is successfully extended for cyclic response using
well-established cyclic accumulation procedure
• A procedure to calculate overall pile response under cyclic loading is
proposed and validated by numerical simulation
• Site-specific soil response and storm load history can be considered explicitly
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Welcome to presentation at OTC:OTC-26942-MSMay 4, 9.30 - 12.00Geotechnics for well design
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