PLAXIS 3D Benchmark for Bearing Capacity of Suction ... · – Around the pile – Underneath the...
Transcript of PLAXIS 3D Benchmark for Bearing Capacity of Suction ... · – Around the pile – Underneath the...
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PLAXIS 3D Benchmark for Suction Anchor Bearing Capacity
Richard WitassePlaxis bv
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Objectives
• Demonstrate the capabilities of PLAXIS 3D to model suction anchor in soft clay easily and quickly
• Accurately predict bearing capacity of suctions anchors including
– H-V interaction
– Load attachment point variation
• Match NGI benchmark results from Andersen et al (2005)*
*Andersen, K.H. et al. – “Suction Anchors for Deepwater Applications”, Int. Symp. On Frontiers in
Offshore Geotechnics, ISFOG, Perth, Proc. A.A. Balkema Publishers, 2005.
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Plaxis 3D FEA Modelling Overview
• Plaxis models defined according case C1 i.e.
– Depth / Diameter = 5
– Normally consolidated clay
• Two calculations series undertaken
– Variation of load inclination θ at optimal load attachment point θ = 0o, 10o, 20o, 30o, 45o, 90o
– Variation of load attachment depth with a fixed 30o load inclination
• Calculations done with PLAXIS 3D Research version
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Model Presentation
• Failure mechanism to fit in soil model
• Half-geometry modeled
• Suction anchor modeled as stiff plate elements
• Very stiff plug
• Interface elements
– Around the pile
– Underneath the pile 50 m
40 m
15 m
D=25 m
B=5 m
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Soil Material Properties
• Undrained behavior modeled in a total stress approach
– Undrained stiffness Eu = 5000 kPa (not relevant as
bearing capacity to be evaluated) and vu=0.495
– Undrained shear strength suC = 1.25 × z (based on DSS
shear with no reduction) and φu = 0o
– Reduced shear strength along skirts su = 0.65 × suC
• Soil unit weight
– Saturated unit weight γsat=16 kN/m3
– Initial effective vertical stress is therefore σ’v,ini = 6 × z
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Meshing
• Auto mesh generation
– 10 noded tet elements
– Coarse mesh setting
– Local refinement inside
and around pile
• Mesh characteristics
– 30000 elements
– 44000 nodes
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Plaxis Calculations
• Loading applied along the center line as a prescribed displacement
• Two calculations series undertaken
– Variation of load inclination θ at optimal load attachment
point θ = 0o, 10o, 20o, 30o, 45o, 90o
– Variation of load attachment depth with a fixed 30o load
inclination
• Use of multicore direct solver on a 8 core machine (two simultaneous runs over 4 cpus each)
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Main Results for Horizontal Loading
Deformed shape at failure Load displacement curve
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Failure Mechanism for Horizontal Loading
Plastic points at failure Plastic shear strain at failure
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Main Results for Vertical Loading
Deformed shape at failure Load displacement curve
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Failure Mechanism for Vertical Loading
Plastic points at failure Plastic shear strain at failure
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H-V Interaction Diagram
0
2000
4000
6000
8000
10000
12000
0 5000 10000 15000 20000 25000
Horizontal Load (kN)
Ve
rtic
al L
oa
d (
kN
)
PLAXIS 3D
PLAXIS results (failure load interpreted as reaction force
at 2m horizontal displacement) Benchmark results
• Results of load inclination variation calculation series
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Variation of Load Attachment Point
• Deformed mesh at failure
Below optimum with load
attachment depth 20 m (z/D=0.55)
Close to optimum with load
attachment depth 17.5 m (z/D=0.70)
Above optimum with load
attachment depth 12.5 m (z/D=0.85)
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Variation of Load Attachment Point
• Load vs displacement curves
F/2
(h
alf m
od
el)
in
kN
Ux in m
FH@opt
FH@20m
FV@opt
FV@20m
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Variation of Load Attachment Point
0
0.2
0.4
0.6
0.8
1
0.5 0.6 0.7 0.8 0.9 1
Load att point at center line z/D
Re
lati
ve
ca
pa
cit
y
PLAXIS 3D
• Results of load att. point variation calculation series
PLAXIS results Benchmark results
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Computational Performance• Calculation run on 2 x Intel® Xeon® Processor E5620
(Quadcore, 12M Cache, 2.40 GHz) so 8 threads in total
• Total wall clock time for load inclination variation study = 11h30m
Load inclination θ # CPU’s CPU Time Wall clock time
0 4 7h35m 3h31m
10 4 8h06m 3h53m
20 4 7h46m 3h44m
30 4 7h35m 3h57m
45 4 8h12m 4h10m
90 4 2h23m 1h08m
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Conclusions
• PLAXIS 3D can straightforwardly deal with modeling of suction anchor without any geometric limitation
• PLAXIS 3D can predict reliable bearing capacities for all relevant loading scenarii
• Effortless definition of geometry, interfaces for soil structure interaction and initial stresses
• Good calculation performance with 6 loadcases fully run in less than half a day for a 130000 dof system
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