Towards Efficient Finite Element Model Review Dr. Richard Witasse, Plaxis bv (based on the original presentation of Dr. Brinkgreve) Journée Technique du CFMS, 16 Mars 2011, Paris
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Topics • FEA in geotechnical engineering • Validation & verification • FE modelling: illustrated traps & pitfalls
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Introduction Simple hand calculations Graphical / analytical methods Conventional design methods Simple numerical methods 2D finite element analysis (1990 ) 3D finite element analysis (2000 ) Journée Technique du CFMS, 16 Mars 2011, Paris
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FEA in geotechnical engineering
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FEA FEA
FEA
FEA
available (soil) data
Design cycle: • Design phase – Preliminary design – Final design • Tender phase – Modified / alternative design • Construction phase – Construction / observation • Maintenance phase – Improvements
FEA
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FEA in geotechnical engineering Key success factors for geotechnical FEA: • Sufficient data – Soil data – Construction details – Competence of engineer – Software features and logic
• Calculation performance – Efficiency and accuracy of software – Computer power
quality control
• Model accuracy
• Interpretation of results – Competence of engineer
• Validation & Verification Journée Technique du CFMS, 16 Mars 2011, Paris
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Validation & Verification Validation is essential in finite element analysis Validation : Matching reality Engineer Verification : Matching known solutions Software Geotechnical Committee (NAFEMS, etc) Document on parameter selection Document on Validation of FEA Case histories Literature reviews Supporting Validation & Verification in geotechnical FEA
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FE modelling: Traps & pitfalls Geometric modelling Loads & boundary conditions Material models + parameters Mesh generation Initial conditions Calculation phases Results (interpretation) Journée Technique du CFMS, 16 Mars 2011, Paris
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Traps & pitfalls: Geometric modelling Type of model? Plane strain Axisymmetry Full 3D What if 2D model is used? Conservative Optimistic Journée Technique du CFMS, 16 Mars 2011, Paris
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Traps & pitfalls: Geometric modelling • Plane strain
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• Axisymmetry
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Traps & pitfalls: Geometric modelling • Pile modelling What is effectively being modeled How it looks like in reality
=
≠
2D Plane Strain Model Equivalent 3D
Real 3D Journée Technique du CFMS, 16 Mars 2011, Paris
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Traps & pitfalls: Geometric modelling Where to put your model boundaries?
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Traps & pitfalls: Geometric modelling Stability analysis Drained deformation analysis Undrained deformation analysis
~ ~
~
~
Dynamic analysis
~ ~
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~ ~ 12/32
Traps & pitfalls: Interface elements Interfaces: Soil-structure Interaction Be careful: 3D situations in 2D Piles Extended interfaces: No strength reduction Improve stress results at tip/corners Journée Technique du CFMS, 16 Mars 2011, Paris
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Traps & pitfalls: Material models Which model to use? Consider stress paths, required features Possibilities & limitations of models
σ
σ
σ ε
ε
σ ε
ε
Selection of model parameters Sufficient soil data? Stress level, stress path, anisotropy
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Traps & pitfalls: Material models choice Simple vs advanced constitutive models Parameters
Moduli
Poisson ratio
Mohr Coulomb
Hardening Soil
E
ref E 50
-
ref E oed
-
Eurref
-
Power m
v
vur
Cohesion
c
Friction angle
φ
Dilatancy angle
ψ
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Traps & Pitfalls: Stress paths • Illustration for excavation problem
Eur ,,E50 Eoed
E0 Eur , E50
Eur
E0 Journée Technique du CFMS, 16 Mars 2011, Paris
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Traps & pitfalls: Undrained behaviour Drained or undrained behaviour? Dimensionless time factor T T < 10-4 T>2
(U < 1%) : (U > 99%) :
k E oed T= t 2 γw D
Undrained conditions Drained conditions
How to model undrained behaviour? A: Effective stress analysis + Kw/n + effective parameters B: Effective stress analysis + Kw/n + E’,ν ν’ + Su C: Total stress analysis + undrained parameters
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Traps & pitfalls: Undrained behaviour Appropriate pore pressure, effective stress, shear strength? Undrained A: Su is a result of the calculation (depending on soil model) q u
2su
ESP
Linear-elastic perfectly plastic TSP p, p’
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Traps & pitfalls: Undrained behaviour Appropriate pore pressure, effective stress, shear strength? Undrained A: Su is a result of the calculation (depending on soil model) q
Strain-hardening u 2su
ESP
TSP p, p’
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Traps & pitfalls: Undrained behaviour Appropriate pore pressure, effective stress, shear strength? Undrained B: Su is an input value q
u? 2su
ESP ?
TSP p, p’
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Traps & pitfalls: Undrained behaviour Appropriate pore pressure, effective stress, shear strength? Undrained C: Su is an input value q
2su
TSP p
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Traps & pitfalls: Mesh generation Element type: • Interpolation order • Locking
εv=0
Shape
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Traps & pitfalls: Mesh generation Global fineness Local refinement
15-node triangles
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Traps & pitfalls: Initial conditions Initial stresses: • Initial total stress • Initial pore pressure • Initial effective stress Initial value of state parameters: • Initial void ratio • Pre-consolidation stress • Other state parameters K0-procedure
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Gravity loading
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Traps & pitfalls: Initial conditions Existing structures: • Requires several phases to set up initial conditions
Existing buildings
our project
New project Initial phase
Phase 1
Phase 2
Phase 3 >
reset displacements Journée Technique du CFMS, 16 Mars 2011, Paris
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Traps & pitfalls: Pore pressures Using general phreatic level
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Using local phreatic level and cluster interpolation
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Traps & pitfalls: Pore pressures Using groundwater flow
Open bottom boundary Journée Technique du CFMS, 16 Mars 2011, Paris
Closed bottom boundary 27/32
Traps & pitfalls: Calculation settings • Tolerated error TE TE = 1%
TE = 20%
Umax = 42.2 mm
Umax = 23.3 mm
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Traps & pitfalls: Safety Factor Analysis • Safety factor based on Phi-c reduction method has a different meaning that safety factor used by structural engineers
∑M
sf
available soil resistance = mobilized soil resistance
∑ M sf = Journée Technique du CFMS, 16 Mars 2011, Paris
failure load working load
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Traps & pitfalls: Phi-c Reduction Analysis • Mesh Sensitivity
Sum-Msf 1.5
1.4
1.35 1.3
1.3 1.2
1.1
1 0
50
100
150
200
250
|U| [m]
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Conclusions FEM: powerful tool in different phases of design process Key success factors: - Sufficient data - Reliable & efficient software - Competence of engineer Plaxis currently working on a visual checklist for efficient model review - Make the engineers aware of the traps and pitfalls - Supported by visual example
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Questions ?
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