Towards Efficient Finite Element Model Review Dr. Richard Witasse, Plaxis bv (based on the original presentation of Dr.

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1 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 1/32

2 Topics FEA in geotechnical engineering Validation & verification FE modelling: illustrated traps & pitfalls Journée Technique du CFMS, 16 Mars 2011, Paris 2/32

3 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 3/32

4 FEA in geotechnical engineering Design cycle: Design phase Preliminary design Final design Tender phase Modified / alternative design Construction phase Construction / observation Maintenance phase Improvements FEA FEA FEA FEA FEA available (soil) data Journée Technique du CFMS, 16 Mars 2011, Paris 4/32

5 FEA in geotechnical engineering Key success factors for geotechnical FEA: Sufficient data Soil data Construction details Model accuracy Competence of engineer Software features and logic Calculation performance Efficiency and accuracy of software Computer power Interpretation of results Competence of engineer Validation & Verification quality control Journée Technique du CFMS, 16 Mars 2011, Paris 5/32

6 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 Journée Technique du CFMS, 16 Mars 2011, Paris 6/32

7 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 7/32

8 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 8/32

9 Traps & pitfalls: Geometric modelling Plane strain Axisymmetry Journée Technique du CFMS, 16 Mars 2011, Paris 9/32

10 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 10/32

11 Traps & pitfalls: Geometric modelling Where to put your model boundaries? Journée Technique du CFMS, 16 Mars 2011, Paris 11/32

12 Traps & pitfalls: Geometric modelling Stability analysis Drained deformation analysis Undrained deformation analysis Dynamic analysis ~ ~ ~ ~ ~ ~ ~ ~ ~ Journée Technique du CFMS, 16 Mars 2011, Paris 12/32

13 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 13/32

14 Traps & pitfalls: Material models Which model to use? Consider stress paths, required features Possibilities & limitations of models σ σ σ σ ε ε ε ε Selection of model parameters Sufficient soil data? Stress level, stress path, anisotropy Journée Technique du CFMS, 16 Mars 2011, Paris 14/32

15 Traps & pitfalls: Material models choice Simple vs advanced constitutive models Parameters Moduli Poisson ratio Cohesion Friction angle Dilatancy angle Mohr Coulomb E v c φ ψ Hardening Soil ref E 50 ref E oed ref E ur Power m v ur Journée Technique du CFMS, 16 Mars 2011, Paris 15/32

16 Traps & Pitfalls: Stress paths Illustration for excavation problem E ur,, E 50 E 0 E oed E ur, E 50 E ur E 0 Journée Technique du CFMS, 16 Mars 2011, Paris 16/32

17 Traps & pitfalls: Undrained behaviour Drained or undrained behaviour? Dimensionless time factor T k E oed T = 2 γ w D t T < 10-4 (U < 1%) : Undrained conditions T > 2 (U > 99%) : Drained conditions How to model undrained behaviour? A: Effective stress analysis + K w /n + effective parameters B: Effective stress analysis + K w /n + E,ν + S u C: Total stress analysis + undrained parameters Journée Technique du CFMS, 16 Mars 2011, Paris 17/32

18 Traps & pitfalls: Undrained behaviour Appropriate pore pressure, effective stress, shear strength? Undrained A: S u is a result of the calculation (depending on soil model) q u Linear-elastic perfectly plastic 2s u ESP TSP p, p Journée Technique du CFMS, 16 Mars 2011, Paris 18/32

19 Traps & pitfalls: Undrained behaviour Appropriate pore pressure, effective stress, shear strength? Undrained A: S u is a result of the calculation (depending on soil model) q u Strain-hardening 2s u ESP TSP p, p Journée Technique du CFMS, 16 Mars 2011, Paris 19/32

20 Traps & pitfalls: Undrained behaviour Appropriate pore pressure, effective stress, shear strength? Undrained B: S u is an input value q u? 2s u ESP? TSP p, p Journée Technique du CFMS, 16 Mars 2011, Paris 20/32

21 Traps & pitfalls: Undrained behaviour Appropriate pore pressure, effective stress, shear strength? Undrained C: S u is an input value q 2s u TSP p Journée Technique du CFMS, 16 Mars 2011, Paris 21/32

22 Traps & pitfalls: Mesh generation Element type: Interpolation order Locking Shape ε v =0 Journée Technique du CFMS, 16 Mars 2011, Paris 22/32

23 Traps & pitfalls: Mesh generation Global fineness Local refinement 15-node triangles Journée Technique du CFMS, 16 Mars 2011, Paris 23/32

24 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 K 0 -procedure Gravity loading Journée Technique du CFMS, 16 Mars 2011, Paris 24/32

25 Traps & pitfalls: Initial conditions Existing structures: Requires several phases to set up initial conditions Existing buildings New project Initial phase Phase 1 Phase 2 our project Phase 3 > reset displacements Journée Technique du CFMS, 16 Mars 2011, Paris 25/32

26 Traps & pitfalls: Pore pressures Using general phreatic level Using local phreatic level and cluster interpolation Journée Technique du CFMS, 16 Mars 2011, Paris 26/32

27 Traps & pitfalls: Pore pressures Using groundwater flow Open bottom boundary Closed bottom boundary Journée Technique du CFMS, 16 Mars 2011, Paris 27/32

28 Traps & pitfalls: Calculation settings Tolerated error TE TE = 1% TE = 20% U max = 42.2 mm U max = 23.3 mm Journée Technique du CFMS, 16 Mars 2011, Paris 28/32

29 Traps & pitfalls: Safety Factor Analysis Safety factor based on Phi-c reduction method has a different meaning that safety factor used by structural engineers Msf = available soil resistance mobilized soil resistance Msf = failure load working load Journée Technique du CFMS, 16 Mars 2011, Paris 29/32

30 Traps & pitfalls: Phi-c Reduction Analysis Mesh Sensitivity Sum-Msf U [m] Journée Technique du CFMS, 16 Mars 2011, Paris 30/32

31 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 Journée Technique du CFMS, 16 Mars 2011, Paris 31/32

32 Questions? Journée Technique du CFMS, 16 Mars 2011, Paris 32/32