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3. Finite Element Packages
engineering/consultancy office FLAC
Plaxis
Abaqus
Geo-slope
Oasys
Others
research institute university contractor
4. Background
Developed at the Technical University of Delft for Dutch DPWWM
Initially was intended to analyze the soft soil river embankments of the lowlands
of Holland
Soon after, the company Plaxis BV was formed, and the program was expanded
to cover a broader range of geotechnical issues
5. What is PLAXIS?????
FEM package - for analysis of deformation and stability
Requires advanced constitutive models for the simulation of the non linear,
time dependent and anisotropic behaviour of soils and/or rock.
Since soil is a multi phase material, special procedures are required to deal
with hydrostatic and non hydrostatic pore pressures in the soil.
Although the modelling of the soil itself is an important issue, many projects
involve the modelling of structures and the interaction between the structures
and the soil.
Plaxis is equipped with features to deal with various aspects of complex
geotechnical problems.
7. PLAXIS Features
Input Features Soil Behaviour
Interfaces Excess pore water pressure
Geogrids Groundwater flow analyses
Anchors Steady state pore pressure
Boundary conditions User-defined soil models
Tunnels Advanced soil models
Loads Mohr-Coulomb model
Hinges and rotation springs Hardening soil model
Plates Soft soil creep model
High-order elements Material set database
Automatic mesh generation Soil tests
Graphical input of geometry models
8. PLAXIS Features
Calculation features Output features
Consolidation Analysis Velocities and accelerations
Staged construction Load displacement curves & stress paths
Calculation Manager Animations
Safety factors Report generator
Updated Lagrangian analyses Cross-sections
Arc-Length control Structural forces & Displacements
Automatic load stepping Stresses
Parameter Variation Deformation
Presentation of results
Stress paths
9. PLAXIS Analysis
Get familiar using Plaxis
Step I
• Define the model
• Input geometry + structural elements;
• Apply boundary conditions;
• Apply loading and/or prescribed displacements;
• Enter material properties;
• Mesh generation;
10. PLAXIS Analysis
Step II
• Initial flow and boundary conditions;
• Generation of initial pore water pressures;
• Define initial stress state;
Step III
• Calculation
Step IV
• Output
11. PLAXIS Usage
engineering/consultancy office Embankment
Trench & excavations
gravity walls
reinforced/anchored earth wall
embedded walls
shallow/ surface foundations
deep foundations
tunnels
water flow
others (soil dynamics,…)
research institute university contractor
12. PLAXIS Usage
engineering/ Back calculation
consultancy
office Full design: ULS calculations
Full design: SLS calculations
Validation of results obtained from analytical methods
Research applications
Others (...)
research institute university contractor
13. PLAXIS Usage
engineering/ Discretisation
consultancy Type of elements
office
Boundary conditions
Initial conditions
Choice of soil model
Choice of type of analysis
Parameters determination
Results interpretation
Others (calculations for ULS...)
research institute university contractor
14. Validation of Results
engineering/ parametric study
consultancy
office comparison with results obtained from an analytical
approach
comparison with examples from literature
experience
Others (monitoring…)
research institute university contractor
15. Why use PLAXIS?
Easy and user friendly?
Versatile and flexible? (you or PLAXIS)
Required?
Advantage?
Value adding or adding value?
Familiar?
16. When to use PLAXIS?
Easy?
Required?
Advantage?
Value adding or adding value?
Familiar?
20. Issues…….
Feel for results?
Time?
Knowledge?
Verification?
Problem identification?
Appropriate model?
21. Solution…….
1. Crude estimation – feel for results
2. Assessment details verified – plane strain/axisymmetric?
3. Model verified – dimensions, elements and boundary
conditions
4. Initial stress state verified – steady state or transient
groundwater, groundwater boundary condition, in-situ stress
conditions
5. Staging verified – changes in dimensions, loading
conditions and load steps
6. Calculation type verified – plastic, consolidation or c-phi
reduction?
7. Output type specified -
22. In a nutshell…….
PLAXIS is not a solution, but a tool, only if used appropriately.
FEM can be too complex and too costly.
The lead engineer need to know whether FEM is really required
or not.
The lead engineer need to make sure adequate verification is
done on the input before analysis is being carried out.
The engineer should have crude calculation/solution/results in
hand BEFORE running PLAXIS.
In many projects, LEM is what really required.
Generally, simple analytical solutions (LEM) would suffice.
Try to avoid doubling the the time and effort – its a waste.
