Plaxis 2D Features

Input features

Interfaces
Joint elements are available to model soil structure interaction. For example, these elements may be used to simulate the thin zone of intensely shearing material at the contact between a tunnel lining and the surrounding soil. Values of interface friction angle and adhesion are generally not the same as the friction angle and cohesion of the surrounding soil.

Geogrids
Geogrids (or geotextiles) are often used in practice for the construction of reinforced embankments or soil retaining structures. These elements can be simulated in Plaxis by the use of special tension elements. It is often convenient to combine these elements with interfaces to model the interaction with the surrounding soil.

Anchors
Elasto-plastic spring elements are used to model anchors and struts. The behavior of these elements is defined using an axial stiffness and a maximum force. A special option exists for the analyses of pre-stressed ground anchors and excavation supports.

Boundary conditions
Fixities are prescribed displacements equal to zero. These conditions can be applied to geometry lines as well as to geometry points in x and y directions. Prescribed displacements are special conditions that can be imposed on geometry lines to control the displacements of these lines. A convenient option exists to use standard boundary conditions that apply in most cases.

Tunnels
The Plaxis program offers a convenient option to create circular and non-circular tunnels using arcs and lines. Plates and interfaces may be used to model the tunnel lining and the interaction with the surrounding soil. Fully iso-parametric elements are used to model the curved boundaries within the mesh. Various methods have been implemented to analyze the deformations that occur as a result of various methods of tunnel construction.

Loads
Two independent load systems can be used to apply point loads and distributed loads. Point loads may be used at any geometry point, whereas distributed loads may be applied to any geometry line, not only limited to the geometry boundary. Loads values can be changed in the “staged construction” mode and/or using load multipliers

Hinges and rotation springs
A hinge is a connection that allows for a discontinuous rotation in the point of connection (joint). Hinges may be introduced to create joints where plate ends can rotate freely, or with rotation springs for connections that are neither pinned nor fully fixed.

Plates
Special plate elements are used to model the bending of retaining walls, tunnel linings, shells, and other slender structures. The behavior of these elements is defined using a flexural rigidity, an normal stiffness and an ultimate bending moment. A plastic hinge may develop for elasto-plastic plates, as soon as the ultimate moment is mobilized. Plates with interfaces may be used to perform realistic analyses of geotechnical structures.

High-order elements
Quadratic 6-node and 4th order 15-node triangular elements are available to model the deformations and stresses in the soil.

Automatic mesh generation
Plaxis allows for automatic generation of unstructured 2D finite element meshes with options for global and local mesh refinement. The 2D mesh generator is a special version of the Triangle generator, which was developed by Sepa.

Graphical input of geometry models
The input of soil layers, structures, construction stages, loads and boundary conditions is based on convenient CAD drawing procedures, which allows for a detailed modeling of the geometry cross-section. From this geometry model, a 2D finite element mesh is easily generated.
 

Soil behavior

Excess pore pressures
Plaxis distinguishes between drained and undrained soils to model permeable sands as well as nearly impermeable clays. Excess pore pressures are computed during plastic calculations when undrained soil layers are subjected to loads. Undrained loading situations are often decisive for the stability of geotechnical structures.

Groundwater flow analyses
Complex pore pressure distributions may be generated on the basis of a two-dimensional groundwater flow analysis. Drains and wells can be modeled using special elements. The groundwater head is conveniently entered as a phreatic level.

Steady state pore pressure
Complex pore pressure distributions may be generated on the basis of a combination of phreatic levels or direct input of water pressures. As an alternative, a steady state groundwater flow calculation can be performed to calculate the pore pressure distribution in problems that involve steady flow or seepage.

User-defined soil models
A special feature in Plaxis 2D is the user-defined soil models option. This feature enables users to include self-programmed soil models in the calculations. This option is primarily of interest for researchers and scientists at universities and research institutes, but it may also be useful for practicing engineers. In coming years, validated and well-documented user-defined soil models may become available via the internet.

Advanced soil models
In addition to the Mohr-Coulomb model, Plaxis offers a variety of advanced soil models. As a general second-order model, an elasto-plastic type of hyperbolic model, called the Hardening Soil model, is available. To model accurately the time-dependent and logarithmic compression behavior of normally consolidated soft soils, a Creep model is available, which is referred to as the Soft Soil Creep model. In addition to these soil models, a special model is available to analyze the anisotropic behavior of jointed rock. Detailed information on these models can be found in the Material Models Manual.

Mohr-Coulomb model
This robust and simple non-linear model is based on soil parameters that are well-known in engineering practice. Not all non-linear features of soil behavior are included in this model, however. The Mohr-Coulomb model may be used to compute realistic support pressures for tunnel faces, ultimate loads for footings, etc. It may also be used to calculate a safety factor using a ‘phi-c reduction’ approach.

Material set database
Material properties for soil, as well as for structural elements are entered in a project database. Material data sets from the projects database may be copied to the global database, for use in other projects.

Soil tests
The soil test option in Plaxis is a convenient procedure to check the behavior of the selected soil material model with the given material parameters. After entering the model parameters, the user can quickly simulate several standard soil tests an compare the results against the results from actual laboratory tests.
 

Calculation features

Consolidation Analysis
The decay of excess pore pressures with time can be computed using a consolidation analysis. A consolidation analysis requires the input of permeability coefficients in the various soil layers. Automatic time stepping procedures make the analysis robust and easy-to-use. The “Staged Construction” option is available in a consolidation analysis.

Staged construction
This powerful Plaxis feature enables a realistic simulation of construction and excavation processes by activating and deactivating clusters of elements, application of loads, changing of water tables, etc. This procedure allows for a realistic assessment of stresses and displacements as caused, for example, by soil excavation during an underground construction project.

Calculation Manager
In the calculation manager several projects can be selected for subsequent calculations. This option allows for efficient computer usage during silent periods, alternatively this option is convenient for use in parametric studies.

Safety factors
The factor of safety is usually defined as the ratio of the failure load to the working load. This definition is suitable for foundation structures, but not for sheet-pile walls or embankments. For this latter type of structure it is more appropriate to use the soil mechanics definition of a safety factor, which is the ratio of the available shear strength to the minimum shear strength needed for equilibrium. Plaxis can be used to compute this factor of safety using a ‘phi-c reduction’ procedure.

Updated Lagrangian analyses
Using this option, the finite element mesh is continuously updated during the calculation. For some situations, a conventional small strain analysis may show a significant change of geometry. In these situations it is advisable to perform a more accurate Updated Lagrangian calculation, which is called Updated Mesh in Plaxis. This option is available for all types of calculations.

Arc-Length control
This feature enables accurate computations of collapse loads and failure mechanisms to be carried out. In conventional load controlled calculations the iterative procedure breaks down as soon as the load is increased beyond the peak load. With arc-length control, however, the applied load is scaled down to capture the peak load and any residual loads.

Automatic load stepping
The Plaxis program can be run in an automatic step size and automatic time step selection mode. This avoids the need for users to select suitable load increments for plastic calculations and it guarantees an efficient and robust calculation process.
    
The Calculations program considers only deformation analyses and distinguishes between a Plastic calculation, a Consolidation analysis and a safety analysis. Optionally, dynamic calculations can be performed. For each project several calculation phases may be specified prior to calculation.

Parameter Variation
This feature assists the user in checking the sensitivity of the calculation results to variations in the material parameters. Based on a given range of possible values for the material data input parameters Plaxis will automatically calculate the resulting variation in selected output values, such as displacements, stresses or structural forces.
 

Output features

Velocities and accelerations
Additional to the output from a static calculation the velocities and accelerations can be shown. Just like any other output it is possible to view velocities and accelerations for the whole geometry at a fixed time in the Plaxis Output program or view the velocities and accelerations in time for a fixed point with the Plaxis Curves program.

Load displacement curves & stress paths
A special tool is available for drawing load-displacement curves, stress and strain paths, stress-strain diagrams and time-settlement curves. The visualization of stress paths provides a valuable insight into local soil behavior and allows a detailed analysis of the results of a Plaxis calculation.

Animations
Video animations can be created for all graphic output including plots of deformation and forces for structural elements.

Report generator
A report generator has been implemented to provide a report of input data and output results that can be further edited in Word.

Cross-sections
This convenient option is available in Plaxis to create graphs for all types of stresses and displacements in any desired cross-section of the geometry.

Structural forces & Displacements
Graphs and tables are available for direct output of axial forces, shear forces, hoop forces and bending moments of structural elements. Besides output of stresses the displacements of the structural elements can be visualized. Both forces and displacements can be plotted per phase or as an envelope to cover all previous phases.

Stresses
Graphical output for stresses is available as: effective stresses, total stresses, Cartesian stresses, pore pressures and excess pore pressures. Stresses can be visualized in contour shadings as well.

Deformation
Graphical output for deformations is available as: deformed mesh, total or incremental displacements and as total or incremental strains. All displacement and strains graphs can be visualized as displacement arrows, contour lines or contour shadings.

The Plaxis postprocessor has enhanced graphical features for displaying computational results. Values of displacements, stresses, strains and structural forces can be obtained from the output tables. Plots and tables can be send to output devices or to the Windows® clipboard to export them to other software.

Presentation of results
The Plaxis postprocessor has enhanced graphical features for displaying computational results. Values of displacements, stresses, strains and structural forces can be obtained from the output tables. Plots and tables can be sent to output devices or to the Windows® clipboard to export them to other software.

Stress paths
A special tool is available for drawing load-displacement curves, stress and strain paths, stress-strain diagrams and time-settlement curves. The visualization of stress paths provides a valuable insight into local soil behavior and allows a detailed analysis of the results of a Plaxis calculations.