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The Program and its Applications

The Program and its Applications. by Ari Cohen. Background. The Netherlands. 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

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The Program and its Applications

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  1. The Program and its Applications by Ari Cohen

  2. Background The Netherlands • 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

  3. Getting Started PLAXIS is comprised of four interconnected but separate interfaces • Input • Calculations • Output • Curves

  4. Tutorial 1: The Modeling the Settlement of a Circular Footing on Sand

  5. 1. Choose new or existing project

  6. 2. General Settings

  7. 3. Plane Strain or Axisymmetry / 15-Node or 6-Node

  8. 4. Plane Strain or Axisymmetry

  9. 4. Plane Strain or Axisymmetry Cont. y x

  10. 5. 15-Node or 6-Node

  11. 5. 15-Node or 6-Node Cont.

  12. 6. Dimensions

  13. 7. Geometry Contour

  14. 8. Loads & Boundary Conditions • Prescribed Displacements • Special Conditions placed on geometry lines that control the displacement of the lines • Drawn over geometry lines • Can be altered by double clicking on the geometry line associated with it • Fixities • Prescribed displacements equal to zero • Can be of horizontal, vertical, or total (horizontal & vertical) • Fixities take priority over displacements and other loads • Standard Fixities • Convenient and fast input option for many applications • Tractions • Distributed loads applied to geometry lines • Input values given in the dimensions force per unit area • Can be altered by double clicking on the geometry line associated with it • Point Forces • Are actually line loads in the out-of-plane direction • Can have vertical and horizontal components • Fixed Rotations • Fixes the rotational degree of freedom of a beam

  15. Select Standard fixities button 8. Loads & Boundary Conditions cont.

  16. Select Prescribed displacement button 8. Loads & Boundary Conditions cont.

  17. 9. Material Properties • Database with material data sets • Soil properties and material properties of structures are stored within the database as four types of material sets • Soils & Interfaces • Beams • Geotextiles & • Anchors • Modeling of Soil Behavior • There exist three types of soil models that PLAXIS supports: • Mohr-Coulomb model • Hardening-Soil model & • Soft-Soil-Creep model • Mohr-Coulomb is most often used as good soil data is not always available to the engineer or scientist • Modeling with the Mohr-Coulomb default requires the following five variables to be input: • Young’s modulus, (E) • Poisson’s ratio, (n) • cohesion, (c) • friction angle, (f) and • dilatancy angle, (y)

  18. 9. Material Properties cont. • All clusters and structural elements in a given model must be assigned a material before a mesh can be generated • The following soil parameters will be used for the footing settlement example:

  19. Click Materials button 9. Material Properties cont.

  20. Select “New” material 9. Material Properties cont.

  21. Type “sand” and leave the other default inputs Select “New” material Enter given data 9. Material Properties cont.

  22. Select “New” material Enter given data and click Ok 9. Material Properties cont.

  23. Drag & Drop the “Sand” material into the cluster 9. Material Properties cont.

  24. 10. Mesh Generation • Global Coarseness • Distinction is made between five levels of global coarseness: Very coarse, Coarse, Medium, Fine, and Very fine. Number of mesh elements generated ranges from about 50 elements for the coarse setting to about 1000 elements for the very fine setting • Global Refinement • Automatically generates a refined mesh; one step per selection • Local Coarseness • In areas where it may not be necessary to have a very refined mesh, the mesh may be made more coarse by adjusting the “Local element size” factor for a particular geometry point. This can be accessed by double clicking on any geometry point • Local Refinement • Instead of adjusting the “Local element size” factor, clusters, lines, or points can be selected and the local refinement option can be used

  25. Select “Generate mesh” Once the mesh window appears select “Update” 10. Mesh Generation cont.

  26. 11. Initial Conditions • Once the geometry model has been created and the mesh has been generated, the “Initial conditions” must be inputted. There are two different modes within the initial conditions tab: Water conditions mode and the Geometry configuration mode • Water Conditions mode • Water Weight:In projects that involve pore pressures, the input of water weight in necessary to distinguish between effective stresses and pore pressures. PLAXIS default water weight is set to 10kN/m3 • Phreatic Lines: Pore pressures and external water pressures can be generated on the basis of phreatic lines. With a phreatic line it is understood that water pressures above the line are zero and increase linearly with depth below the line. The phreatic line can be a general or user defined type • Groundwater Flow: In addition to generating water pressures using a phreatic line, water pressures can also be generated using groundwater flow calculations. This requires the input of groundwater head boundary conditions • Water Pressure Generation: After a phreatic line or groundwater boundary conditions are specified, the generate water pressures button is selected to complete the water conditions process • Geometry Configuration mode • Deactivating Geometry Components: In projects where embankments and structures are to be constructed the geometry model will have some elements that are initially not active. These elements must be deactivated in this mode. By default PLAXIS activates all elements of the model outlined in the previous geometry setup stage. Clicking on a particular element in this mode determines if it will be on or not • Initial Stress Generation (K0-Procedure): Initial stresses in a body are influenced by the weight of the material and the history of its formation. The stress state is generally characterized by an initial vertical stress, sn,0 which is related by the coefficient of lateral earth pressure K0. A default value is given based on Jaky’s formula (1-sinf).

  27. 1.Select “Initial conditions” button and accept default water weight 2. Water levels will not be generated in this example so proceed to generate initial stresses 11. Initial Conditions cont.

  28. Accept default K0 parameters 11. Initial Conditions cont.

  29. Length of lines note relative magnitude of principal stresses Orientation of lines note principal direction 11. Initial Conditions cont.

  30. Finally it is time to proceed to the Calculation phase of the program 11. Initial Conditions cont.

  31. Questions?

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