Education

Modeling Answers

  1. Refer to the document:

 

http://www.rocscience.com/downloads/phase2/SlopeStabilityUsingPhase2.pdf

Phase 2 data files for the slope stability analysis can also be downloaded:

Phase 2 slope stability data files. The data files are compatible with the

 

Phase 2 demo so you can look at them and run them using the demo. The data files are in a zip archive so require a program such as pkzip or winzip to extract them.

2. It is possible in

Phase 2 to change the properties of a material, modulus, strength etc. between stages. Remember that changing the modulus in a finite-element scheme has no effect if the model is in equilibrium in that stage. You must also have a change in loading or excavation in which case the incremental deformation will be a function of the new properties.

Phase 2 has the ability to do load splitting between stages to allow you to simulate such a case.

3. You can use joint (slip) elements as the interface between 2 materials.

4. In version 4 of

Phase 2, the Drucker-Prager material model has been added. You could also use the Mohr-Coulomb or Hoek-Brown criteria.

5. The program currently does not have this capability.

6. Getting the correct initial stress state for gravity loaded near surface excavations is one of the most difficult parts of a near surface FE analysis. The first step is to figure out what the initial stress state in the ground should look like. If you want an inclined surface with vertical stress equal to overburden pressure and you also want a stress ratio not equal to 1, and maybe locked in stresses as well, you should excavate the slope first. To do this you have to make a multi-stage model with a horizontal surface, then an excavation that intersects the ground surface representing the cutting out of the slope. In the first stage, do nothing but define a gravity-based block of material with the correct stresses and boundary conditions. In the second stage, excavate the slope. In the third stage excavate your underground or surface excavations. Make sure that the stress state is what you want prior to excavating in all stages. An example of a near surface excavation under a non-horizontal stress field can be found in the project gallery.

7. Yes, the bolt model that we use for plain strand cable bolts handles faceplates and pre-tensioning.

8. The issue of the modeling of composite support systems such as steel sets and concrete/shotcrete is currently being considered with the purpose of defining a method that people can use in

Phase 2. Currently you may use a tributary area method over a meter length of your tunnel to generate a single liner material that models the composite system.

9. Phase 2 is a total stress package. We do not currently support groundwater and effective stress analyses.

10. For simple cross-sections a rough estimate of the displacements and stresses can be determined from an axisymmetric analysis.

11. Phase 2 handles body forces, initial stresses, applied tractions (pressures), nodal forces etc. Basically any stress/force boundary condition you can think of. However it does not handle pore pressure or temperature boundary conditions.

12. The finite-element engine uses a small strain formulation.

13. There is no restriction on the number of elements or nodes.

14. The initial stress state for elements with these loading conditions is a combination of the initial stress and the self-weight of the material. The gravitational initial stress in an element is a function of the unit weight and depth below surface while the self-weight (body force) of the material is also a function of the unit weight. Since body force and initial stress are separate loading mechanisms, the unit weight is defined separately for each. This also ensures that different materials can have different unit weights.

15. Yes, use stage boundaries, an external surface equal to the final state with the completed retaining wall and an excavation whose geometry is the same as the retaining wall. The exterior boundary and the excavation boundary will overlap in spots. This is acceptable.

16. This is only possible if you want to add a vertical nodal displacement to a node with an existing “Restrain X” boundary condition, or a horizontal nodal displacement to a node with an existing “Restrain Y” boundary condition. What you should do is free the node (Restraints->Free) with the “Restrain X” or “Restrain Y” boundary condition. You should then add the nodal displacement that you want. If the nodal displacement has a horizontal component of zero this will be equivalent to having the “Restrain X” condition. For example: A nodal displacement of (0.00, 5.00) is the same as having a nodal displacement of (0.00,5.00) and a “Restrain X” condition. If you are interested, you can verify this for yourself by examining the section of the FEA file marked “restraints:”. Create a model with a “Restrain X” boundary condition on a node. Save this file. Free the Restraint on this node. Change this node to a nodal displacement of (0.00, 5.00), save to a different file, and then compare the files.

17. The initial element loading is one of the most difficult concepts to grasp in Finite-Element (FE) modeling. Basically, in FE an element can have two initial internal loadings, initial stress and body force. Body force is just self-weight. If you use just body force in a model then you will notice that a component of the displacements is due to your model settling under it’s own weight. If you use materials with initial element loading set to body force only then the Field Stress defined in the Loading menu is not used at all. This brings us to the case of initial stress. An initial stress locks in a particular stress in an element. The stress that is locked into the element is defined in the Field Stress menu. Think of a finite element as a sponge, applying an initial stress is like compressing the sponge. If you release confinement on an edge of the sponge it will expand in that direction. This is basically what happens when you open up an excavation in a material with an initial stress. Now how do body force and initial stress relate. They work to balance each other out. Think of an element with initial stress, then put a layer of material on top of it such that the material has enough body force to balance the initial stress. As a result the element is in equilibrium and won’t expand or contract. As an example, let’s think of a bucket (a rectangular region) of material represented by finite elements. The boundary conditions are free on the top surface and rollers on the left/right/bottom sides. If the material has just body force then it settles under it’s own weight and the top surface moves down. If the material has just initial stress then it expands and the top surface moves up. If you have both defined then the material is in equilibrium and there is no displacement of the top surface. To have the body force and field stress balanced you must use a gravitational Field Stress with unit weight equal to the materials unit weight and a ground surface elevation equal to the top of the bucket.

18.

  1. a) Create the excavation as a single CLOSED polyline using the pline command. Create the exterior boundary in the same way. Create materials and stage boundaries with the pline command but not closed. Don’t use arcs, just plines.
  2. b) You might try exporting only the excavation pline by using the dxf export option to pick the entity you want to export. If the excavation pline is the only entity in the dxf file,

Phase 2 should have an easier file,

Phase 2 should have an easier time importing it. Do the same for material stage and exterior boundary plines.

  1. c) Bring in the geometry one dxf file at a time starting with the exterior boundary then excavations, then material boundaries and stage boundaries. Only have one box checked (i.e. excavations) in the DXF import dialog in

Phase 2 .

  1. d) Try exporting using a release 12 or 13 dxf file instead.
  2. e) Start simple to get the process down. Draw a simple external boundary in autocad, export it. Draw a simple excavation, export it. Draw a couple of material boundaries, export them. Now bring them into

Phase 2 in the same order, one at a time.

19. How do I simulate the three-dimensional advance of a tunnel using

Phase 2 ? How do I get the proper deformation prior to support installation? 3D tunnel simulation using the Material Softening method in Phase2

20. How do I define the initial stress field under a non-horizontal ground surface? How do I set up the initial stress field under an embankment using Phase

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