Phase2 FAQs: Modeling

Below are answers to Modeling FAQs for Phase2. Click on the question to read the answer.

  1. How do I use Phase2 to do slope stability?

    As of version 6, Phase2 has built-in, fully automated slope stability using the SSR method. See the Shear Strength Reduction Overview topic for more information.

    Also see the Phase2Slope Stability Verification manual for many examples of slope stability analyses using Phase2. The example files are installed with the program so that you may run the verification examples yourself.

  2. How do I simulate the three-dimensional advance of a tunnel using Phase2? How do I get the proper deformation prior to support installation?

  3. 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 Phase2.

    See this Phase2 Developer's Tip:

    Setting up the Initial Stress State for Surface Models

    Also see the Gravity Field Stress topic. The Use Actual Ground Surface option can now be used to obtain a much better estimate of the initial stress field under a non-horizontal ground surface. Previously it was only possible to define a single datum elevation from which to measure the vertical stress.

  4. Is it possible to model hardening of Shotcrete in Phase2?

    It is possible in Phase2 to change the properties of a material (modulus, strength etc.) between stages. However, 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. Phase2 has the ability to do load splitting between stages to allow you to simulate such a case.

  5. Is it possible to model contact between structural members, e.g. between primary shotcrete and secondary concrete lining?

    You can model a liner/joint/liner scenario if you model one (or both) layers as a solid layer of finite elements, and then use joint (slip) elements as the interface between the 2 materials.

    You can model a joint/liner/liner... scenario using the Composite Liners option.

    You can model a joint/liner/joint scenario using the Structural Interface option.

  6. Does Phase2 provide a material model for concrete?

    The Drucker-Prager material strength model could be used for modeling concrete properties. You could also use Mohr-Coulomb or Hoek-Brown criteria.

  7. Can you define your own material model?

    It is not currently possible in Phase2 to define your own material model.

    However, you can take a Shear-Normal strength function, and use the program RocData to convert this to an equivalent Generalized Hoek-Brown function, which could then be used in Phase2.

  8. Can Phase2 model bolts with attached face plates and pre-tensioning?

    Yes, in Phase2 there are 3 different bolt models which allow for faceplates and pretensioning: Plain Strand Cable, Swellex/Split Set, and Tieback.

  9. How do I model steel sets embedded in shotcrete?

    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 Phase2. 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.

    The following document discusses how to obtain equivalent liner properties for composite support systems such as steel sets embedded in shotcrete.

    Equivalent Properties for Steel Set / Shotcrete Liners

  10. How can groundwater be modeled in Phase2?

    There are several ways of modeling groundwater in Phase2:

    • Phase2 has steady state finite element seepage analysis built right into the program.

    • You can also model groundwater pore pressure using piezometric lines, a water pressure grid, or Ru values.

    If groundwater is incorporated in the analysis, Phase2 will use the pore pressure information to calculate effective stress.

  11. How can 3D-stress and displacement distributions at the face of a tunnel be modeled with Phase2?

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

    Also see FAQ #2.

  12. What external loading is possible in Phase2?

    Phase2 handles body forces, initial stresses, distributed loads (pressures), concentrated loads, pseudo-static seismic load, ponded water load, groundwater pore pressure etc. Basically any type of stress/force/displacement boundary condition.

  13. Can large strain be considered in Phase2?

    In general, NO. The finite-element engine uses a small strain formulation.

  14. Is there any restriction as to the number of elements or nodes that can be used?

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

  15. What is the difference between the unit weight entered for a material with body force and the unit weight entered for a gravitational stress field?

    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.

    Also see FAQ #17.

  16. Can I build the external surface up in layers to model the construction of a retaining wall?

    Yes, this could be done in various ways, using stage boundaries and/or material boundaries.

    For an example of a retaining wall model, see the following tutorial:

    Retaining Wall Tutorial

    Note that the external boundary is used to enclose all other boundaries, so that you can freely define the retaining wall and backfill using any combination of material and/or stage boundaries, and carry out the construction of the model over any number of stages.

  17. What is the logic behind the Initial Element Loading for materials and how does this relate to the Field Stress defined in the Loading menu?

    See the Initial Element Loading help topic for a discussion of this issue.

    Also see FAQ #15.

  18. I'm having trouble importing DXF files into Phase2. Do you have any suggestions for making DXF import easier?

    See the Import DXF topic for complete details about importing DXF files and troubleshooting problems with DXF import.

  19. Are material properties saved with DXF files?

    DXF files only store the geometrical information associated with the object stored. Material properties are not stored.

  20. When we apply a distributed load to a stage boundary, the load is still visible even when the elements on the boundary have been excavated at a later stage. How can these loads be removed?

    Stage the load. When you add a distributed or line load, there is a check box in the dialog which allows you to stage the load. Set the Stage Factor = 0 for stages after the boundary has been excavated.

    See the Add Uniform Load help topic for more information.

  21. What guidelines are there for determining an Expansion Factor for the external boundary?

    The External boundary should be at least three typical diameters away from the zone of interest (i.e. Expansion Factor = 3). A typical diameter being the diameter of a circle that encloses the zone, or excavation, of interest. This is based on experience and analytical solutions (such as the Kirsch solution) which show that typical induced displacement profiles die out in around 3 diameters. NOTE: if there is plasticity and a failed zone in your model, then the typical circle diameter should include the zone of plasticity.

    If you are concerned that the proximity of the External boundary may be influencing the analysis results, then try a larger Expansion factor (e.g. Expansion Factor = 5), to verify that the boundary conditions are not significantly influencing the excavation behaviour. The only drawback to using larger Expansion factors is the increased number of elements, leading to increased computation time and larger file sizes.

  22. How do you select the most appropriate mesh type, element type, gradation factor, discretization etc?

    We always recommend that you experiment with these parameters in order to review the issue of mesh sensitivity. Basically how sensitive are your results to the finite element mesh? This is a common issue in finite element modeling and is well documented in the literature. There are many tools available in Phase2 for customizing the mesh, see the Mesh Overview topic for more information.