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Steps for a Typical Probabilistic Analysis

In a Probabilistic Analysis, statistical information can be entered to account for uncertainty in joint orientation, shear strength and other parameters. This results in a Factor of Safety distribution for each wedge from which probabilities of failure are calculated.

The following general procedure outlines the typical sequence of steps used to carry out a Probabilistic Analysis with UnWedge:

  1. Project Settings: Make sure you select a Probabilistic Analysis type in the General tab. Select the desired Unit System (metric or imperial) and if you are analyzing End Wedges, make sure the Compute End Wedges checkbox is selected. You may use a Design Standard from the Design Standard tab, and you may also specify a Project Title from the Project Summary tab. Additionally, with a Probabilistic Analysis, you may specify a Sampling Method and Number of Samples as well as a Random Number Generator.
  2. Define Opening Section (Add Opening Section / Import DXF): Define the opening cross-section of your excavation with the Add Opening Section option or import the coordinates from a DXF file. You can also define or import a Ground Surface or a Truncation Surface at this step.
  3. Input Data > Tunnel Axis Orientation , Unit Weight: Enter the Tunnel Axis Orientation and the rock unit weight under the General tab of the Input Data dialog.
  4. Input Data > Joint Orientations: Enter the joint orientations under the Joint Orientations tab of the Input Data dialog. Joint orientations can also be imported from a Dips file.
  5. Input Data > Joint Properties: Enter the joint properties (strength, water pressure, structure) under the Joint Properties tab of the Input Data dialog.
  6. Assign Joint Properties: If your joints have different properties, assign the desired properties to the joints under the Joint Orientations tab of the Input Data dialog.
  7. Probabilistic Input Data: To carry out a Probabilistic Analysis with UnWedge , at least one input parameter must be defined as a random variable. The Probabilistic Input Data options are found in the Statistics menu.
  8. Statistics > Joint Orientations: Enter the variability in the Dip and Dip Direction of the joints or use a Fisher Distribution.
  9. Statistics > Joint Properties: Enter the variability in any joint properties (tensile strength, phi, etc.).
  10. Compute: Select Compute Calculator Icon to compute the results.
  11. Probability View - Switch to this view using the drop-down menu on the toolbar. You will see the Maximum Support Pressure, as shown in the Sidebar. To display a specific result, select it from the drop-down menu on the Sidebar.
  12. Plots: You can plot histograms, cumulative plots, or scatter plots of the results you are interested in by using the toolbar buttons or the Statistics menu. You can also plot the results of a specific trial (i.e., a "picked wedge") by right-clicking on the desired segment and selecting the necessary plot.
  13. Wedge Information: The Wedge Information Panel displays the results of the Deterministic Analysis. When you click on a certain segment, the panel displays the results of the specific "picked wedge."
  14. Scale Wedges: By default, UnWedge initially calculates the maximum-sized wedges that can form around the excavation. Use the Scale Wedges options to scale down the size of the wedges according to actual field observations (e.g. observed joint trace lengths).
  15. Support Design (Bolts / Shotcrete / Pressure): If any wedges require support to achieve the design Factor of Safety, you can add bolts, shotcrete, or pressure support to the model to determine the support requirements that are necessary. Use the Perimeter Support Design View to define support around the perimeter of the excavation. Use the End Support Design View to define support for the End Wedges of the excavation. You can add variability to the support as well:
  16. Statistics > Bolt Properties: Enter the variability in any bolt properties (tensile capacity, plate capacity, etc.). You can also use a random bolt offset by checking the appropriate checkbox.
  17. Statistics > Shotcrete Properties: Enter the variability in any shotcrete properties (shear strength, thickness, etc.).
  18. Advanced Features:
    • Compute All Joint Combinations: If you have more than three joint planes of interest, you can analyze all possible combinations of three joint planes by checking the Compute All Joints Combinations (Probabilistic) checkbox in the Joint Orientations tab of the Input Data dialog. You will then be able to view the results (e.g., Maximum Support Pressure) from all the possible combinations of three joint planes in the Probability View.
    • Field Stress: With the Field Stress option on the toolbar or the Analysis menu, you may include the effect of in-situ stress (clamping stress) on the Factor of Safety of the wedges. You can add variability to these values as well by selecting Statistics > Field Stress.
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