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Strength Envelope for a Tunnel in Undisturbed Rock

1.0 Introduction

In this example, we’ll be deriving the principal stress and normal-shear strength plots for an undisturbed in-situ sandstone tock mass surrounding a tunnel at a depth of 100 meters using the Hoek-Brown strength criterion. With no test data available, we’ll be using reference data to determine the Hoek-Brown classification parameters.

Finished Product:

The finished product of this tutorial can be found in the Tutorial 1.rsdata data file. All tutorial files installed with RSData can be accessed by selecting File > Recent Folders > Tutorials Folder from the RSData main menu.

2.0 Creating the model

  1. Open RSData.
  2. project template

  3. Select the Rock Template for Project templates and click OK.

By default, strength graphs are presented, and a rock template is given for Rock Project Template. Within the rock template, a rock material model and an empty failure state are included. The rock material is also depicted on the strength graphs.

Strength graph

3.0 Define Material Properties

  1. Select the Define Material Models Define Material Models icon in the toolbar. A dialog will appear.

3.1 STRENGTH

  1. In the Strength tab, make sure the Failure Criterion is set to Generalized Hoek Brown.
  2. <

    Setting Failure Criterion to Generalized Hoek Brown in the Strength tab

    Ensure the Define by field is set to GSI, mi, D to see the GSI calculator fields. With the GSI Calculator users have access to reference tables for deriving classification parameters. Enter this data in the dialog by doing the following:

  3. Input a value of 75 for UCS of intact tock (intact) (MPa).
  4. Click on the GSI Picker icon GSI Picker button beside the GSI input. Based on the chart, we will input a value of 55. Click OK to close the dialog.
  5. GSI input dialog box

  6. Click on the list button beside the mi (aka Intact Rock Constant mi) input and select Sandstones from the list to change the Mi Value to 17. Click OK to close the dialog.
  7. Keep the D (aka Disturbance factor) input as 0.

The dialog should look as follows:

Define Material Models Dialog

3.2 STIFFNESS

Switch the Stiffness tab and enter the following:

  1. In the Defined by field select MR.
  2. Click on the Pick MR Value button Pick MR Value button beside the MR input, and select Sandstones from the list.
  3. Pick MR value dialog box

  4. Click OK to close the dialog.

The dialog should look as the following:

Dialog box proceeding step 3

3.3 FAILURE RANGE

Switch to the Failure Range tab.

  1. Enter the following data to determine the Failure Envelope Range (sig3max), which is used to calculate the equivalent Mohr-Coulomb parameters for the Hoek-Brown model.
    • Application = Tunnels
    • Unit Weight (MN/m3) = 0.027
    • Tunnel Depth (m) = 100

    The dialog should look as follows:

    Dialog box proceeding step 1 regarding failure range

  2. Click OK to save and close the Define Material Models dialog.

4.0 Results

The Normal vs. Shear Stress plot will be updated as follows:

Normal vs Shear Stress plot

Select the arrow on the right side of the screen to open the results panel. Then go to the Visibility Tree and select Material Model (Rock).

Material rock model

The resulting outputs and equivalent Mohr-Coulomb parameters are calculated and displayed in the data table as shown below.

Outputs and equivalent Mohr-Coulomb paremeters

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