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# Strength Properties

The Strength Properties which are entered in the Tunnel and Rock Parameters dialog, will depend on the Solution Method selected in the Project Settings dialog.

In the Project Settings dialog, a small description of each solution method is displayed to the right of the Solution Method section once a specific method has been selected.

## Mohr-Coulomb Properties

The Duncan Fama, Vrakas and Anagnostou, Barbosa, and the Lee and Pietruszczak solution methods are formulated in terms of the Mohr-Coulomb failure criterion, therefore the following strength properties are required:

• Compressive Strength or Cohesion of the rock mass
• Friction Angle of the rock mass
• Dilation Angle of the rock mass

Other parameters may be required for specific methods. With the Barbosa method, for example, you may also input residual strength parameters.

## Calculate from GSI

It is important to note that the Mohr-Coulomb parameters can be calculated from Hoek-Brown parameters, by selecting the Calculate from GSI button in the Tunnel and Rock Parameters dialog. If you select this button, you will see the Parameter Calculator dialog, which allows you to compute Mohr-Coulomb parameters from Hoek-Brown parameters. See the Parameter Calculator topic for more information.

## Hoek-Brown Properties

The Carranza-Torres and Lee and Pietruszczak solution methods are formulated in terms of the Generalized Hoek-Brown failure criterion, therefore the following (peak) strength properties are required:

• GSI (Geological Strength Index)
• mi (Intact rock constant)
• D (Disturbance factor)
• Compressive Strength of Intact Rock
• Dilation Angle

### DEFINE PEAK STRENGTH PARAMETERS AS

In most cases, you will define the peak strength parameters using GSI, mi and D. In this case, the Generalized Hoek-Brown parameters mb, s and a, are automatically computed from GSI, mi and D, and the values are displayed in the dialog.

However, the Define Peak Strength As option, allows you to define the peak strength parameters directly in terms of mb, s and a. If you set Define Peak Strength As = mb, s, a, then you will be able to enter mb, s and a directly, rather than GSI, mi and D.

### RESIDUAL STRENGTH PROPERTIES

The Carranza-Torres solution (2004), can account for residual strength parameters of the rock mass. If you wish to define residual strength for the rock mass, then select the Residual Strength Properties check box, and enter values of mb, s and a which define a residual strength envelope. Note: the residual properties can only be defined in terms of mb, s and a. It is NOT possible to define residual strength using the GSI, mi and D parameters, as these are only applicable for the peak strength envelope.

### PARAMETER ASSIST DIALOGS

Whenever you see a button beside an input edit box, if you select this button you will see another dialog, in the form of a chart or table, which allows you to estimate a suitable value for the parameter. These "assist" or "pick" dialogs are available for the following variables: Intact UCS, GSI, mi and D.

NOTE: Other inputs may be available depending on the specific solution method selected