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.
If the Solution Method = Duncan Fama, then Mohr-Coulomb strength properties are entered.
If the Solution Method = Carranza-Torres, then Hoek-Brown strength properties are entered.
Mohr-Coulomb Properties
The Duncan Fama solution method is formulated in terms of the Mohr-Coulomb failure criterion, therefore the following strength properties are required:
Compressive Strength of the rock mass
Friction Angle of the rock mass
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 solution method is 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 checkbox, 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.