Generalized Hoek-Brown

The Generalized Hoek-Brown strength criterion is described by the following equation:

where:  

is a reduced value (for the rock mass) of the material constant (for the intact rock)

and are constants which depend upon the characteristics of the rock mass

is the uniaxial compressive strength (UCS) of the intact rock pieces

and are the axial and confining effective principal stresses respectively

In most cases it is practically impossible to carry out triaxial or shear tests on rock masses at a scale which is necessary to obtain direct values of the mb, s and a parameters in the Generalized Hoek-Brown equation. Therefore some practical means of estimating the material constants mb, s and a is required. According to the latest research, the parameters of the Generalized Hoek-Brown criterion [ Hoek, Carranza-Torres & Corkum (2002) ], can be determined from the following equations:

where:

Consequently, the input for the Generalized Hoek-Brown criterion in Slide offers two options:

Define Strength using mb, s, a

If this option is selected, then you can enter the parameters mb, s and a directly.

NOTE: the parameters mb, s and a can be calculated from GSI, mi and D using the above equations, by selecting the GSI button image\b_gsi_calculator.gif in the Define Material Properties dialog. See the Parameter Calculator topic for more information.

Define Strength using GSI, mi, D

If this option is selected, then you can enter the parameters GSI, mi and D directly.

NOTE: if you select the Pick button beside an edit box, you may access interactive tables and charts for:

Select the Pick button for the parameter that you wish to estimate, and you will see a table or chart which allows you to select an appropriate value for the parameter, for your material.

Parameters vary with depth

If you are defining strength using the GSI,mi,D option, then you will have the additional capability of defining these parameters as a function of depth. To do this:

  1. Select the Parameters vary with depth checkbox and select the Define button.

  2. You will see another dialog which allows you to define the parameters GSI, mi, D as Constant, F(Depth) or F(Datum) by selecting from the Type drop-list and entering the following.

Constant

GSI, mi or D is constant throughout the material.

F(Depth)

GSI, mi or D is a function of depth, where depth is measured from the top of the material layer, to the center of a slice base. For example:

F(Datum)

GSI, mi or D is a function of depth, where depth is measured from a user specified Datum (y-coordinate) to the center of a slice base. For example:

UCS (intact)

The intact UCS (ultimate compressive strength) is required input for the Generalized Hoek-Brown criterion, regardless of which method you have chosen to define the strength envelope (mb,s,a or GSI,mi,D).