Strength Properties

In Examine2D there are two options for defining the strength properties of your material:

Mohr-Coulomb
Generalized Hoek-Brown

To define the Strength Properties:

Select the Strength Criterion (Mohr-Coulomb or Generalized Hoek-Brown) from the Project Settings dialog, or by clicking on the Strength button in the Sidebar.
Enter the required Strength parameters in the Sidebar. See below for more information.

NOTE: Strength Properties in Examine2D are ONLY USED to compute the Strength Factor contours (i.e. degree of overstress, based on the elastic stress distribution). Plasticity and stress re-distribution due to failure cannot be modeled in the Examine2D boundary element elastic analysis. If you want to model progressive failure (i.e. plastic analysis) then use the finite element program Phase2.

Mohr-Coulomb

For the Mohr-Coulomb strength criterion the following input parameters are required:

Cohesion
Friction Angle
Tensile Strength

The Mohr-Coulomb criterion is applicable for either rock or soil. If you are using the Mohr-Coulomb criterion for rock, then your input parameters should represent the rock mass strength, and not the intact rock properties.

TIP: the program RocData can be used to obtain equivalent rock mass Mohr-Coulomb parameters, based on Generalized Hoek-Brown rock mass classification parameters.

Generalized Hoek-Brown

For the Generalized Hoek-Brown strength criterion, the rock mass strength parameters can be defined in two ways:

by entering the parameters mb, s and a (and intact UCS)
by entering the parameters GSI, mi and D (and intact UCS)

Generalized Hoek-Brown (mb, s, a)

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

Eqn.1

where:

mb is a reduced value (for the rock mass) of the material constant mi (for the intact rock)
s and a 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 principal stresses respectively

If you choose the Generalized Hoek-Brown (mb,s,a) option in Examine2D, then the parameters mb, s and a are entered directly.

Generalized Hoek-Brown (GSI, mi, D)

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 parameters (mb,s,a) in the Generalized Hoek-Brown equation. Therefore some practical means of estimating the material constants mb, s and a is required. If you choose the Generalized Hoek-Brown (GSI, mi, D) option in Examine2D, then you must enter the parameters GSI, mi, D and intact UCS, from which the parameters mb, s and a will be automatically calculated according to the following equations [Hoek, Carranza-Torres & Corkum (2002)]:

Eqn.2

Eqn.3

Eqn.4

where:

GSI is the Geological Strength Index
mi is a material constant for the intact rock
the parameter D is a "disturbance factor" which depends upon the degree of disturbance to which the rock mass has been subjected by blast damage and stress relaxation. It varies from 0 for undisturbed in situ rock masses to 1 for very disturbed rock masses.

NOTE: this method still uses Equation 1 to determine the rock mass strength, with the addition of Equations 2, 3 and 4 to calculate mb, s and a from GSI, mi and D.

Estimating Parameters GSI, mi, D and intact UCS

If you are using the Generalized Hoek-Brown (GSI, mi, D) option in Examine2D, the input parameters can be estimated using convenient charts and tables, by selecting the Pick button located beside each of the input parameter edit boxes in the Sidebar.

When you select a Pick button, a table or chart will appear, allowing you to determine a suitable value for the desired parameter. For example, the dialogs for estimation of mi and GSI (Rock Type = General) are shown below.

Dialog for estimation of mi value

Once you have determined a value, select OK in the Pick dialog, and the value will be automatically loaded into the Sidebar.

GSI chart (Rock Type = General)

Notice that there are TWO distinct GSI charts:

One for general rock mass types, and
A second for weak, heterogeneous rock mass types such as flysch, which extends the useful range of GSI down to values as low as 5.

Select the Pick button for each of sigci, mi, GSI and D, and experiment with the dialogs.

For further information about the Hoek-Brown classification parameters sigci, mi, GSI and D, please consult Hoek, Carranza-Torres, Corkum (2002).

The Generalized Hoek-Brown strength criterion is strictly applicable for modeling rock mass strength, and does NOT apply to soils.