button, and paste applicable data from RocData
or RocLab by selecting the 
button. See below for further information.In the Define Material Properties dialog, the Strength Parameters allow you to define:
the failure (strength) criterion for a material
the material type (elastic or plastic)
Failure Criterion
The following strength criteria are available in Phase2 for defining the strength of your rock mass or soil:
See below for information about each failure criterion.
NOTE: for the Mohr-Coulomb, Hoek-Brown or Generalized Hoek-Brown criteria, you can link directly to RocData or RocLab to help determine values of input parameters.
Material Type
You may select either Elastic or Plastic for the Material Type.
Elastic Material
If you choose Material Type = Elastic, then the failure criterion parameters which you enter will only be used for the calculation and plotting of strength factor within the material. Although an Elastic material cannot "fail", the failure envelope allows a degree of overstress to be calculated.
Plastic Material
If you choose Material Type = Plastic, the strength parameters you enter will be used in the analysis if yielding occurs. This is unlike Elastic materials, where the strength parameters are only used to obtain values of strength factor, but do not affect the analysis results (i.e. stresses and displacements are not affected).
If you define a material to be Plastic then you may also define residual strength parameters and a dilation parameter, depending on the strength criterion.
If the residual strength parameters are equal to the peak parameters, then you are defining an "ideally" elastic-plastic material.
The dilation is a measure of the increase in volume of the material when sheared (see below for more information).
NOTE: if you define a material as Plastic, then you are restricted to Isotropic elastic properties for that material. You cannot combine plasticity with Transversely Isotropic or Orthotropic elastic properties.
For the Mohr-Coulomb criterion you must define the following parameters:
Cohesion
Friction Angle
Tensile Strength
If you are not considering pore pressure in the analysis, then the cohesion and friction angle are total stress parameters. If you are considering pore pressure, then cohesion and friction angle are effective stress parameters.
If the Material Type = Plastic, you will also be able to define:
Dilation Angle
Residual values of cohesion and friction angle
Link to RocData / RocLab
For assistance with determining Mohr-Coulomb
parameters you can startup RocData
or RocLab by selecting the button, and paste applicable data from RocData
or RocLab by selecting the button. See below for further information.
The Hoek-Brown strength criterion in Phase2, refers to the ORIGINAL Hoek-Brown failure criterion [ Hoek & Bray (1981) ], described by the following equation:
Note that this is a special case of the Generalized Hoek-Brown criterion, with the constant a = 0.5. See below for definition of the parameters in this equation.
The original Hoek-Brown criterion has been found to work well for most rocks of good to reasonable quality in which the rock mass strength is controlled by tightly interlocking angular rock pieces.
For lesser quality rock masses, the Generalized Hoek-Brown criterion can be used.
Link to RocData / RocLab
For assistance with determining Hoek-Brown
parameters you can startup RocData
or RocLab by selecting the button, and paste applicable data from RocData
or RocLab by selecting the button. See below for further information.
For the Generalized Hoek-Brown criterion you must define the following parameters:
The intact uniaxial compressive strength (UCS) of the rock.
parameters mb, s and a
If the Material Type = Plastic, you will also be able to define:
Dilation parameter
Residual values of mb, s and a
The Generalized Hoek-Brown strength criterion is described by the following equation:
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 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 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:
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.
Parameter Calculator
The parameters GSI, mi, D and UCS can be
estimated for your material using the Parameter Calculator dialog, which
is available by selecting the GSI button 
in the Define
Material Properties dialog. Values of mb, s and a are automatically calculated
from the above equations, and the rock mass modulus is also calculated.
See the Parameter Calculator topic
for more information.
Link to RocData / RocLab
For assistance with determining Generalized
Hoek-Brown parameters, you can startup RocData
or RocLab by selecting the button, and paste applicable data from RocData
or RocLab by selecting the button. See below for further information.
The Drucker-Prager strength parameters are:
Tensile Strength
q parameter
k parameter
If the Material Type = Plastic, you will also be able to define:
Dilation parameter
q (residual), k(residual)
NOTE: if you wish to calculate equivalent Drucker-Prager parameters based on Mohr-Coulomb parameters, click here for the appropriate equations.
Specification of the Cam-Clay model requires five material parameters, and the initial state of consolidation. These parameters are summarized below. For a theoretical overview of the Cam-Clay and Modified Cam-Clay strength models, see the Theory section.
Lambda
Lambda (
) is the slope of
the normal compression (virgin consolidation) line and critical state
line (CSL) in 
space.
Kappa
Kappa (
) is the slope of
a swelling (loading-unloading) line in space.
Critical State Line Slope (M)
The slope (M) of the Critical State Line
(CSL) in 
space.
Specific Volume (N or Gamma)
There are two possible methods for defining
the specific volume parameter. The N
parameter defines the specific volume of the normal compression line at
unit pressure. The Gamma (
) parameter defines the specific
volume of the CSL at unit pressure. The choice of parameter can be selected
by the user in the Cam-Clay Options
dialog (see below).
Elastic Parameters
There are two possible methods of defining the elastic parameter for a Cam-Clay material. You may enter either the Shear Modulus or Poisson’s Ratio. The choice of parameter can be selected by the user in the Cam-Clay Options dialog (see below).
Initial State of Consolidation
There are two possible methods for defining the initial state of consolidation. The Overconsolidation Ratio (OCR) is the ratio of the previous maximum mean stress to the current mean stress. Or you can specify the Preconsolidation Pressure (Po). The choice of parameter can be selected by the user in the Cam-Clay Options dialog (see below).
Cam-Clay Options
If you select the Options button in the Define Material Properties dialog, you will be able to choose the method of defining the following Cam-Clay parameters:
Elastic Parameters (Shear Modulus or Poisson's Ratio)
Specific Volume at Unit Pressure (N or Gamma)
Initial State of Consolidation (OCR or Preconsolidation pressure)
The selections in the Cam-Clay Options dialog determine the Cam-Clay parameters which can be entered in the Define Material Properties dialog, for a given material.
The Modified Cam-Clay strength model in Phase2 has the same input parameters as the Cam-Clay model, but uses the Modified Cam-Clay equations. See above for a summary of input parameters.
For a theoretical overview of the Cam-Clay and Modified Cam-Clay strength models, see the Theory section.
Dilation Parameter
A dilation parameter can be defined for Mohr-Coulomb, Hoek-Brown and Drucker-Prager materials, if the Material Type = Plastic.
Dilatancy is a measure of how much volume increase occurs when the material is sheared.
For a Mohr-Coulomb material, dilatancy is an angle that generally varies between zero (non-associative flow rule) and the friction angle (associative flow rule).
For Hoek-Brown materials, dilatancy is defined using a dimensionless parameter that generally varies between zero and m.
Low dilation angles/parameters (i.e. zero) are generally associated with soft rocks while high dilation angles/parameters (i.e. phi or m) are associated with hard brittle rock masses. A good starting estimate is to use 0.333*m or 0.333*phi for soft rocks and 0.666*m or 0.666*phi for hard rocks.
Link to RocData / RocLab
If you have the programs RocData or RocLab installed on your computer, you can start up these programs directly from the Define Material Properties dialog. You can then use RocData or RocLab to help determine parameters for the Mohr-Coulomb, Hoek-Brown or Generalized Hoek-Brown criteria (e.g. by curve fitting lab test data, for example).
To start up RocData
or RocLab select the button in the Define Material Properties dialog. If you have
both programs installed on your computer, then RocData
will be opened.
To paste applicable results from RocData
/ RocLab into Phase2,
first select the Copy Data option
in RocData / RocLab,
and then select the paste button in the Define Material
Properties dialog in Phase2.
Applicable data will be pasted into the dialog. If applicable results
for the selected strength criterion are not found, an error message will
be displayed.
For information about RocData and RocLab see the Rocscience website.