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Strength Criteria
RocData offers four of the most widely used strength criteria for geotechnical engineering applications involving rock, soil or joint strength. The following options are available:
The strength criterion is selected in the Project Settings dialog. When a strength criterion is chosen, all input and output parameters will pertain to the selected criterion.
Strength criterion options in Project Settings
The following table summarizes the range of applicability for each strength criterion.
Applicability of strength criteria in RocData
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Strength Criteria: Generalized Hoek-Brown
The Generalized Hoek-Brown criterion is an empirical failure criterion for rock which establishes strength in terms of major and minor principal stresses. It predicts strength envelopes that agree well with values determined from laboratory triaxial tests of intact rock, and from observed failures in jointed rock masses.
The Generalized Hoek-Brown criterion is non-linear and relates the major and minor effective principal stresses (sigma1 and sigma3) according to the following equation:
The parameters of the Generalized Hoek-Brown criterion (mb, s, a) can be obtained from empirical relationships based on observations of GSI (Geological Strength Index), sigci (uniaxial compressive strength of intact rock), mi (intact rock m parameter) and D (rock mass disturbance factor).
For a given set of input parameters (sigci, GSI, mi and D), RocData calculates values of mb, s, a, and other rock mass parameters. Results are displayed in the Sidebar as shown below.
For more information see the Generalized Hoek-Brown Criterion help topic.
Hoek-Brown input and output parameters
Curve fitting of Hoek-Brown envelope to triaxial test data
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Strength Criteria: Mohr-Coulomb
The Mohr-Coulomb criterion is the most common failure criterion encountered in geotechnical engineering, and can be used to describe rock, soil or joint strength. Many geotechnical analysis methods and programs require use of this strength model. The Mohr-Coulomb criterion describes a linear relationship between normal and shear stresses (or maximum and minimum principal stresses) at failure.
Mohr-Coulomb parameters
The direct shear formulation of the criterion is given by the following equation:
where c is the cohesive strength, and phi is the friction angle.
The Mohr-Coulomb criterion for triaxial data is given by the following equation:
The Mohr-Coulomb criterion in RocData can be used to analyze either direct shear or triaxial test data.
RocData also calculates equivalent Mohr-Coulomb parameters for non-linear failure envelopes, over a specified stress range. See the Equivalent Mohr-Coulomb Parameters topic for more information.
Curve fitting of Mohr-Coulomb envelope to direct shear test data
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Strength Criteria: Barton-Bandis
The Barton-Bandis failure criterion is an empirical relationship widely used to model the shear strength of rock discontinuities (e.g. joints). It is useful for fitting a strength envelope to field or laboratory shear test data of discontinuities. The Barton-Bandis criterion is non-linear, and relates shear strength to normal stress using the following equation:
where phi_r is the residual friction angle of the failure surface, JRC is the joint roughness coefficient, and JCS is the joint wall compressive strength.
For more information see the Barton-Bandis Criterion help topic.
Barton-Bandis parameters
Curve fitting of Barton-Bandis envelope to direct shear test data
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Strength Criteria: Power Curve
A substantial amount of experimental evidence suggests that the failure envelopes of many geotechnical materials, ranging from clays and rockfill to rock discontinuities and rock masses, are not linear, particularly in the range of small normal stresses. The relationship between shear and normal stresses of such curved envelopes can be described with the Power Curve model:
In non-dimensional form the Power Curve criterion can be expressed as:
where Pa is the normalizing constant, usually atmospheric pressure.
The Power Curve model in RocData can be fit to either triaxial or direct shear data.
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Strength Criteria: Stress Units
In RocData you can choose from 8 different metric or imperial stress units for the analysis of strength data. The following options are available, as selected in the Project Settings dialog.
Stress Units option in Project Settings
The selected Stress Unit determines the units of all stress (strength) input and output data for the current file (e.g. principal stress, normal stress, shear stress, cohesion, etc).
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