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Failure Envelope Plots
RocData plots the failure envelopes corresponding to the current strength criterion and strength parameters which are in effect. Failure envelopes are plotted in both principal stress space (sigma1 versus sigma3) and shear-normal space (shear stress versus normal stress).
Display of principal stress plot and shear-normal plot
By default, both the principal stress envelope and the shear-normal envelope are displayed, as shown in the above figure. You can choose to display either plot type on its own, by selecting the desired option (principal stress or shear-normal) from the toolbar, the Analysis menu or the right-click menu.
Shear-normal strength envelope
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Failure envelopes: Interactive Data Analysis
As input data is changed, the failure envelope plots are instantly updated, allowing you to see the effect of changing strength parameters on the shape of the failure envelopes and the calculated output parameters.
Click on spinner arrows to change input data
Failure envelopes are updated as input data is changed
Other interactive failure envelope features include the Stress Sampler and Instantaneous Mohr-Coulomb Sampler, described below.
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Failure envelopes: Stress Sampler
The Stress Sampler option allows the user to graphically obtain the stress coordinates at any point along the failure envelopes. Just click the mouse anywhere on the plot and the corresponding stress coordinates on the failure envelope will be displayed.
Stress Sampler on shear-normal stress plot
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Failure envelopes: Instantaneous Mohr-Coulomb Sampler
The Instantaneous Mohr-Coulomb Sampler option allows you to graphically obtain the instantaneous Mohr-Coulomb parameters (cohesion and friction angle), at any point along a curved (non-linear) failure envelope (i.e. Generalized Hoek-Brown, Barton-Bandis or Power Curve).
Just click the mouse anywhere on the plot and the instantaneous Mohr-Coulomb envelope (tangential line) will appear on the plots, for the value of sigma3 or normal stress at which the mouse was clicked. The instantaneous cohesion and friction angle will be displayed, as well as the stress coordinates. A vertical dotted line will be displayed on the plots, to mark the location.
Instantaneous Mohr-Coulomb sampler on shear-normal stress plot
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Failure envelopes: Equivalent Mohr-Coulomb Envelope
If you are working with one of the non-linear strength criteria in RocData (Generalized Hoek-Brown, Barton-Bandis, Power Curve), you can display the equivalent Mohr-Coulomb envelope on the failure envelope plots by selecting the Mohr-Coulomb Envelope option.
For more information see the Equivalent Mohr-Coulomb Parameters topic.
Equivalent MC Envelope (blue) for non-linear shear-normal envelope (red)
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Failure envelopes: Display Options
Various options are available for customizing the appearance of the failure envelope plots. The Display Options dialog is shown below. Other options such as zooming and grayscale are also available.
Display Options dialog
For example, Mogi's Line can be displayed, which defines the ratio of major and minor effective principal stresses at which there is a transition from brittle to ductile failure. This line is defined by sig1/sig3 = 3.4, and is plotted as a green line on the principal stress plot, when the Mogi's Line option is selected in the Display Options dialog.
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If the principal stress failure envelope lies ABOVE Mogi's line, this indicates
a brittle failure mode.
If the principal stress failure envelope lies BELOW Mogi's line, this indicates
a ductile failure mode. This may occur with low values of GSI, for example.
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Display of Mogi's Line
The Graph Icons help you to quickly identify the type of graph you are viewing. The display of the Graph Icons can be turned on or off in the Display Options dialog.
Principal stress and shear-normal stress graph icons
By default, the Graph Icons are displayed at the lower right corner of the failure envelope plots. They can be moved to a different location by clicking on them with the mouse and dragging them to the desired location in the view.
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