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Stress Analysis
The default Unwedge analysis is based upon the assumption
that the wedges are subjected to gravitational loading only (i.e.
self-weight of the wedges). The actual stress field in the rock
mass surrounding the excavation is not taken into account.
While this assumption leads to some inaccuracy in the analysis, the
error is generally conservative, leading to a lower factor of safety.
You may include the effect of in-situ stress (clamping stress) on
the wedges, with the Field Stress option.
Normal stress contours on wedge planes.
To include Field Stress:
1. Select the Field Stress option from the Analysis menu.
2. In the Field Stress dialog, select the “Use Stresses in
Analysis” checkbox.
3. Select the Field Stress type (Constant or Gravitational) and
enter the
required parameters.
4. When you select OK, a boundary element stress analysis will be
computed.
This will determine the stress distribution
around the excavation.
5. The stress distribution is then used to determine the induced
normal and
shear stresses on each wedge plane,
and these stresses will be included
in the wedge safety factor calculations.
It is important to understand that the inclusion of Field Stress in
the Unwedge analysis will (in general) increase the safety
factor of a wedge. The Field Stress acts to "clamp" the
wedge in place (it can never lower the safety factor). If long term
behaviour is being considered, then the Field Stress option should
not be used, so that the analysis and design of support is conservative.
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Stress Analysis: Constant Field
Stress
Use the Constant Field Stress option if you wish
to specify an initial in-situ stress field (prior to excavation),
which is constant and does not vary with depth. Only the direction
and magnitude of the principal stresses need to be defined.
Constant Field Stress parameters.
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Stress Analysis: Gravitational
Field Stress
Use the Gravity Field Stress option if you wish to
specify an initial in-situ stress field, which varies linearly with
depth from a specified elevation.
Gravitational
Field Stress parameters.
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Stress Analysis: How does it work?
The stress analysis in Unwedge uses the boundary element
method and also uses a Complete Plane Strain formulation. This means
that although the excavation model is 2-dimensional, the field stress
and the stress analysis results can be 3-dimensional.
For further details about the assumptions and implications of including
Field Stress in the Unwedge analysis, please see the Field
Stress topic in the Unwedge Help system.
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Stress Analysis: Advanced Settings
The stress analysis in Unwedge is not a “black box”
– advanced users who wish to configure the settings of the analysis
can do so with the Advanced Stress Settings dialog. This allows you
to configure the rock mass elastic parameters, boundary element discretization,
solver type and other parameters.
Advanced
stress settings dialog.
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Stress Analysis: Viewing Stress
Contours
If you have included Field Stress in your Unwedge analysis,
then you can view the actual stresses computed on the wedge planes.
To do this, right click on any Perimeter wedge in the 3D wedge view.
From the Field Stress Visualization sub-menu, select a stress type
(Normal Stress, Shear Stress etc.) and the joint planes of all wedges
will immediately display contours of the selected stress type.
Selecting stress contours to view on wedge planes.
When you view stress contours on the wedge planes, a Field Stress
Legend will indicate the contour values. The stress at any point on
a wedge face can also be obtained as a “data tip” –
simply hover the mouse over the stress contours and the value of stress
at that point will be displayed, as shown in the figure below.
Data tips display of stress on wedge face.
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