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Wedges
Using block theory, Unwedge determines all of the possible
wedges that can be formed by the intersection of 3 joint planes and
the excavation. The method used for finding the wedges is described
in the text by Goodman and Shi (1985), “Block Theory and Its
Application to Rock Engineering”.
Generally the wedges are tetrahedral in shape, i.e. the 3 joint planes
make up 3 sides of a tetrahedron and the fourth “side”
is formed by the excavation boundary.
Tetrahedral wedges (wireframe viewing mode).
However prismatic wedges can also be formed. This can occur if two
of the joint planes strike in the same direction, so that the resulting
wedge is a prismatic rather than a tetrahedral shape. In this case,
the third joint plane will truncate the prism at either end.
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Wedges: Perimeter Wedges
Unwedge always determines the wedges which can form around
the perimeter of the excavation. A maximum of six wedges can be formed
around the perimeter. Less than six wedges may be formed, depending
on the joint orientations and the shape and orientation of the excavation.
Perimeter wedges around access ramp.
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Wedges: End Wedges
Unwedge also finds the wedges that can be formed at both
“ends” of the excavation. If the Opening Section represents
the plan view of a horizontal excavation, then the End Wedges will
be the roof and floor wedges.
End wedges (maximum size) at ends of cavern.
End wedges (scaled size) – roof wedge of drawpoint
example.
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Wedges: Failure Modes
The following wedge failure modes can occur:
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Falling
Sliding on single joint plane
Sliding on two planes, along line of intersection
Lifting (e.g. high water pressure on joints)
Stable
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The wedge failure mode is reported in the summary of wedge
information analysis results.
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Wedges: Scaling Wedge Size
Unwedge always initially calculates the maximum sized wedges
which can form around an excavation. The Scale Wedges
option allows you to scale down the size of the wedges, according
to actual field observations, including:
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Joint trace lengths
Joint persistence
Wedge volume / weight
Excavation face area
Apex Height
Tunnel length |
Scale Wedges dialog.
To scale the wedge size, select the desired parameters in the Scale
Wedges dialog, using the checkboxes, and enter the scaling values.
If you enter multiple scaling parameters, Unwedge will automatically
use the parameter that gives the smallest wedge size for each wedge.
You may choose to apply the scaling to All Wedges, or individual wedges
can be independently scaled.
Scaled roof wedge.
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