When you define the Opening Section boundary, a local X-Y coordinate system is used, where:
the X-axis is horizontal and positive to the right (in the plane of the 2D section).
the Y-axis is vertical and positive upwards (in the plane of the 2D section).
The relationship of the Opening Section X-Y coordinate system to the 3-dimensional orientation of the excavation you are modeling, is as follows:
If your Tunnel Axis Orientation is horizontal (i.e. Plunge = 0 degrees), then the positive Y-axis of the Opening Section boundary corresponds to the Global Vertical Up direction.
If your Tunnel Axis Orientation is non-horizontal (i.e. Plunge > 0), then the positive Y-axis of the Opening Section boundary must correspond to the Trend of the Tunnel Axis Orientation.
Note: this is also true for vertical excavations (Plunge = 90 degrees)
In all cases, the X-axis of the Opening Section boundary is perpendicular to the Tunnel Axis Orientation.
The absolute values of the X-Y coordinates which you use to define the Opening Section boundary, is not important, except for the following cases:
If you are defining a Gravitational Field Stress, then the Y-coordinates of the Opening Section must be consistent with the Ground Surface Elevation which you define in the Field Stress dialog.
If you are defining an Elevation based Water Pressure for your Joints, then the Y-coordinates of the Opening Section must be consistent with the Elevation of the water table which you define in the Input Data > Joint Properties dialog.
The absolute values of the X and Y coordinates of the Opening Section boundary, should have a similar order of magnitude. For example, if the X coordinates are small (near zero) then the Y coordinates should also be small. Or if the X coordinates are large, then the Y-coordinates should be large. If there is a very large difference in the order of magnitude between the X and Y coordinates, then numerical problems may result. For example, if X coordinates were small (near zero) and Y coordinates were very large (> 1 million), then numerical problems may occur with the analysis.
Except for these cases, you may define the Opening Section boundary coordinates anywhere in the X-Y plane, and this will not affect the results of the Unwedge analysis.