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# Opening Section Coordinates

When you define the Opening Section boundary or a ground/truncation surface, 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).

## Opening Section Orientation

The relationship of the Opening Section X-Y coordinate system to the 3-dimensional orientation of the excavation you are modelling is as follows:

### HORIZONTAL EXCAVATIONS

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.

### INCLINED OR VERTICAL EXCAVATIONS

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.

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.

## Coordinate Values

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.