# Tunnel definition

When defining a tunnel, you create a TXL file and then key in tunnel components from construction drawings and plans to complete the tunnel definition.

The tunnel definition must include the horizontal alignment, vertical alignment, templates, and template positions. Other elements are optional.

- The horizontal alignment defines a line that runs along the center of the tunnel.
- The vertical alignment defines the changes in the elevation of the tunnel.

- The template defines a cross section of the tunnel at a point across the tunnel to define how wide it is at different points.

  Add a template for each change in width. The template may consist of any number of surfaces.

- Add template positions to assign the appropriate template at different points along the tunnel.

- Add rotation to tilt or rotate a tunnel template and the associated set out positions around an origin point.

  Rotation is mainly used around a horizontal curve to represent superelevation. However, it can be used anywhere in the tunnel alignment provided that there is a valid horizontal alignment, vertical alignment, and template assigned.

- Add set out positions to pre-define blast holes, bolt holes, or pipe insertion points to be set out in the tunnel.

- Station equations define station values for an alignment.

- Alignment offsets offset the horizontal and/or vertical alignment, typically for carriage clearance around curves in a rail tunnel. See Alignment offsets.

Keyed in tunnels are saved to the current project folder as TXL files.