With every road scheme you make in AutoCAD Civil 3D, you have to tie it into an existing road somewhere. This can be with a roundabout or a T-Junction, and in this Blog I’ll tackle it with a T-Junction.

What you normally receive is a bunch of 3D polylines to describe the existing centreline and the existing channel lines. If you don’t get a centreline then that’s a debate or blog for another time. From these 3D polylines you need to tie in the new centreline to the existing centreline, and the new channel lines to the existing channel lines.

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The Junction Wizard is great for new road junctions and can handle this type of scenario if you know what you are doing with it. You have to recreate the existing road alignments and profiles then tell the wizard to use them. However, there is a faster way…

You can simply convert the 3D polyline that describes the centre of the existing road into a feature line. Create your existing road corridor on the feature line and create a corridor surface. Remember to target the channel polylines both horizontally and vertically. This gives you a tie in point for the crown and also a point on the channel of the existing road. The channel is now being used as the Bellmouth.

To get the existing channel levels, you can extract from the Corridor an alignment and profile, selecting the channel feature line. In the UKIE country kit, this has the point code ‘Lane’. Now you have this, simply create the junction as normal with offset and connected alignments and profiles.

The result? A dynamic corridor that can be easily updated as the design changes.

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I’ve created a video of the process which can watch below if you can spare 20 minutes.

Tying Into An Existing Road With AutoCAD Civil 3D

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