Civil 3D Intersections – Fixing an Alignment that Crosses Itself

If you have an alignment that loops around and creates an intersection on itself, you have a situation to say the least. These are rare but they do exist in the real world and in real design. The AutoCAD Civil 3D Intersection command doesn’t know what to do with this intersecting alignment. It doesn’t give you a reply saying, “I can’t resolve a self-intersecting alignment.” It doesn’t even crash or give you an error. The command just does nothing, as if it is waiting for you to pick another location so it can go to work. So, there are a few work-arounds.

The DIY Intersection

One work-around would have you create the intersection manually. Although challenging, it isn’t a good use of time. It takes longer and everything that the intersection tool does automatically, has to be done manually:

• Creating offset and curb return alignments.
• Creating offset and curb return profiles.
• Defining quadrant baselines.
• Defining quadrant regions.
• Creating the primary through region.
• Configuration of targets.
• Altering the secondary alignment to reflect the geometry of the primary.
• Inserting manual sections in quadrants at the intersection point.

That’s a lot of room for manual error.

Spackle with Grading

Another work around is to grade in the intersection with feature lines. This is a manual effort also:
• Create feature lines at the locations of edges (pavement lines, curb lines, sidewalk lines, crowns of the secondary; and crown or edge of pavement of the primary depending on intersection type).
• While creating the feature lines, be careful not to OSNAP to pavement layers that are not part of the top layer of the corridor.
• Match end of feature line elevations to adjacent corridor regions.

Maybe something magical could be done involving the extraction of corridor feature lines to “influence” the elevation of the intersection feature lines. However, staying on top of those endpoint elevations can be like herding cats.

The Short Alignment

Another method is to duplicate the loop alignment that does not involve herding cats but still takes advantage of much of the Intersection automation. This is not a copy-paste thing. This is something done diligently, but resulting with a quicker and more intelligent outcome. Here are the steps.

To duplicate the alignment:

1. Execute the Create Alignment from Existing Alignment tool from the Home ribbon, Create Design panel, Alignment tool.
   
2. Select the loop alignment.
3. At the Specify Split Point prompt, OSNAP to the point of intersection of the loop road or type the station value at the prompt.
   
4. At the Specify Next Point prompt, drag the cursor up about 50 feet past the curb return endpoint of the secondary alignment or type the station value at the prompt.
   
5. Type F for Finish.
6. In the Create Alignment from Existing Alignment dialog, type in a name for the shorter version of the alignment and click OK.
   
7. Click on the short alignment. Select Surface Profile from the Launch Pad panel of the alignment contextual menu.
   
8. Create the surface profile and the profile view for the short alignment.
   
9. Select the profile view of the short alignment and on the profile view contextual ribbon, Launch Pad panel, click Profile Creation Tools. Then click OK to the Create Profile dialog.
   
10. Click on the Draw Tangents without Curves tool and trace over the top of the surface profile in the short alignment profile view. Just OSNAP to the start and end of the profile.
    
11. Pan over to the intersection. Click on the loop alignment, grab its endpoint grip at the intersection, and pull it over to the endpoint of the short alignment.
    

Now we are ready to run the Intersection command.

12. On the Home tab of the ribbon, on the Create Design panel, Intersections tool. Click on Create Intersection.
    
13. Click on the location of the intersecting alignments (loop and short), then step through the Create Intersection wizard configuring your settings, parameters, current corridor, and subassembly sets. Then click Create Intersection.
    

Here is the intersection:

14. Return the loop alignment’s grip to its original location to “undo” the edit in step 16.

Here is the short profile:

The next step will be to superimpose the loop alignment’s design profile into this profile view. The superimposed profile will be dynamic with the loop alignment’s design profile. This will help us to match up the final PVI of the short alignment with the loop alignment.

15. Click on the short alignment’s profile view and click Superimposed Profile.
    
16. Using the pickbox, pan over to the loop alignment’s profile view and select the design profile.
    
17. Next, select the profile view of the short alignment.
    
18. In the Superimpose Profile Options dialog, fill in the check box next to Select End and type in a station value about 5 feet greater than the start value. Then click OK.
    

In the short alignment’s profile view, we can see the superimposed version of the loop alignment displayed below highlighted. This provides a visual reference of the loop profile so we can ensure that the loop and the short profile match. Displayed on the left, the superimposed profile is displaying two markers, the start and end. Since 10+05 for the loop profile exists outside the profile view, the program is forcing it to display at the end of the limits of the short profile view. This can be remedied by applying a style that does not display the end profile marker, as depicted in the before after images below.

Now can grip edit the end of the short profile to match up with end of the superimposed loop profile and rebuild the corridor.

In AutoCAD Civil 3D, looped alignments introduce challenges in our model-based design workflows. Using the Short Alignment method, allows you to keep most of the automation of the intersection and at the same time allows you to keep an eye on the vertical design in both the looped versions and the short versions of the design profiles. Being able to adapt with sound processes to keep the design efficient and buildable is key to establishing best practices in your design environment.

– Cyndy Davenport