Surface Instruction Tips and Tricks

I was modeling an example for a training class yesterday and I did the following

  1. Created a Surface Model for Finished Grade from a bunch of 3D strings to create the Road Model
  2. Used the Site Improvements Function to box out the Subgrade for the Pavement Areas
  3. Used the Site Improvements Function to box out the Topsoil and seeding areas of the embankments in Cut and Fill.
  4. Used the Create Subgrade Surface to compute the bottom of subgrade (bottom of engineered materials)
  5. Then I built a Corridor Model and used a Surface Instruction for the Finished Grade and a surface instruction for the subgrade

I noticed that on the embankment areas - there were a number of places where the Embankment Fill Material in the Box Outs was not being computed - I guess this is down to a zero tolerance between the edge of Finished Grade and the edge of subgrade being in the same place.

So here is the trick

  1. Create a Surface Edge Breakline around the Finished Grade Model or around the subgrade model - either works
  2. Offset the edge breakline inside by e.g. 0.1’
  3. Make the Offset line elevation undefined
  4. Add the offset line to the subgrade surface as a surface boundary. This trims back the edge of the subgrade by 0.1’ all around the model and allows the two surfaces to tie at the edges of the model thereby getting your Cut and Fill and Subgrade Quantities correct.

Another Tip
I also noticed that at the very start (0 Station) and at the very end of the job, the subgrade material layers disappear at exactly the 0 station and the exact end station. This is because when we box out the subgrade, there is a very small offset applied to the top and bottom of the box out so as to not create vertical faces in the model. However this may give you erroneous volumes if your first Template happens to be at 0 station and the last section drop could be at say 20’ short of the end of the road - so here are two more tips - check this - if it happens then do this

  1. Make your first Template location at e.g. 0.1’ station
  2. Apply a Null Template at the end station -0.1’

This will ensure that the end section volumes have a full section at the start and end of the job to compute with.