For me this is a simpler approach.
Create strata surfaces with at least both materials.
Strata for utilities is a little tricky. You build the strata layers using site based boreholes.
Then you run the Define Corridor Strata command, set the method to surfaces and pick the site based strata surfaces
Utilities are based on corridor technology so it needs the corridor type of strata.
Create two templates one for each condition. You can copy the first and change the offset from 6" to 12".
Apply the 6" offset template to each run and run the utility takeoff report
The calculation tab of this report will detail where the 6" offset bottom will cut into the Ledge or Rock material
Add templates to transition in and out as needed
Remember that strata is a big guess and you are working for an estimate (keep the transitions to a minimum #)
In the following table, where you see an area for Rock this is where the template cuts into the Rock material
Now you know the stations where you need to change templates.
In the following run I would start with the 12" template insert it again at 225 and then insert the 6" template at station 250. This will transition the change in depth over a 25' distance. You can make this transition as short as needed.
Use the surface slicer command, profile viewer and view the surfaces in 3d to confirm your decisions.
Keep the maximum sampling distance to 10' for greater detail.`

The feature I like the best about the utility modeling is that you can visually confirm your design with each adjustment.
tim