That depends on the slope strategy, but if there are IGN elements for both cut and fill, and the ground happens to go through them, the interface will never be found, and you will not get a solution.
For example, let's say in CUT you have a 2m 1:1 slope (IGN), and then 1:2 to ground, while in FILL you have a 2m -1:1 slope (IGN), then a -1:2 slope to ground. That would mean that there is a 4m 'height' band - from 2m in cut to 2m in fill - where the ground will never be found, i.e. if the road is within 2m of the ground.
I hope that makes sense.
For example, let's say in CUT you have a 2m 1:1 slope (IGN), and then 1:2 to ground, while in FILL you have a 2m -1:1 slope (IGN), then a -1:2 slope to ground. That would mean that there is a 4m 'height' band - from 2m in cut to 2m in fill - where the ground will never be found, i.e. if the road is within 2m of the ground.
I hope that makes sense.