The trench is 8ft deep. I'm running this pair of conduits so deep because I'll be running my sewer main across here at 4 ft deep along with my water main, wich will be 3ft deep. There will also be electrical runs and some one inch water lines crossing it.
My reasoning is to make it deep enough that it won't be in the way when I start installing those other lines.
I'll keep it this deep for about a hundred feet, then bring it up to about 4 ft until I hit the workshop.