I have a ditch about 4 feet wide at the top, 2 feet wide at the bottom, and 2 feet deep. I put in a 18" double-headed plastic culvert pipe. I had some extra topsoil, and used that to backfill the ditch. I added about 6 inches at a time, tamping each new layer. It took about 4 tons of soil.
Now, I'm afraid I made a mistake using the topsoil. I was planning on putting a couple inches of gravel on top, as vehicles will be crossing this ditch. If the topsoil will settle and pack eventually, meaning I'd need to add more gravel, I'm okay with that. Will the topsoil ever stabilize, or will this continually give me problems? I'll dig everything out and replace with gravel or fill dirt, if I have to, but not looking for extra work if it's not necessary.
I may be reading this incorrectly for comprehension, but...
If you used top soil for back fill down inside the ditch to stabilize the culvert, I would be concerned with washout. If you are doing gravel anyway and don't mind the extra effort, I would flip the two. Gravel for the bedding. Top soil above the pipe to hold the driveway material. I am not sure the top soil will stablize or not. Depends on the amount of clay content. However, it has a better chance to stay put on top of the ground than in the ditch.
Depends on what your "topsoil" consists of. For example, my subsoil is orange clay, and my "topsoil" is more like brown clay which will compact. If your topsoil was more of a sandy loam mix, I would have reservations with it continuing to settle or washing out. Ideally back fill with a clay based fill dirt. That said it would be hard for me to dig out a finished culvert.
Yes Lane, I think you screwed up...
There's a first time for everything right!
At this point I agree with John, its already buried leave it be for a year and see what happens.
Add gravel as it settles.
Unfortunately, topsoil is a giant sponge that expands, moves, contracts, and never really packs well. Not great road material.
As mentioned, much depends on your subsoil. If it's very sandy, you're not going to win anyway. But pure clay packs like concrete.
If the traffic is light vehicles, you may be okay. Any kind of heavy truck traffic will pound your road all to heck.
A soft road can soak up infinite volumes of gravel. Given the cost of gravel these days, it may be worth looking into geotextile. This is like mondo industrial grade landscape fabric. Put this down before adding gravel, and at least it won't mix in.