Peas or radishes might work. Even supermarket purchased peas will sprout and become a cover crop. At this time of year nothing is probably going to sprout quickly. I'm assuming you're in zone 4 to 6.

Very common with dirt diggers who are concerned with speed and not soil structure and given no instruction to save topsoil. Surely you can do a cover crop but you need to fix the deep clay after the trench settles. Personally I'd try a red clover & just plow under when grading.

Dan

You know, I have had trouble 10 years later in my garden where this same thing happened. That deep sub-soil, here anyway, contains some element that just doesn't seem to leach away, and the only way I can grow something in that spot is by adding a huge amount of compost every year. It finally better when I dumped a couple dozen wheelbarrows of really good pond soil on top, creating a raised bed. Well, mound, with leached out RR ties on one side.

The trouble I have had with the same situation is that the squeegee that was originally used under the pipes was quite large and, when brought up and mixes, made it impossible to grow anything. I finally enlisted the help of my sons and we sifted the soil with 1/4 x 1/4 inch hardware cloth. Once that was done we mixed it with compost and the soil seemed to be back to normal.

You will have issues with settling, but personally I'd figure out if the soil has squeegee mixed in it and sift it before it settles.

Quite a lot of work!