There's probably a two foot cylinder of pea gravel starting about a foot deeper than the elbow and going up above the valve/drain. Found the same when I dug around the original.Glad it all worked out Tim. Thank goodness they didn’t have to spend all day digging and back-filling. Labor is the biggest expense.
I think it was good to go poly all the way to the hydrant. Will never corrode. I’m sure they dumped some washed rock around the base. Serves a few important purposes.
Number 1 - allows draining of the water out of the standpipe, and into the stone, when the hydrant is shut-off. If it’s only dirt, the drain holes eventually clog.
No.2 - the stone generally keeps soil away from the steel elbow and hydrant at the base. Some soils are corrosive to steel and the washed rock is typically not.
No 3. - stability. The rock is typically shoveled around the base and a couple feet up around the hydrant and the the hydrant is held plumb when backfilling. Much better than relying on soil by itself.
If you let it go through a winter, the ground should settle pretty well with the rain and snow. Will be ready for maybe a little topsoil and seed in the spring.
I'll definitely spread some grass seed on it, but not going to bother with any topsoil, if what grows stays green, that's fine by me. As it is, I'll have to spray it a couple times before winter to keep the weeds down. After it dries, I'll probably run the tiller over it to even out the soil and break up clods, and may just seed it this fall and leave it. Not in an area that is visible from the house, and as long as I don't trip walking over the area, don't care that much. It was a mix of grass, knotweed and misc other weeds that was mowed every couple of weeks to begin with.