Re post 2273 - do you suppose that there might have been 2 clay pipes running through that section of slab, but that one has been broken away? That almost looks too perfect for the pipe to have moved during the pour and still retained the round shape.
The cross-sectional area answers this.
(i.e. can't fit two adjacent pipes plus the gravel into this same cross-sectional [CS] volume).
The Polyethylene plastic sheet left a perfect forensic outline of the CS volume. What I suspect happened is that when the heavy concrete pour was vibrated, the plastic "tent" design deformed where the "marble like" round rock actually went under the drain pipe to the other side, forming the adjacent hump. That's how the plastic ended up wrapped around the one side of the drain pipe & no drain rock on that side - and most/all of the rock on the other side (an voila, the "hump").
This design is very unstable to overpour concrete & vibrate it - as you can see from the results.. I'm surprised someone didn't think of what would happen.