Ironically enough, most of the plumbers I have worked around aren't qualified, don't understand or don't care about proper trench backfill and compaction in parking lots.
On a Taco Bell I had the site work for, the plumber had 300' of 4" PVC mainline and dumpster drain to run. I dug the ditch for the mainline, grease trap and backfilled, all by the hour. We had to level the stone for the grease trap as they couldn't grasp the use of a rotary laser.
Super told me the site sewer and water (PVC not copper) was a $20K contract.