Do I need to cover the pipes that will be under the shower base with concrete?
The only requirements are that the pipes and shower base be properly supported. To accomplish that I wouldn't go less than 3&quot; thick with the slab. 4&quot; is more standard. It's only going to take a few bags anyway, so there's no good reason to cheap out. I would want a good air/moisture barrier under my shower.
If you add fill soil be sure to use inorganic material (not black dirt). Sand or gravel are good, as is washed rock. I would run some water in there after the plumbing is finished to compact the soil--just enough to saturate it. That will help prevent settling after the concrete is installed.