I am planning to build a shower in my downstairs bathroom (see first attached picture). The floor of the bathroom is built on a slab foundation. When I pulled out the old shower, it exposed the plumbing beneath (see second attached picture). As I hope you can see, there is a hole in the slab with a PVC trap that runs into a cast iron pipe. The cast iron then disappears into the slab towards the main waste pipe.
My first question is - what should I do with the hole in the slab? I assume that I want to make a solid, flat sub-floor since the books I have all show a base layer of plywood beneath the shower. A friend suggested that I should pour sand over the plumbing and then fill the rest with concrete. Any thoughts?