I removed a shower stall and there was a hole for the plumbing as expected but once I got a good look the entire area below the cement slab is open. I want to fill the whole as the bathroom is being converted into a mud room. I was assuming I would just have to get a bag or two of gravel and/or sand and fill the hole and then add 3-4" of concrete but since everything is open underneath I am not sure how to fill the hole now. Seems weird it is all open underneath. Any tips would be appreciated.
Note the soil on top of the pipes. You should have this looked at by a good remodeling general contractor or structural engineer. Residential slabs do not generally span those distances even with rebar.
Filling with sand may be the answer but figuring out what’s going on should be the priority