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.