To concrete or to not concrete!

I'm installing a whirlpool tub where a shower use to be on a concrete slab. We had to move the previous drain by jackhammering, have now installed the drain. We have concreted all but the 8" x 14" hole for the drain connection. Now that I've finished the drain plumbing and tested it - should I concrete the remaining hole? I am leaning toward just filling it in with sand.
Brad

fill it with sand until you get within one inch of the top and cement the reset. This way you will keep out bugs and maybe some ground water.

I think I'd use patching concrete probably more like 3" or 4" on top of the sand. Reason for the patching concrete is that since it contains no rock, it would be much easier to chip out if needed.

cement

Our "inspectors" are worried about gophers digging under the house and getting under the tub, so we have to concrete the hole. It just takes a thin coating of mortar, concrete, or whatever.

Cement

Thanks guys - it's nice to know the right thing to do and the reason!
Brad