I'm about to close in the back side of a basement bath tub. Beneath this tub is what I'm told is called a bath trap or tub trap, a 1' by 1' square hole through the floor slab for the drain pipes. I'd like to seal this in to prevent termites and help an existing radon problem. Any ideas on how to do that?
I've thought about concrete. One bag would make quick work of it, but the day would come when I sorely regretted such a permanent solution. Another thought is spray foam but that probably wouldn't last very long. Perhaps a thick base of gravel with a thin layer of concrete over top. That might be removable enough for when the tub drain leaks years down the road. Any other ideas?