I have a roughly 20"x20" hole under the guest bathtub in my house where they chipped out and replaced the old cast iron drain lines at some point in the past. Whoever did the replacement didn't patch the hole so it's just dirt under the tub where the hole is. This hole is maybe 3-4' from the outside wall of the house so I figure it doesn't take too much work for moisture to migrate from the edge of the foundation to this area. I have an access panel to the back of the faucet valve and the closet where the panel is located always smells dirty/musty. I am wondering if I bought some two part urethane and poured it down there if that would help keep the moisture and stank at bay? Anybody dealt with a similar issue or think this would be a majorly bad idea? My thought is that it might make it a pain to get to any plumbing repairs in the future but that is going to be a pain regardless.

No need to fill

Note that when large voids are to be foamed, the amateurs always have regrets, but a new respect for the over-expansion and power of the foam.

UmustBKidding said:

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Every tub i have ever seen on grade level slab has a hole to soil level for the drain. The ones that cause problems are the ones that have the wood forms still in place that attract termites inside.
My experience only numbers a dozen or so but have never seen one sealed.

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Agree with this. To add, I think it's common for the top layer to be made of sand. Sand would help with moisture in that layer and could reduce the smell.

Not sure what your access is like, but if you are able to replace the top layer of soil with sand, or just fill to top of concrete, it would a cheap option to try and it's not permanent. Very easy to take back up with a shop vac. I'd try that before foam if it were me.