Support Needed for Under an Acrylic Tub

[jimmi]

I've been asked to fix a problem with a tub that was installed 2 years ago. Customer cannot make contact with installer. I've made it clear that I MAY be able to help, but my work is certainly after the fact.
Okay, a Sterling acrylic tub (with surround) was installed in a 1st floor bath. At first glance, all looks pretty good. Everything is solid, no leaks, drains well, etc.
The problem is with the support of the tub. The tub appears to be a pretty decent model from Sterling (not as flimsy as many tubs are) but when you stand in the tub (whilst taking a shower, for example) there is a slight creaking when you walk about. It's not severe, and the problem may be somewhat psychological as much as structural.
They certainly don't want me to tear out the tub or anything, so here was a thought I had...
The basement has an unfinished ceiling and I can get to the subfloor directly under the tub w/o difficulty. Can I (carefully!) drill a few 3/4" or 1" holes and spray some kind of closed-cell foam to seal the space? Of course, I'm concerned with expansion, but I was thinking that I would spray over several visits to allow the foam to expand and dry before doing more. Also, I thought if it was a low-expanding foam it would help.
Thoughts on this method? Any other way to fix this?

 

[breplum]

Jimmi you have a good imagination, but getting the foam in would be challenging.
I would not fear expansion, except out the holes you drill.
Most plastic tubs are supposed to be set in mortar.

 

[jimmi]

Agreed, however, the customer isn't cool with a major job here - they'll live with it as is (hell, he and his wife are both skinny - it probably creaked more with my fat butt in there... , and yeah, I thought the holes I drill would work for the expanding foam. Just looking to offer a solution for them.

Actually, the most challenging part (to me) is the hole drilling. Go too far and well, there'll be more than 1 drain...ha

 

[CT18]

You can add some sand bags to the tub to help with the expansion during foaming.

 

[frodo]

Do not use foam, it is a temp fix at best, foam will wear out as time goes on
the fix is to concrete that tub in.

spray foam the drain opening in the floor to stop concrete from dripping onto the ceiling below
then mix up a batch of soupy concrete,,,not stiff but not water,
slide tar paper under the tub on top of the floor
pour the concrete under the tub, you are using the 2x4 rat plate and the skirt as your form.
fill it up, if it ozzes out from under the skirt, clean it up with wet sponge and water in a bucket