Long story short, we have a whirpool tub (Lyons brand) that's been sitting uncompleted and unusable for 4 years. I didn't read the instructions properly and we didn't put the mortar underneath it To try to solve the problem, we drilled through the plywood base (away from the fiberglass tub) and attempted to "pipe" the mortar underneath. This left an air bubble in the center with no mortar.
I called Lyons and they suggested drilling through the basement subfloor and filling with an expanding foam (one that would stop expansion when it reaches the tub). Is there such a thing that would also hold the weight of the water and an occupant?? DH wants it completed professionally, but we don't really have the money for that and I think with the proper info we could do it ourselves.