how to support whirlpool tub...

Our whirlpool tub was installed after framing was complete and apparently they didn't pay attention to the fact that the base of the tub is not supported. If you step into the tub, the bottom is very squishy and you can hear crunchy sounds. Everything has been hooked up and secured so taking the tub out seems impossibly. Our plumber suggested either spraying foam or using sand. I don't get how we can get either thoroughly under the tub. My thought is to slide a sheet of water-resistant sheet rock, or whatever you use in a bathroom. Clearly, I need HELP!

Assuming you have access, you could mix up some deck mud (say 5:1 sand to portland cement) and stuff it underneath. The stuff should be mixed so it is just wet enough to clump together when you grab a handfull and squeeze - it should not drip water when you do this. This is the mix that is used when building a shower base. It will pack down. Think wet beach sand, not runny concrete. A high density/low expansion foam that is designed to be flexible rather than get hard and crumble if compressed might work, but put too much, and you could crack the tub when it expands or cause it to bow when it lifts.

I used floor leveling material under a basement shower stall. This is the stuff that mixes with water and is self leveling when poured out on a floor. I caulked around the stall with rags until the stuff set up. The shower floor is solid, so I know it worked. I have serious doubts that foam or sand will provide the support you need.

How would I get the cement-like mixture completely underneath the tub? I can see packing it around but not enough to get all the way to the back and center of the base.

You'd need a board or something to push it under.

It comes as a power in a bag. Mixed with water, it becomes a creamy liquid and will flow into low spots. When used as a floor leveler, it is self leveling. Used under a tub, you would need to caulk around the edges of the tub to hold it in until it sets up. Then just clean up around the edges. It is a product made by Armstrong and you can find it in floor covering stores and/or departments.

Thank you everyone for all your feedback and suggestions. Once I muster the courage to start shoving the mixture under the tub, I'll post updates.