I have hit this situation in the past. Took out a pre-fab shower to install a fill tub with tiled walls. There was a big hole. Worse, the foot of the new tub had nothing to sit on. Knowing the hole was for plumbing access, we dug out and poured a sonotube (reinforced with re-bar) at the point where the tub foot had to rest. Also installed a ledger around the tub preimeter to add support.
Finished the installation and moved on. Wish I had pics because the hole was a lot bigger than 1'X1'
ALSO ... you might want to do a little pre-reading: