It looks to me like you'll have to remove the floor of the room since its so close to the tilt-up walls foundation and create a separate foundation for the combination of walls and floors and now water load.tiltups foundations are usually 30" wide x 18" deep (in California earthquake country) and that's a serious weight load at any depth..a structural engineer will have to be involved to calculate minimums.It sounds like a helluva lotta fun though!