Imagine a 9'x9'x6' cavity. The top is my concrete pool deck (spanning 9' w/ only a couple of 33 yr old 2x4s supporting it) & 3 of the sides are my pool walls. The top of the pool wall on one side is compromised w/ a crack bc the top deck sank a little years ago pressuring it. I won't get into the whole story bc it's fairly long but we just bought this old house & pool 18 mos. ago & I'm trying to figure out what the best method & material would be to fill this cavity in a way that will support the top deck but not put too much pressure on the pool walls while it cures out. Obviously economics is always a bonus too. I plan on filling the pool before I fill the cavity so that the interior water pressure supports the walls. I've considered a grout pump, vermiculite or some mix w/ it or finally if I can find a foam that would spread out, fill voids, not expand to much & provide structural support to the top deck. If you have any experience w/ such things I'd love to get your ideas. Thx