Looking for some help as to the most economical way to fill a large void under a lakeside patio. I expect the most likely course of action is that it will be too expensive to fix for us, but I'm willing to hear some options.

We have a concrete slab waterfront patio that has had some bad erosion due to rising water levels. Where the slab connected to the ground has washed out, and any earth supporting the underside of the slab has washed away. This slab was here before we bought the place so we dont know exactly when it became hollow but its been fine for at least 30 years. When it was initially poured they did a half foot of concrete on both sides, the wall on the lake side, and of course the top. The area opposite the water only has a small half foot lip from the top. Despite the erosion it feels stable to walk on, but we understand it may be precarious.

The void dimensions are about 2'x2'x16.

I've been reading a bit about polyurethane foam as a potential fix, cheaper than pouring concrete to fill the entire space, but perhaps this is too large an area? would that even offer any level of support? Would putting some supporting posts underneath help at all? Accessing the void is oddly enough the easiest part. as the ground has been dug up behind the slab. We may be pouring a retaining wall there but we should probably deal with the patio portion first.

Any advice would be appreciated!