Thanks for the additional information. The seam between the “slab” concrete and the concrete that was under it is very straight. Almost like a finish layer was poured atop a smoothed finish layer.
And I agree, no evidence of any anchoring below the big hunk of concrete. But the way that they stuck together, would that indicate that there was anchoring between the top layer and the piece of fill below? I suppose that would easily be determined on inspection, and I somehow doubt that there was It’s just that they stuck together so nicely.
There’s been so much information in this thread that I’ve forgotten a lot of the previous posts. To anybody joining late, take a gander back in the thread’s history. There’s a ton of information there.
That is the most interesting facet. One thing is for sure, the seam junction is "very clean" to have such a condition. I have run across conflicting "as-built" verses the original blue prints (such as the lateral transfer bars between slabs).
Here's one for you. The original anchor bars, which would have been protruding below the original slabs & into the highly erodible material that washed away into this very large void = This is what forms the "tight bond". The anchor bars are now attached to the "fill void" blob instead of the original highly erodible material. It's possible that the clue of one or two of the anchor bars are hidden in the shadows (underside).
Given this condition, the block would tumble in the harshest conditions and would stay together.
Good job! way to "stick" with this puzzle!