Take a look at the upper image in Post 64, above. Look closely at the two broken edges where the concrete surface has sheared/ripped/slipped off its substrate. There are no apparent pieces of rebar but note the parallel groves in the substrate a few inches apart running in the spillway longitudinal direction. They look to me to be impressions remaining from rebar which has been pulled from the slab as the lower slab pieces washed away. They appear to be just barely below the surface of the slab, positioned at a depth within the slab so shallow as to provide minimal reinforcement. If so, an element of the spillway failure may have been inadequate concrete due to improper positioning of the rebar within the slab. (Perhaps the rebar crew built the cage too high, then the concrete pourers poured to the form line notwithstanding that barely covered the rebar.)
That looks to be well within code. Keep in mind it's a 15" slab if I remember correctly; it is supposed to have 2" of cover depending on the side of the slab (exposed to weather, or 3" if cast against earth, in the case they used 2 layers of steel, or at the construction / expansion joints). It is placed where you would want it for it's intended purpose- temperature and shrinkage cracking.
To get to minimum steel of 0.0020 or 0.0014 (not sure what steel they used), if it is 15" thick would require #6 @ 14" O.C. top only. Looks about like what is in the pictures, and matches the detail shown above.
Everything I see in the pics points to a compliant design constructed correctly- but I'll leave that seal up to the licensed engineers!
Also note in detail A that the dowel is wrapped to prevent bonding on the top side- this is normal, allows for temperature and shrinkage expansion and contraction. But if the subbase under the bottom panel erodes and is no longer there to support the panel, it will drop out and slide away from the top panel unrestrained. And then you will see what happened here, a relatively clean line across the top of the failure, and a big mess on the bottom edge of the hole.
There was probably a similar detail along the sides to allow movement without compromising the walls. Keep in mind, this thing is something like 180 feet wide, 2/3 of a football field! So when panels started to drop out, the left side was still somewhat supported and the panels broke off irregularly, exposing where the rebar was, and across the top and right side, the dowels simply pulled out of their slots.
Interesting how things behave when they fail, even when behaving exactly as designed and intended- intentionally allowing movement of the slab.
