This piece may turn out to be invaluable for actual construction forensics as it hasn’t been pulverized/destroyed into debris as in the river. IT HAS been surface worn from spillway flow containing small rock pieces. You can see the surface damage this from a “small rock sand blasting effect” on the huge aeration block surfaces.
Only would a close up inspection reveal better answers. IT is possible this “backfill block” was further down from the failure area. It may just have happened to be hydraulically ripped out from the continuing damage due to the high pressure turbulent hydraulic scouring.
In any case, a large backfill chunk, that hasn’t been destroyed (evidence) may provide great information. (it may take a long time before any official information is discussed or released - early analysis, especially without “hands on” access is fraught with potential missing information.)
Re: Cavitation damage: Keep in mind that a sub-structure erosion void may have created a recurring stress at the slab top region. This stress could have caused complex crack/horizontal void pressures (which could lead to a ‘chip’ off of the surface to break). Then cavitation damage would exploit this defect, leading to more exacerbating conditions - including an eventual shift in a slab if there was a sub-structure erosion/void present.
Oh I know. I was just enjoying the fact that as an old retired construction fellow, I was 30% there sitting at my computer.
The first time the spillway was stopped to examine the damage, this big chunk was not down in the teeth of the dispersion blocks. It appeared later. That says to me that it is a slab torn loose subsequently down hill from the initial large opening. It is however a good record of the variations we have in the spillway sub-grade and its lack of uniformity.