A quick update for those who followed the Oroville Dam spillway collapse in this thread earlier this year.
Our guesses about the failure have been confirmed in several expert analyses, concrete was too thin, no re-bar (there were short dowels of rebar between the slabs but that’s all). What we didn’t know is that the concrete was poured on a foundation of fragmented rock and worse so once water started passing under it, from the side or through cracks, the thin concrete lost support and failure was inevitable.
The repair work is underway now. All the old concrete is out, much of it blown up (boys will have their fun 
). They are cleaning up rock and debris, and pumping concrete in where necessary to form a solid new foundation. This Sacramento Bee page has several videos.
The repair contract with Kiewit is for $275 million over two years (expect the final cost to be higher with incentives and changes etc.), and the total including the emergency fixes will be well over $500 million. The state is hoping that the feds will pick up a large part of the cost :-"