The location of the cracks raises another question that I have not seen addressed.
The rebar is presumably very solidly anchored in good rock. It is also well established that the area is geologically active.
An often overlooked fact is that solid rock can and does move. If it moves an easily detectable amount we can measure the energy released (an earthquake). But the ground can and does bend elastically as well. The movement is NOT necessarily uniform.
This may seem irrelevant, and most of the time it is. However we have just added enormous weight on a slope, and anchored that weight into the rock below. This weight could easily be enough to cause elastic deformation of the rock as it adjusts to the new conditions. On a steep slope with variation in the slope the load would not be uniform, and I would not expect the deformation to be uniform.
In most situations the weight would be added much more slowly, giving the rock more time to adjust. Movement would still occur, but the movement would be transferred only to the concrete that had already been poured. Subsequent layers would be affected by any movement that had occurred prior to when they were poured.
If this is happening I would expect the solidly anchored rebar to transmit minute differences in movement to the concrete. Since it was all done so quickly, it is easy to see how this movement would be transmitted through the entire thickness of the concrete.
Exacerbating the situation, in this case the concrete may not have enough time to fully cure. If there is minute differential movement in the base this would make it more vulnerable to cracking.
This is bound to seem far fetched to engineers who have vast experience in the field. However, this is certainly an unusual situation and that’s when the unexpected is most likely to occur.
For those of you who consider this possibility nuts, consider the location of the cracks. They are NOT over the rebar. If the cracks are caused solely by shrinkage of the concrete as it cures then they should be more evenly distributed. Something about the rebar must be affecting their location.
There is also a way to test this hypothesis. The slope of the spillway varies. If minute differences in movement of the rock the spillway is anchored into are a significant contributing factor, then you would also expect the distribution and size of the cracks to be correlated in some way with the slope of the spillway.
Final comment: Yes, I realize I may, in fact, be nuts. I am not a civil engineer. My first two degrees were in unrelated fields of engineering (mostly math) and I have only a tiny bit of hands on engineering experience. Worse, I’m so ancient I used a slide rule and analog computer all the way through my undergraduate years.
I was following your argument and nodding in agreement but you lost me when started trying to correlate the location of the cracks with the location of the rebar.
Concrete will crack, that is a given, the trick is to control the cracking. My assessment just by looking at last couple pictures, is it appears that it is a mass weight problem causing strain (tension) on the slab placed on the slope. Concrete does not resist tension very well, which is why we use rebar, and cracking will occur throughout the slab, not because of the rebar location. I would take a look at the slab pouring plan. In other words are they skip pouring individual slabs, by skipping every other one and then coming back and pouring a slab in the gaps after the other slabs have cured, or is this just one long continuous pour? What is the size of the slab? Slab panel joints are usually quite small and not greater than 15ft x 15ft. Anyway just some thoughts.