Huge slabs like runways, giant aprons and roadbeds have special issues when placing.
It needs to be understood that if we disregard temperature expansion and contraction, which is not as severe in concrete as it is in steel and many other materials, then what we need to concentrate on is that concrete takes up the greatest volume the day it is placed. This causes reinforce concrete to have joint failure and to crack (and to curl) in almost all situations and minimization of that issue is what is attempted with good design and procedures. These involve continuous placements with very low slump mixes and/or checkerboard placements.
The spillway in the photos is very deceiving because it is actually much larger than it looks. Imagine a 12 lane interstate and you have a closer idea as to its width. This means you have longitudinal placement joints to consider as well as transverse.
Absolutely, KC. The project looks as simple as pouring a concrete driveway, but it’s much more complex. Drainage. Enormous changing pressures depending on use and quantity of water. Temperature extremes. A driveway can crack and it poses no real issues. I can lose its flatness - a corner can lift or drop and the result is really insignificant. I can drive up a bumpy driveway as easily as I can drive up a gravel road.
I think that the original spillway was well designed and built, but time takes its toll. There’s going to be a lot of discovery behind what all failed when things settle down. They will incorporate the original terrain into the design. There’s so much involved.