This link https://www.youtube.com/watch?v=nOnDeAIgUo8 is a short video of the construction of the dam. It shows that machine in operation. As add supposed,”They must have been classifying the aggregate prior to conveying to the dam site.” They mention that and you can see two conveyor belts.
This link is to a longer version. It has more info on the train and material placement. Worth watching to the end. BVB
https://www.youtube.com/watch?v=how56Jem9ZU
Kewel