The sides are made elsewhere, and craned in. They are then pile-driven into the riverbed. You can then pump the new hole dry, and start working on the foundations.
This thread is just like reddit threads. All quips and one liners and minimal actual conversation, these posters are indistinguishable from karma-farming bots.
Literally all but 1 top level comment at the time I posted this one.
Why is the quality so damn low? It's honestly depressing, part of the reason for leaving reddit was to get away from what is essentially auto-generated drivel/spam to actually interact with other humans interested in interacting with other humans.
This is done by pounding metal pilons & sheets into the riverbed side-by-side to create a semi-sealed off section surrounding the work site This can be done in two layers, an outer and an inner wall, and the gap filled with soil. Then the water is pumped out and workers can effectively work in the now exposed riverbed.
Well it's supposed to be Reddit for the tankies. Only they've mostly gone over to lemmygrad now, but there's still enough wingnuts hanging around to remind us of them.
Long time and unlimited resources is usually the way to go. Sure machinery would make it fast, but you can get 10 guys to hammer for 2 years to drive it. Then move on. Even today it's a huge logistics and financial problem to build bridges which is why typically it's always governments that did it.
The machine that shows up about 3 seconds in looks to me like a manual piledriver, uses pulleys and cranks yo pull the weight up, then just gravity to drop it down the track.