I saw a guy weld 1.5 or 2 inch plate once to repair the bucket on a giant bulldozer. That was… interesting. I was trying to figure out why he left such a big gap until he started welding. He had to fit the head in to lay down layer after layer of welding bead to join the pieces at full depth.

How would you weld even one foot of steel?

For a butt joint V it out on both sides and do many, many passes. Maybe have a few guys with rosebud torches working heat into it as you go. Big rods make it go faster (https://youtu.be/j61ezBX-EyA). For a lap joint it's the same idea, you're going for a great big fillet. But if you have a gigantic press at your disposal there are other options. For inspiration: https://youtu.be/k_LA_R4ifYk

The idea behind forge welding is you get both parts nearly molten (e.g. "welding heat") then your hammer blow (or the pressure from a huge press) puts enough energy into the weld area to briefly melt it.

Also, hot rivets might be a better option than welding if you can get away with it.

Explosively? https://en.wikipedia.org/wiki/Explosion_welding

OMG. Are you required to slap it and say, "That'll hold" afterward?

A lot of the armor plate on the big battleships like New Jersey were welded. Some of those are over a foot thick.

It would take a LOT of passes.