I havent been yet so this is from my general knowledge.
Its common practice to take ladders underground in pieces, the timbers can be bundled together and all the rungs carried in a bag. Once undergound you only have to knock them together.
I work like this, beauty of it is that the timbers float so can be towed along through wet sections, but the rungs of refo bar are heavy carrying.
Check out my efforts at Frongoch, photos on here.
I am very doubtful if any mine laders were assembled on surface.
My avatar is a poor likeness.