They didn’t if it was pressurized. It’s called Caisson’s disease
They discovered that they had to depressurize slowly, just like modern day divers.
America’s closest equivalent to the Brunels were the German immigrant John Roebling and his sons and grandsons. Responsible for the Brooklyn Bridge (1867), workers (and Roebling himself) were badly injured by the ‘bends’ when coming up from the pressurized footing-construction caissons extending below riverbed level. They shortly rigged up decompression chambers to combat the problem.