The hydraulic rivets were the kind of rivets that were used in construction of the Titanic. There were about 3 million rivets in the Titanic but some places were fitted with hull quadruple rivets. See more details at http://www.titanic1.org/articles/brittle-steel.asp.
On the hull sides they used mild steel rivets in triple rows, for bow and stern wrought iron rivets in double rows. The Olympic was build the same way and held together fine, despite several collisions.