The Roman arch prevented rigidity entirely, maintaining each of the masonry in compression, with the keystone into the piers. Stone in compression has terrific toughness, and the Romans built massive arched bridges and aqueducts in big numbers. Extending their arch right into a tunnel, they invented the barrel vault, with https://digitalasset.us-lax-1.linodeobjects.com/westhillmasonry/Hardscape-Contractors.html