Romans loved the shape of a circle because they thought the shape of a circle is the perfect shape. That is where the Romans got the inspiration for arches. The Romans got the love of the shape of a circle from the greeks who thought the circle was also a perfect shape next to a sphere. The Romans built the foundations of the bridge using cofferdams, cofferdams were used to build the bottom of the pillars for the Roman bridges. A cofferdam is made by tying big wood logs together to form a wall, then they pump out the water from inside the barrier. once they are finished the water is pumped back into the cofferdam, and then they remove the barriers. I used wondered how workers were able to build the pillars underwater it makes a lot more sense now. Here is a video about cofferdams: