There are eleven breakable objects to destroy on Founders' Island. Eliminating them all doesn't mean you complete eleven riddles as the game increase the counter of solved riddles every five broken objects. You can increase the counter by three on the Founders' Island because four "missing" objects can be found in the underground construction of Gotham Metro City (separate chapter of this guide).
Use Batmobile's 60mm cannon on the objects. They are painted in red and can be found mostly on building's walls. You shouldn't have much trouble with finding them, help yourself with zooming the screen.