"As designed, these arches will be 30 meters long, or one-half the length of a standard viaduct span." -- Note that "dovelas" can also be transtated as "sets of arch stones" (if the viaduct is a masonry structure, which seems unlikely, given its length). -- The French term "voussoir" is also used routinely in English and American civil engineering to refer to the elements of an arch or span. (Incidentally, the English word "dowel" usually refers to a cylindrical piece of wood, of a size appropriate for use in cabinetry and carpentry. If the Spanish text were referring to a concrete dowel (which is extremely unlikely, given the context), it would have probably used the term "barra de trabazon.") Best, HC