In reality every arch will be built with a single curve to the vault; each arch will be a straight unit. The pillar cross section will taper in plan so that it is wider on the outside of the curve. The parapet wall will be built on a continuous curve. (That's all generalisation, doubtless some engineer somewhere has built structural arches with compound curves just to show off his prowess!) Not so easy to translate to model form when the slight curve of the illustrated prototypes is replaced by an 85cm radius. If the viaduct is to be viewed from the inside of the curve only, I would build it up from straight arch units, and put a separate curved parapet wall along the top. Don't know how best to tackle it if seen from the outside of the curve, that would be 'interesting'.