Multiple-span arch bridges are not necessarily several arch bridges back to back. The cause is quite simple; flatter arches need good, solid abutments to resist the horizontal thrust of the arch, and, unless one is willing to greatly constrict the waterway being spanned, any piers must be much thinner than a good abutment should be.
Possibilities in Multiple-Arch Construction
A simple solution to achieve waterway without needing thick piers would be to use rounded arches, like a Roman arch; after all, a Roman arch has negligible horizontal thrust. However, sometimes this solution is a tad impractical due to the Roman arch’s relatively high height (½ the span), meaning that either a steep grade will be present as one climbs the bridge or even more, smaller arches must be used, which will mean more piers and hence more waterway constriction again.
Another possibility would be building the arches as close to the streambed as possible. This not only would allow for more rounded arches to be used practically, but will also eliminate the toppling tendencies of a pier. The problem with this solution is that the waterway will be constricted by the spandrel walls between the arches.
Arguably the best solution ever used on the multiple-arch problem is brilliant in its simplicity. If we have two identical arches, no matter how flat they may be, as they are the same shape and size, it follows they have the same horizontal thrust as each other and these two opposing equal forces will cancel each other out. The French did extensive work along this line, building chains of flat arches atop thin piers with success. The primary drawback of this solution is that both arches on a given pier cannot stand independently, meaning that one cannot just pull the formwork out of an arch when it is completed; the next arch in the chain must be sufficiently finished to prevent a collapse when the forms are removed. (Walter Sharp discovered this when building the triple-arch Esch’s Spur Bridge in Cowley County, Kansas. All went well until his workmen pulled the formwork out of the first arch when it was completed, but before there was an adjoining arch up yet. The resultant collapse was immediate and pretty much complete; fortunately, it would appear that there were no injuries.)
But what about building dissimilar arches? When building dissimilar arches back to back on a bridge, the pier will need to be heavy enough to resist the sliding tendencies of the flatter of the two arches. The pier does not necessarily have to be as thick as an abutment would be, however, for the rounder of the two arches will still be presenting a significant amount of weight, roughly equal to the weight of one half its span. And, of course, the weight of the fill above the springing of the two arches will definitely help. A bridge with dissimilar arches is, of course, possible to build, though frankly rather uncommon, as it is usually possible to simply build two identical arches. Typically, dissimilar arches are used to accommodate certain streambed conditions, such as shallow outcroppings of bedrock in an otherwise deep stream.