Stone Arch Viaduct / Bridge

[railandsail]

I considered the 'pink foam' construction methods, but I keep running into the idea of having to carve all those stones details into the surfaces, plus the non-reproducibility of all those multiple arches.

This foam-core mock-up was originally going to be just that,...a mock-up. But I have discovered that this particular foam-core product is much more deserving of being the actual structural portion of the finished product,....then cosmetic skins added to this backbone structure.
My foam-core is actually expanded foam PVC sheet material, very easy to fashion/cut with standard wood working tools, very durable, totally waterproof, etc.
 
I started out using some 1/4” thick material for the roadbed. Then I experimented around with the arch dimensions. I ended up with 2 arch dimensions,...a 6” inch center-to-center column dimension with a 5” dia arch,...and a 5” cen-to-cen one with a 4” dia arch

….the larger arch for the double track portion and the smaller arch for the single track portion.

In order to make those arches line up I spaced them along the inboard edge of the rising bridge roadbed,...every 6'' for the double track portion. So now I knew where my risers needed to be,...behind the vertical columns of those arches. I cut these risers out of 1/2” PVC foam,..the same type of material I had on hand for the roadbed. AND then I doubled up the thickness of the risers by gluing two side-by-side making them 1” thick risers...overkill, but I will explain my reasoning later.

Now I had to get the height of each of those multiple risers correct. I measured my ultimate rise, my minimum rise. And the curving lineal distance along the roadbed between those 2 points. I plotted those two heights on the edge of my plywood work table and drew a straight line between them. Now I could go to any point along that line and measure what height riser I needed.

(BTW, my plywood work table was NOT long enough to get the whole distance in there, so I had to add a temp alum beam to the edge of that table to get an extra 40 + inches in there).

I am going to make all of the risers the same width as my roadbed, 5”, so I cut a 5” wide strip of 1/2” PVC foam, then proceeded to cut off the different height risers. Turns out I need to cut 2 pieces for each riser location so I could double them up to get my 1” thick riser.