Right: Adding rocks to the form for the inner wall of the curved bridge. Note that for this wall, the form is convex.

Because the wall on the inner curve of the bridge faces away from the viewers, I decided to use only a single row of square cut stones along the top edge.

Once the second wall of the bridge has been cemented in place and the cement has set, you can begin to fill in the space between the two walls. Start by cutting and bending strips of foam core to be hot-glued to the underside of the arch. At the ends, it will be necessary to cut the foam core to fit the rocky slopes of the canyon. An exact fit isn’t necessary. Small gaps can be covered later.

A cement core would be simplest, and in cases where the underside of the bridge can’t be seen, you might want to do that. But in this case, visitors can see the underside of the bridge from the lower level of the yard, so the underside had to be made with stone too.

I placed roughly shaped, random stones into the foam core arch. These didn’t have to be as small or as carefully fit as on the walls. I only needed to give it the impression of stone masonry. Beginning at the low spots at each end, I placed larger stones where needed to cover any gaps. A bit of mortar was poured over the stones at the ends, before placing stones over the rest of the arch. This stabilizes it. If you try to place too many stones at one time, they may slide down on top of each other.

Right: Stones were placed across the top of the arch after the ends had been mortared in place.

Left: After all the stones are covered with a layer of mortar, hardware cloth and aluminum bars are placed into the middle of the bridge for reinforcement.

Right: To finish it off, the core of the bridge was filled with high strength mortar. A few larger stones were inserted into the mortar as it was poured, helping to fill in the deep ends of the bridge.

Do not pour the mortar all the way to the top of the walls! You want to create a channel into which ballast can be added. Then the track is “floated” on the ballast. This eliminates the need for creating a precise floor on the bridge.

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