I’ve built several stone arch bridges for my railroad, using the same technique as the miniature stone retaining walls. Here’s a brief look at the process:

I use 3/16” thick foam core art board to make a form for the wall. I begin by cutting a piece to fit the location.

One way to do this is to lay a board across the gap, with 1” or 1/2” increments marked on it. Then measure down at each increment. Then simply transfer these measurements to the foam core and cut it out. It doesn’t have to be perfect.

In some cases, a profile gauge (aka “contour gauge”) can be used to measure the shape of small gaps.

If the wall must be curved, gently crease the foam core and bend it to shape.

Above: The completed bridge.

Left: A piece of foam core board, cut and bent to fit the location for the outer wall of this curved, stone arch bridge.

Next, use hot glue to attach strips of foam core around the perimeter of the cutout shape. You may need to angle these strips inward along the bottom edges of the form, in order to make the wall fit better into the slope of the terrain.

Next, place small rocks into the form. These can be random shapes, or cut stones. For this wall, I used cut stones along the top, and random stones for the rest of the wall.

Small cut stones can often be found in the tile department of Lowe’s or Home Depot, attached to a flexible backing. In most cases you will need to peel them loose from the backing.

Use thin stones along the lower edges of the form, where clearances may be tight.

Below: I found these fancy stone tile strips at Home Depot, and used them along the top edge on the outer facing wall of this bridge.

Below: Three strips of stone tiles were placed in the form, along the top edge. The rest of the form was lined with random rocks. Note that for this wall on the outside curve of the bridge, the form is concave.

Next, cut a piece of 1/4” hardware cloth (wire screen) to fit into the form. Again, this need not be perfect, just a loose, approximate fit. Lay the hardware cloth over the stones, being careful not to move them out of position.

When it’s removed from the form, the wall will be weakest at the arch, so I add a 1/4” diameter aluminum rod for reinforcement. In this case, I also included a pair of large, galvanized nails for extra strength.

Now mix up some mortar. I like to use a roughly 50/50 combination of Rapid-set “high strength” mortar and vinyl concrete patcher. The darker vinyl cement tones down the whiter mortar for a more realistic appearance. You could also use mortar alone, and add concrete coloring agents to darken it.

You’ll want to make the mortar about the consistency of thick pancake batter. Too thin, and it’ll run under the stones, ruining the effect. Pour the mortar into the form, and carefully spread it so that it fills the form out to the edges and just covers the hardware cloth.

At this point, you may wish to gently press some thin stones into the mortar along the top edge of the wall, since that part of the wall will be visible. Doing so will make the wall look like it’s built entirely of stones rather than only on one side.

After the mortar has set, carefully peel back the edges of the form. Then turn it over and let the wall drop out into your hand.

Now test fit the wall on the layout. Small bits of stone can be used to raise the wall at either or both ends. If necessary, you can carefully sand away some of the mortar along the edges to achieve a better fit. Use very coarse sandpaper wrapped around a small block of wood.

Once you are satisfied with the fit, cement the wall in place. For this I use the high strength mortar with a bit of concrete coloring tints added, to match the color of the surrounding rock. Push this mortar down around the back of the wall, letting it squeeze out a bit through the gaps. Small stones can be used to fill in any large gaps.

Gently rinse away excess mortar with a hose, using a fine spray setting.

Continued on Part Two...

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