Last week I built some miniature retaining walls along the edge of a “cliff” on my In-ko-pah Railroad. These turned out nicely. The way I built them was to make a form out of foamcore board, cut to fit into the existing full-size rocks of the cliff. I place small bits of stone into the form, topped with a piece of 1/4" hardware cloth, and then pour in a mix of mortar and vinyl concrete patcher. When the mortar sets up I remove the wall from the mold, clean up any excess that bleed under the rocks, and cement the wall into place on the cliff.
I liked the way this worked, so I decided to try building a stone arch bridge over a small canyon using the same technique. The span was about 15", with an 8" diameter arch. I built a mold for facing side of the bridge and placed the stones into the mold. I used brown stones, with a band of white stones for a fancy trim along the top edge. It took me about 3 hours to cut and place all the tiny stones. It came out looking great, with only minimal cleanup needed after demolding.
But key is to remove it from the mold at the “green” stage. If the mortar is too hard, you won’t be able to clean the excess mortar off the face of the wall. On the other hand, remove it from the mold too soon and it’ll be too fragile. Hard to tell though exactly how long is long enough.
Yep, I pulled it out too soon. Just as I was about to place it into position on the layout, it suddenly broke right down the center. All the stones in the center of the bridge wall fell apart, the mortar disintegrated, and the remaining sections on either side threatened to do the same. I barely managed to get it set down on a nearby step before the entire thing could be destroyed.
Well, it took a few days but I finally managed to piece it back together and get it successfully installed. It doesn’t look quite as nice as it would have, but it’ll do. Fortunately this bridge is not in a prominent location – which is part of the reason why I chose this spot for my first stone bridge. The lessons learned will be applied to more visible areas of the layout.
I should have pics of the bridge and retaining walls in a few days, plus photos of some other cool new additions to the layout.