While building the arch of a stone arch bridge is the most challenging part the project, it actually is not as difficult as it looks. There are several simple tricks that make the construction easier.
Thin Stones Mean Easier Arc Creation
As mentioned in our article on turning the arch, the use of thin stones allows for easy creation of the curve of the arch. The thinner the stones, the closer they are to flat slabs and the less of an angle they need to have. Fairly flat slabs are much more easily obtained than wedge-shaped ones. As long as the tapered end of the slabs are placed downwards, they will still act like wedges. If the flat slabs are a tad less angled than desired, stone fragments in the joints can be used to shim it up. And, of course, if the slab is a tad wobbly, stone fragments can be used to help fix that, too.
The Bottom Gaps in the Arch are Significant
It is obvious that the stones should be placed on the arch centering such that their tapered end is downward. However, another key point to remember in arch building is to make sure that the bottom joints of the arch are more or less closed. The idea is that, even if there is a noticeable gap between a pair of arch stones, this gap can be filled in with chips and pieces of stones if the bottom of the gap is fairly closed. If, due to the shapes of the stones, the gap abruptly widens at the bottom, these stones will be very difficult to wedge in.
Flush and Flat Arch Faces Make for Easier Construction
The faces of the arch should be fairly flat and flush. In high-velocity streams this can prevent damage to the arch, but the main reason for keeping the faces of the arch flat and flush in a DIY project is that if the arch is irregularly shaped, the spandrel walls can be rather challenging to place on the arch. The arch stones are best cut before being placed into the arch, as, otherwise, they can actually be knocked out of the arch again when trying to trim them.
Thick Arches are Forgiving
Just because your arch stones do not fit perfectly doesn’t mean the arch is weak. Arches are surprisingly forgiving.
Furthermore, the thicker the arch is, the more room for error you have. Making the arch ring thicker than strictly necessary ensures that the structure will be strong, even if the joints are not perfectly tight.