While building any stone arch bridge is quite rewarding, nothing quite compares to a drystack (mortarless) structure. The basic principles between mortared and mortarless work are the same. The primary difference is that more care must be taken when laying the stones for a mortarless structure. This is especially true for the arch.
Challenges of Mortarless Work
In general, an arch is built like a wall, though in a curved shape. When building a drystack wall, the builder has access to both faces. This means that you can add stone shims to tighten the joints. However, when building an arch you have access to only one face when adding shims.
During construction the bottom side of the arch is obscured by the formwork. After construction, gravity prevents stone chips wedged in from the underside from staying put.
An obvious solution is to choose the stones so that the top joints are tighter than the bottom ones. After all, if the bottom joints are closed, you can always add stone chips into the top joints to help make the arch angles as needed, resulting in tight joints. If the bottom joints are open, however, little can be done to tighten them and stone chips added in from the top may drop out the bottom. This means much care must be taken when laying the stones.
Prevention is the Solution
The primary thing to remember when building a mortarless arch is to lay the arch stones correctly.
Friction is much greater between stones in a mortared build than a mortarless one. If thin arch stones are used to help turn the arch, friction is vital for keeping stones from being knocked loose. Obviously, the stones should fit tightly to maximize friction and stability.
However, based on experience, there is another factor at play. Stones with very smooth faces are not ideal for an arch. Occasionally limestone slabs with a naturally smooth, almost polished surface can be obtained. At a glance, they seem perfect for an arch — thin width and perfect shape. But these slabs are almost slippery in nature.
Stone shims have a hard time staying in place against these stones and can very easily loosen up. Unless your arch stones are fairly well-cut wedges, it is best to choose fairly flat stones with a rough surface for an arch.