All stone bridges need some form of maintenance sooner or later, and mortarless bridges have their own unique set of needs. For some basics which apply to any type of stone bridge see our post on maintaining stone bridges; note, however, that the section on repointing mortar does not, of course, apply to a mortarless bridge.
How a Mortarless Structure Works
A mortarless stone bridge relies entirely on the correct distribution of forces throughout the structure to stand. A mortarless stone structure is perpetually shifting, albeit slightly. The idea, then, when building a mortarless stonework is to ensure that gravity will gently hold the stones in place, and shift back into place any stone lightly displaced by the elements. This means that how much repair a mortarless bridge needs is directly related to how well it is built. This also means any repairs must be done with how gravity will effect the work kept in mind. Friction between stones is crucial in a mortarless bridge. In a mortared bridge the mortar essentially forms a custom-shaped stone fragment that not only adheres to the stones but causes maximum friction. In a mortarless bridge, the amount of friction between stones is directly related to their size and how well they fit together. If the friction between stones is low, the bridge can be easily reduced to rubble.
A Concern with Mortarless Stone Arch Bridges: Vibration
Because a mortarless stone bridge has no mortar, the structure as a whole tends to be somewhat vulnerable to vibration. This can lead to some difficulties stemming from the vibration induced by heavy vehicles crossing the bridge. One problem vibration can cause is arch stones working free from the arch and falling out. How likely this is depends entirely on the quality of work. If the joints in the arch are very tight, it is highly improbable any arch stones will be loosened. On the other hand, many mortarless bridges have somewhat loose stones that are held in with stone fragments. Such a stone can be worked loose rather easily; the vibration can work out the stone fragments rather easily. Sometimes, a mortarless bridge will have to be closed to vehicular traffic to prevent this from happening.
Vibration-related problems can, of course, occur to any part of the bridge. When it comes right down to it, how much damage the vibration of vehicles crossing the bridge can cause is directly related to the quality of the build. Large stones are not so easily loosened due to their sheer size. Also, if the joints between stones are tight, the friction between the two surfaces is maximized, and the stone becomes much harder to shift. Even if the joints between stones are not, perhaps, as tight as might be wished, and even if the stones are not massive, there are ways to greatly reduce the amount of vibration the structure will have to sustain.
Preventing Vibration-Related Damage to a Mortarless Bridge
One very simple and reliable method of reducing vibration on a stone bridge is to build up the amount of fill above the arch. The higher the fill, the wider the area loads are spread over the bridge. The fill also tends to dampen vibrations, which, as has already been indicated, can be highly beneficial to a mortarless bridge. Now, of course, there are many instances where piling more fill onto an existing stone bridge is impractical. An alternative solution to increasing the amount of fill on a bridge is to pour a reinforced concrete slab on the bridge. This slab greatly distributes load over the entire bridge. This can be a large help for a mortarless bridge. This distribution of the load and its associated vibrations greatly reduces the strain any one part of the bridge has to sustain. This is especially important for a bridge with poorly fitted arch stones. A vehicle concentrates its weight (and vibration) from its four wheels over a relatively small portion of the arch. Spreading this load over the entire arch greatly reduces the chances of stones being worked loose.