Stone bridges rarely fail all at once; normally subtle signs of trouble give warning that maintenance will need to be carried out well before an actual failure. That said, most bridges are regularly inspected for trouble to ensure user safety. The findings of an inspection are useful for determining the nature of any trouble with the bridge, how serious it is, and what, if anything, needs to be done to ensure the structure’s stability.
Scour is a problem with any type of bridge. Scour is the gradual undermining of a structure. Generally, probing of the streambed at the base of the bridge is the best way to detect scour. That said, the presence of a heavy silt deposit at the bridge often merits further investigation, for these silt deposits are often associated with scour. It is worth pointing out that a scour hole often fills in with silt after a flood, which is why probing is important, as it will reveal whether or not the ground around the bridge is truly solid. Scour can begin to occur fairly suddenly, so it is important to never assume scour won’t be a problem.
One of the best indicators of trouble with a stone bridge is movement. This movement is a literal shifting of parts of the structure. Sometimes it is indicated by the presence of new cracks, other times by a slight shifting of stones, or perhaps by a slight change of shape in an arch. A common example of movement is the bulging of failing spandrel walls. The movement of parts of a bridge tends to be quite slow and subtle, but regular monitoring will reveal if anything is changing. Using the spandrel wall example, it should become obvious from inspection to inspection if the wall is continually moving by how far it is out of line compared to the previous inspection. If the spandrel wall is stationary from inspection to inspection and the bulge is slight, it is probably not a problem, though it should always be specifically monitored. Broadly, when it comes down to it, an unchanging defect in a stone arch bridge is likely not serious, as long as it isn’t an imminently critical defect that affects the bridge’s ability to carry loads. When something starts shifting, however, action should probably be taken soon to correct the problem.
Deterioration of Material
Damaged stone and mortar are additional signs of trouble. Obviously, mortar will need to be reworked sooner or later as it tends to wash away and fall out over the years. This is hardly unusual, and repointing should be considered routine maintenance. Deterioration of stones is a more serious problem that often indicates a drainage issue. Often, drainage issues will also leave stains on parts of the bridge, commonly on the underside of the arch. Regular inspections should determine if the trouble is ongoing. Another type of stone deterioration common in bridges built of chalky limestone is waterline deterioration of the masonry. If this type of deterioration is found, it can be safely assumed to be steadily worsening until protective measures to the foundations are taken.
One advantage of regular monitoring is that it can reveal unusual problems. One such problem is the plugging up of a stone bridge after a flood with dead trees. Though a total obstruction of this type is not particularly common, heavy accumulations of dead trees against the bridge are quite possible. Dead trees piling up in quantity at a bridge are a threat to the structure, though such a problem is highly unlikely to be missed during an inspection.
Most defects in a stone bridge are usually not overly serious if they do not grow worse. That said, these subtle distress indicators should be listened to, as a little bit of preliminary maintenance early on can save expensive, catastrophic failures later. Stone bridges are fantastically forgiving, but this beneficial characteristic of such bridges should not be allowed to cause the structure to be neglected, either. Regularly inspecting a bridge can quickly reveal the nature of defects, and photographs and measurements will give clues to the progress of trouble. Corrective action can then be taken well before more serious problems set in. In the end, of course, how serious and what, if any, repairs should be carried out on a stone bridge will be a bit of a judgment call to be made by a knowledgeable person.