Bulging spandrel and approach walls on a stone arch bridge are common defects. Though these masonry walls were, no doubt, once in straight lines, they often began traveling outwards from their original locations. Obviously, this can pose a bit of a maintenance problem as a bulging wall, if it keeps moving, will eventually simply collapse and be done with it. To remedy the situation, it is helpful to understand why exactly the wall is bulging.
Excessive Force Against the Wall
A bulging wall in a stone bridge can almost invariably can be tracked down to excessive outward force imposed on it from whatever it is trying to contain. Sometimes this can result from a rather common design defect where the wall was made too thin relative to its height. However, as most stone arch bridges in use today are quite old, it is likely that if the original wall was too thin it would have failed long ago. Of course, it is possible for any and all repairs to a failed wall to be built too thin as well, causing a renewal of problems. Also, if the wall was originally rather thin, modern, heavy, vibrating traffic loads can cause it to work out and fail over time.
Another common problem is a wall that fails abruptly after a flood. If this catastrophic failure is not due to scouring, the culprit almost invariably is the waterlogging of the fill behind the wall. Once waterlogged, the fill weighs considerably more and is far less homogeneous, resulting in a massive load for a relatively thin stone wall to retain.
Remedying the Situation
First, the root cause of a problem must be determined. If the fill is waterlogged, it is important to determine why this is occurring, and rectify the drainage problem if possible. Sometimes the waterlogged fill problem will be due simply to an excessive flooding problem, where extended periods of high water totally soak the fill. Another point to consider is whether or not the bulge is becoming worse. If the situation is not changing, it is likely that no immediate action is required, though this defect should certainly be kept under surveillance to catch any worsening of the problem.
A common historical approach to the bulging wall problem was to run metal rods through the structure that end on both sides with metal plates. This solution helps to tie a section of spandrel and/or approach walls together. Unfortunately, this method not only can negatively impact the bridge’s aesthetics, but is not foolproof. Especially if the metal plates on each end of the tie-rod are small, it is rather easy for the masonry walls to break up around the tie-rods and began moving again — except for the few firmly bolted stones immediately around the tie-rods, of course! That said, this method has been used extensively with success and can be done reasonably unobtrusively. It is best for minor problems or for reinforcing already stable walls to help them accommodate heavier loads.
A far better solution is to attack the root cause of the problem, namely the fill. While replacing the original fill with anything that can’t easily become waterlogged can be a big help, replacing the fill with a homogeneous mass of concrete is arguably the most effective method of stabilizing a bulging wall. This solid mass of concrete exerts little to no thrust against the original masonry walls, and, being neatly tucked away beneath the road, won’t negatively impact a historic bridge’s appearance. The end result is the failing walls are nearly completely relieved of any major strain and, furthermore, the whole bridge itself can be strengthened in this fashion, allowing for better load-handling capacity.