Many stone arch bridges were designed with wing walls. The wing walls are essentially retaining walls that keep the soil along the stream from slumping into the stream. Obviously, having the soil slump into the stream is undesirable, as it can make a wreck of the road. However, for some stone bridges, the stone walls that form the approaches actually rest in the soil. If the soil slumps in, the bridge’s approaches are likely to collapse. The wing walls keep this from happening and are important for a bridge’s stability.
Wing walls, as a general rule, seem to be the most unstable part of a stone bridge. In the southeastern Kansas area, there are very few stone arch bridges that have all (if any) of their wing walls left. The wing walls can be scoured just like any other part of a bridge and apparently are quite frequently.
Happily, while the wing walls’ function can be important, there are other ways to stabilize the streambanks. Ideally, of course, the wing walls would be protected from scour (scour aprons will work here). Heavy weights added atop the wing walls, such as large concrete caps, helps keep them from being knocked out by debris impacts. But even if the wing walls fail or are missing, there are still viable options for keeping the streambank stable.
When the Stream is Stable
Occasionally, the stream is so well behaved that the banks are gently sloped, grassy, and stable.
In such circumstances, nothing needs to be done if there is no erosion occurring around the bridge. A bridge may not originally have had wing walls, if the stream was deemed sufficiently stable.
Stabilizing Stream Banks
More often, though, the banks of a stream have a strong urge to erode away.
If the wing walls have failed, a common and surprisingly effective solution is riprap. This riprap consists of large amounts of stone (and sometimes concrete as well) thrown down the stream bank. This amount of heavy material tends to greatly hinder erosion.
Another solution is to pour a concrete layer on top of the soil on the stream bank. When the concrete sets up, it forms a large, almost impenetrable erosion barrier that can last for years.
Both these solutions are relatively inexpensive; however, the primary downside is that riprap and concrete can be very unsightly!
Rebuilding the Wing walls
Of course, collapsed wing walls can be rebuilt; even if not with stone like the originals, concrete blocks or poured concrete walls could be used instead. The main thing here, of course, is to design the foundations of the wing walls deep enough to keep them from being undermined again.
How Significant are Wing walls?
When it comes right down to it, though, the significance of the wing walls is directly related to how the bridge was originally built and how much erosion is taking place.
If the entire length of the approach walls rests on solid rock, the bridge is likely to be stable even if the banks are badly eroded. If it becomes clear that some of the stonework rests in the soil and is being significantly undermined, the bridge needs to be repaired.
In some cases, with undermined approaches, a basic scour apron can be added to protect the bridge. And, of course, if the roadway is vanishing into the stream, the banks probably should be stabilized.