One of the important parts of a stone arch bridge is the spandrel walls. The spandrel is the part of the bridge that is built up over the arch to form a reasonable roadbed grade. Unlike the approach, the spandrel walls are located over the arch.
Except for a very flat arch, something must be done to make the arch practical to drive over. A Roman arch, for instance, would be absolutely impossible to mount with any kind of wheeled vehicle, and so something must be done to make a more level road bed. The answer is the spandrels. Occasionally the spandrels will be solid masonry, but typically they will be hollow walls that hold a fill of dirt, rock, or some other substance.
In Cowley County, Kansas, spandrel walls are frequently pairs of mortared walls that hold crude unmortared masonry with some dirt and stone chips thrown in.
Building Spandrel Walls
The spandrels are rather tricky to build up. For one thing, shaping the wall around the arch takes some effort, and the bond between the two is often weak, except for the occasional instances where the arch stones were precision cut to accommodate the spandrel stones. If (as is often the case) the spandrels are a pair of walls designed to hold some form of fill, special care must be taken to ensure they can indeed actually hold the fill in, without being shoved out by the fill. This movement of the fill is also aided by freezing/thawing cycles. To this end, spandrel walls must be fairly thick, and often must be peculiarly shaped to hold back the fill.
Typically the walls will be wider on the bottom than at the middle (though you don’t actually see this as the increase in width happens to be on the inside of the bridge). At the same time the top of the spandrels is often wider than the middle. This, of course, creates a slightly awkward shape. However, since the top of the wall is wider than the middle (again with the extra width pointing to the inside of the bridge), the wall will have a slight tendency to lean inward — thereby resisting the fill’s outward-leaning tendencies. These two instabilities tend to create stability, which helps.
That said and done, bulging spandrel walls often are a common defect in stone arch bridges.
Repairing and Maintaining Spandrel Walls
Bulging spandrel walls need to be remedied. If they bulge far enough, a vehicle on the bridge can cause a sudden wall failure, resulting in the vehicle, an assortment of dirt, gravel, pavement, etc., and a few tons of masonry plummeting into the stream. An example of this occurred on the Rockville railroad bridge over the Susquehanna River, with the result that some rail cars carrying coal, as well as an assortment of bridge pieces, went plunging into the water.
A common method of rectifying the spandrel wall problem is to use tie rods to, in effect, “bolt” the two opposite walls together to keep the fill in.
For this method to be effective, large metal plates are used to distribute the “bolting power” over a larger area. Most masonry consists of numerous independent units stacked carefully upon each other, making it all too easy to simply bolt a handful of stones in place while the rest continue in an outward movement.
Other options include rebuilding sections of the spandrels and replacing the fill with more stable material and adding drainage features to help prevent the fill from getting waterlogged, thereby reducing the weight to be contained as well as significantly reducing frost heaving.
Frost action is not the only way the spandrels are taken out, however. When flood waters go over the top of the bridge, not only can they remove the fill but, occasionally, can even take out the spandrel walls.
While the power of raging creeks is not to be underestimated, thicker spandrel walls help prevent the bridge pieces from being removed by flooding. Judging from old newspaper records and the number of stone bridges in Kansas featuring various concrete sections in the spandrel walls, this occurrence must have happened more often than one might think.
In some cases this loss of fill and spandrel walls can even lead to collapse of the arch — the weight of the fill may partly be responsible for keeping an arch stable.
Ideally, water should never go over the top of the bridge at all, which will mitigate bridge damage during floods. However, in some places (like Kansas) when it rains, it pours, so that the stone arch bridges go under water periodically.
If you have never been in Kansas near the Grouse Creek in Cowley during — or at least right after — a flood, you may find this hard to envision. According to Walter Sharp, Cowley’s foremost stone arch bridge builder, during a large flood in 1922 the Grouse became miles wide in places. For perspective, the longest stone bridge over the Grouse Creek has three 50-foot arches — and a single mile wide is more like the Mississippi on a normal day.
Building up the roadbed to a height over flood stage helps prevent the loss of spandrel walls and fill (though at considerable expense) but, when it comes down to it, there is only so much one can do. While spandrel wall failure due to bulging is more easily preventable, when the water is up, impact from debris can do an amazing amount of damage, and can lead to spandrel wall failure.
In the above-mentioned flood of 1922, only one of 14 stone arch bridges over the Grouse sustained damage. This damage to the top layers of stone was caused by the accumulation of debris. In this case, the term “debris” unfortunately included crops.
When the spandrel walls do fail, restacking the original stones and replacing any missing or otherwise unusable stones is the preferred repair method for historic bridges. Otherwise, concrete tends to be a more affordable material, and often makes its way into bridge repairs.