Deciding Where to Build a Stone Bridge

Some locations are more ideal for building a stone bridge than others. In this post, we look at what sort of stream characteristics are ideally suited for easy stone bridge construction.

There are several stream characteristics to look for when seeking a good place to build a stone bridge. Due to the bridge’s geometry, considerable time and expense on the approach can be saved by building the bridge in the right spot. Obviously, for strict applications, such as a public road where the top concern is keeping a straight (and safe!) alignment, the options are limited. But for many small-scale builders, there is a degree of flexibility available in the precise location.

Stream Geometry

Due to the curved shape of a stone bridge, and the height relative to the span, by far the easiest type of waterway to bridge is a narrow gorge. So if a stream runs through a gorge and then opens out into a wide, low area, the gorge is by far the better place to build.

Stone Culvert Near Town of Oak Valley — In this area (Chautauqua Hills of Kansas) narrow gorges are common, making stone arch bridges an excellent choice. This small stone arch bridge near the town of Oak Valley, Elk County, Kansas, presents a large amount of waterway with no hump. In fact, as can be seen, the arch was placed on fairly sizable abutments, showing that the builder wanted more rise than could be made with a Roman arch alone. Locations like these are ideal.

The biggest difficulty when designing a stone bridge is the hump in the road invariably left by a relatively tall arch over a shallow stream. While for some footbridges this feature of a steep hump may be considered a quaint addition, for any kind of bridge carrying vehicles, this hump can easily be a nuisance. Granted, by building a long, possibly sloping, approach the hump can be eliminated, but this requires much more work and material; in some historic bridges the approach required was found to cost as much to build as the bridge proper.

DIY Mortarless Rubble Bridge — This stone arch bridge is over a shallow stream and as a result presents quite a hump. This hump could have been considerably reduced with longer approaches, but this would have required more than double the material. As it is a footbridge over which the occasional lawn tractor passes, it was decided to leave the grade as it is.

Foundations

A rock foundation is highly desirable. If the streambed is bedrock, so much the better. Happily, narrow gorges are also the more likely streams to have rock at the bottom.

Cedar Township 291st Road Culvert(2) — Rocky gorges abound in the southeast corner of Cowley County, Kansas, and many of them are spanned by crude stone arch culverts of relatively recent origin like this one. The large amount of stones available and the nature of the streams themselves make stone arch bridges an excellent choice for these winding roads.

The rock bottom helps prevent settlement. If rock is not available, gravelly clay is a fair choice. Sediment should be avoided where possible, and sand foundations are simply asking for trouble. In short, the better foundation is the one that compresses the least and is not easily scoured.

Conclusion

In the end, it still is very likely that the range of stream geometry options is limited, but the work will be easier if:

The stream is narrow relative to its width (within reason).
The foundation is as firm as possible.
A rock foundation is located at or near the streambed level.

Even if the above considerations can’t be addressed, a stone arch bridge can still be built over practically any type of waterway; it will just require more careful planning and possibly additional spans.