One common difficulty that arises when planning a major rehab for a stone arch bridge is understanding what is happening within the structure. Surprisingly frequently, stone arch bridges were built differently on the inside than what the outside visible portions of the structures might lead one to expect.
Subtle Arch Features
Just because the arch of a stone bridge features a single layer of stones of a set thickness on the outside does not mean that the same construction was used on the inside. For one thing, the facing stones are frequently found cut to an even size, which not only helps aesthetics, but makes building the spandrel walls far easier. On the inside, however, the arch stones are often a wide variety of sizes, and then neatly covered with the fill. Another possibility is for an arch to be built a single stone thickness on the outside, yet constructed to be two stones thick on the inside. This is significant as an arch that is two stones thick behaves differently from an arch that is only one stone thick.
Another common difference seen between the inside and the outside of an arch is the difference in care taken. It is not uncommon for the outside ring to consist of ashlar, while the inside is, in fact, rubble work. Another related possibility is for the outside of the arch to feature tight joints while the inside joints are quite a bit looser.
For an interesting example of how the inside portions of an arch can differ from the outside faces, see Arch Building Simplified: Walter Sharp’s Secret to Building Stone Arch Bridges.
Fill and Backing
The fill in particular provides wide room for possibilities in construction methods. What type of fill was used makes a world of difference. Gravel, for instance, is a loose fill that cannot be readily waterlogged, while a soil fill is homogeneous to a degree but can become saturated. Another possibility is a solid stone fill or even concrete in some instances. Such a fill makes an arch bridge rigid and strong, yet hides beneath the surface completely out of sight.
Sometimes there really isn’t any fill; the spandrels are hollow. Many bridges feature sizable hollows on the inside, which greatly change the arch’s behavior depending on the size and placement of the hollows. While these hollows may pass all the way through the bridge to the outside and therefore be obvious, many stone arch bridges have hollows hidden entirely by outside spandrel walls. For an example of one reason why hollows may be added in a stone arch bridge see our post on The Challenge of Long-Span Arch Bridges.
The spandrel walls themselves are not always one thickness from top to bottom. Spandrel walls were often deliberately built thicker at the bottom than they were at the top in order to improve stability. Furthermore, the spandrel walls may sometimes interweave with the fill, though not always.
To accurately determine the properties of a stone bridge with an eye towards restoration and coming up with at least a reasonable idea of how much weight it can actually hold, it is helpful to do a detailed investigation of the structure. Historical research can be a huge help here, digging up facts on a builder’s construction methods or, better yet, some form of bridge plans. Though a more advanced procedure, removing the roadway and surface fill can allow access to at least some hidden portions of the structure. There are also various methods, such as drilling cores and using ground-penetrating radar, that can aid in determining how a stone arch bridge was built without resorting to drastic measures. Happily for weight-limiting and therefore safety purposes, the vast majority of stone arch bridges have a large reserve of strength, meaning that the average stone bridge on the road system is underrated as opposed to overrated.