Master Class articles appear monthly to share technical expertise on specific topics. Some articles provide further understanding of dry stone walling basics. Others address advanced topics of particular interest to professionals engaged in designing and building stonework for functional purposes in infrastructure. Still others present information valuable to people creating in dry stone for spiritual and artistic reasons. To access previous Master Class posts, please click here.
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Photo courtesy of Dan Snow
By Michael Weitzner
There are many wonderful stone arch bridges in New England. Most of them were built in the 19th century; some may be a little older and a few date from early in the 20th century. Many of them are dry stone structures. While it’s difficult to know for certain how many there were, one can hazard a guess that perhaps half of them have survived. We know that some of these bridges failed because of undermining; for example, the double-barreled arch built in 1907/8 by James Otis Follet over the Cold River in Walpole, NH, was destroyed by a flood in 2005.
Follett was a son of Townshend, Vermont; as a self-taught engineer and builder, he is reputed to have built as many as forty stone arch bridges. Fewer than ten of them survive, including the handsome span over the Tannery Brook in West Townshend, seen below from upstream.
I have had the good fortune to be able to take a good look at some of these bridges. Some of them are extremely well-built and have withstood the ravages of time, extreme weather, and heavy use. There are some that carry traffic the likes of which was never imagined when they were built: the heavy goods vehicles of their time were wagons drawn by teams of horses or oxen.
One thing has stood out for me: while many of the bridges have strongly built arches, more than a few of them seem to have problems with their spandrel walls. The spandrels are the walls that rise from the arch to support the roadway above.
Parts of a Typical Arch.
Source: “Cyclopedia Of Architecture, Carpentry, And Building” James, C., et al. American Technical Society, 1912.
I have carried out repairs to the spandrel walls of two of James Follett’s bridges, including an emergency repair on the West River bridge (which will have its outward-leaning spandrels completely rebuilt in the next few years) and the bridge over the recently-renamed Hussy Brook on State Park Road in Townshend (see photos below).
Two other bridges that I have looked at recently also have spandrel problems: the Old County Farm Road bridge in Wilton (NH) and the bridge over the Middle Branch of the Swift River in New Salem (MA). In both cases, spandrel walls have collapsed, leading to the closure of the bridges pending repairs. The New Salem bridge suffered a spandrel wall collapse on the upstream side of the bridge in 1945; the wall was not rebuilt but was simply buttressed with dumped rubble.
J.O.Follett’s bridge over the Hussy Brook before repairs…
… and after spandrel wall repairs in 2020
Old County Farm Road bridge in Wilton (NH). Rip-rap has been dumped into the gap left by the collapse of one of the spandrel walls. The spandrel wall on the other side of the span (on the right in this image) remains intact although it is bulging outwards. Similar bulging is evident on the upstream spandrels.
Failing spandrel wall, New Salem (MA).
Judging by the quality of the walling, it seems reasonable to conclude that these failures are partly a result of poor technique (especially in the case of the New Salem bridge) and partly a result of minimal specifications. As far as I can ascertain, many of the spandrel walls were built as a thin, single-skin retaining wall, i.e., there is very minimal tying-back, if any, of the face. In effect, the walls are thin containers for the infill, and over time, the compressive effect of traffic on the infill behind the spandrel walls is to squeeze the walls outward. It’s very likely that frost heaving also has played a significant role in the process.
There is a strong desire to preserve and protect these bridges while still keeping them open to wheeled traffic. In order to achieve this goal, it seems clear that, as has been the case with covered bridges, it will be necessary to make some modifications that will provide a greater margin of safety. One approach is to limit the loads allowed to use these structures and the speed at which vehicles travel over them should be very low (ideally, no more than a fast walk). If modern traffic is to use the bridges safely, in many cases, the spandrel walls will have to be replaced with much stronger walls that are several feet deep, reaching well across the width of the roadway, with stout infill material that cannot easily be squeezed downwards and outwards. This is an expensive process because, in order to remove and replace the existing spandrels and infill, the vault must be stabilized by building a centering (also called falsework or form).