I recently got a chance to check out two wonderful examples of dry stone infrastructure with Michael Weitzner in Cavendish VT. Likely made from the stone excavated in near by cuttings these stone features show excellent work with the local level bedded gneiss. This section of railway line was built in or around 1848 connecting Burlington to Boston.
The bridge abutments support a timber bridge that was built to allow the farmers access to their fields land on the other side of the rail line. Now everything is grown up in forest, and the casual observer may be mystified as to the point of a bridge in such an apparently remote location. The span of the bridge is about 12′. The abutments show excellent fitting and shaping of the stone with no mortar. There is also good use of batter and dressed corner stones.
Unusual for structures of this vintage no one has made attempts to “repair” cracks by smearing portland cement based mortars or adding concrete. The remote location of the bridge is probably to thank for that. Some of the stone blocks at the top of the wall have fallen off or been shifted. This appears to have been done when the timber bridge was replaced at some point in the past.
While still structurally sound, the abutments exhibit a number of “pressure cracks” in the face stones, particularly directly under the load points from the bridge supports. The sides of the abutments ‘wing walls’ show some pushing out as well. Overall, considering it has already lasted over 170 years when most concrete bridge abutments have a design life of 50 to 100 years, it is a great example of just how strong dry stone structures can be.
A few hundred feet away from the bridge abutments is a fantastic example of a dry stone culvert to allow the stream to pass under the railroad. Aproximately 30 inches wide and 5 ft high it is similarly very well crafted of the local stone. The sides of the culvert are built as retaining walls and the top is created by thick slabs of stone spanning the space. The culvert is approximately 50 ft long from end to end.
A common feature of old bridges and culverts, now often hard to find, is a ‘pitched’ floor Pitching is often described as building a stone wall on its side, such that the face of the wall becomes the floor. As such the length of each stone goes into the ground with only end face visible. Set in courses and crossing the joints, there is a tremendous amount of friction between the stones and it becomes very difficult for rushing water to erode the floor of culvert. Because of the friction between stones it is more durable than flagstone that is laid flat. The same technique is also seen used on some hiking some modern trails to prevent erosion from foot traffic.
The culvert is in very good condition. The walls and floor are still true and strait. There is some damage to the header walls at both ends that could easily be repaired. There is one lintel stone that has cracked and slightly sagged in the roof of the culvert. But the structure overall is remarkably sound considering the 100’s of floods, freezing temperatures, and train vibrations it has endured over the last 171 years.