Restoring a stone arch bridge can sometimes be something of a challenge. Often the results are worth the effort. Restoring a stone arch bridge may even cost less than replacing the bridge. The question is how do you restore a badly damaged bridge?
Pudden Bridge Example
Let’s look at Pudden Bridge as an example of a bridge that needs restoration. Pudden Bridge, also known as Esch’s Spur Bridge, is a triple-arch bridge located in Cowley County, Kansas. The structure’s primary defect is that about half of the upstream facing stones of the middle arch have fallen, taking with them the spandrel wall above and much of the fill in the area of the collapse.
Other more minor defects include numerous “dry” joints between stones, and some young trees trying to sprout between stones. Though we are unaware of any plans to restore this historic bridge, a study of how it can be restored can nevertheless provide valuable insight into what sort of repairs would be required in seriously deteriorated stone arch bridges.
Determining the Cause of Failure
Before anything else can reasonably be done, it must be determined what caused the partial collapse of the middle arch in the first place.
Are the Piers Undermined?
An obvious question would be, are the piers undermined?
While it appears that there may be some slight signs of scour around the edges of the concrete aprons protecting the piers, the fact that no part of the piers themselves have collapsed would indicate that the substructure is firm. Undermined piers is almost certainly not the problem.
The problem with Pudden Bridge appears to have strictly originated with the middle arch itself, starting when, during a flood in October 2016, a few stones were knocked out of this middle arch approximately where it ends at the westernmost pier.
Why Did the Arch Stones Fall Out?
Why did these stones fall out? As the collapse occurred during a flood, it was probably largely due to high-velocity debris. (Grouse Creek, which Pudden Bridge spans, moves large trees and limbs on a regular basis.) And, once these stones were knocked loose, more of the arch was made vulnerable, hence the further collapse in May 2019 during another large flood.
Clues to the Failure: Weak Arch Joints
Another significant detail relevant to the state of Pudden Bridge is provided by Walter Sharp, who built Pudden Bridge in 1913. Sharp explained in an article written for the Wichita Daily Eagle in 1922 that he relied heavily on mortar in the arches of the bridges he built. While he kept the facing stones cut fairly precisely, the interior stones of his arches used the mortar to make the angles.
As the mortar appears to have largely crumbled and leached out of Pudden Bridge, this means that all of the arch joints are likely fairly weak. And, while the facing stones are cut more precisely than the interior arch stones, the loss of mortar would almost certainly mean that the bond between the interior stones and the facing stones of the arch is poor.
Clues to the Failure: Broken Stones
Another interesting detail with Pudden Bridge is the fact that many of the upstream arch facing stones are, in fact, broken. Surprisingly large pieces of the stones have broken off, usually at an angle from the top of the stone face down to some point on the underside of the arch. The way they are broken off seems to suggest high-velocity debris at high water levels smacking into the arch before being forced under it by the water, occasionally shearing the facing stones in the process.
While this in and of itself may not be too serious, it is possible that the stones that fell out of the arch were actually broken out by the water. Furthermore, as the arch faces are no longer flush, they may be more vulnerable to flood action; in extreme cases of broken stones, there are jagged ends for debris to catch on and the friction between stones — so critical for the bridge to hold together — has been reduced. Perhaps the arch stones which fell out were so weakened.
Reason for Failure: Conclusions
It appears that deteriorating mortar joints and deteriorating stones caused the partial collapse of the middle arch of Pudden Bridge. Of course, a detailed, professional inspection would be required to determine this for absolute certainty.
Determining What is Necessary for Restoration
It is important to recognize and determine the cause of the original failure in a stone bridge that is to be restored. If the collapse was indeed caused by deteriorated stones coupled with deteriorated mortar joints, it would follow that, as part of a restoration effort, repointing the mortar joints and perhaps even reworking the joints between arch stones would be a valuable investment of time to prevent further difficulties. This would also go a long way toward preventing trees from rooting between stones. Deteriorated stones would need to be dealt with on a case-by-case basis, and badly broken stones replaced.
A final thing to note is the value of looking at old pictures of the bridge to be restored. Old pictures provide valuable clues as to what the structure originally looked like, and, in meticulous restoration projects, can allow for exact replacing of the stones so that all the locations of the masonry joints match how they originally were.
Old pictures can also be of aid in determining prior defects which led to further problems with the bridge. Care needs to be taken in this, however. One photograph of Pudden Bridge led us to once believe there was a three-stone running joint about exactly where the arch was originally damaged in October 2016; that said, a close look at the photograph shown above reveals that this could not have been the case. The significance of this is that a three-stone running joint would certainly have been vulnerable to debris, and perhaps a factor in the collapse. As it turned out, though, that was not true. Especially if the photograph of a bridge was taken at a distance, it can be difficult to accurately tell the placement of the joints between stones, as lighting factors and stains on the stones can confuse the issue.