So how do you restore a stone arch bridge? In Part 1, using Pudden Bridge (Esch’s Spur Bridge) in Cowley County, Kansas, as our hypothetical example, we concluded that the partial demise of this massive structure’s middle arch was likely caused by breaking stones and deteriorating mortar joints. So now we will use this information to explore what would be required to restore the bridge.
It is important to note that a detailed inspection by professionals is necessary to accurately determine the state of the bridge, to confirm that deteriorated mortar and stones caused the problem, and to see if there are other problems hiding out of sight. With this information, the group of professionals would be equipped to restore the structure, and would have the best ideas for doing so. This information is for example purposes only.
Adding the Falsework
In order to repair Pudden Bridge, a falsework specifically designed to fit the arch’s curve would need to be built. In the case of Pudden Bridge, while an exact measurement of the arches would be necessary to accurately determine the shape, based on pictures of the bridge and information in newspapers, the arches all appear to be 50-foot-span segmental arches, the segments being 90 degrees of a full circle.
Do All of the Arches Need Falsework?
When restoring multi-span stone arch bridges, one must factor in how much the arches depend on each other for stability, as well as how important the fill is to the arches. In the case of Pudden Bridge, while the fill is probably not crucial for the stability of the bridge, the three relatively flat arches balance each other out on the piers; therefore for safety purposes it would best to put a falsework under all three arches.
Removing the Fill
Once the arches are supported, the next step would be to remove the fill from the bridge. This is necessary to access the collapsed arch. Technically, only the fill over the center arch would need to go; however, having determined that deterioration of the mortar joints played a role in the demise of Pudden Bridge, it would be best to uncover all three arches so as to correct any hidden problems in the arch joints.
Walter Sharp designed his bridges such that the mortar in the arch joints plays a crucial role in making the arch angles.
Another thing to note is that sometimes the spandrel walls rely rather heavily on the bridge fill for stability. Therefore, it may be necessary to remove portions of the spandrel walls. In the case of Pudden Bridge, the spandrel walls would probably have to be removed anyway, to allow for complete access to the arches.
Pictures of the bridge before this removal and numbering of the stones would ensure the bridge was restored to its original condition.
Ensuring the Arches are Stable
The arch work would consist of two parts. Missing arch stones should be replaced back into the middle arch. Badly broken stones in any of the arches should be repaired or replaced.
Deteriorated stones could be repaired with certain kinds of mortar, or even epoxy; however, replacing the stones is often the surest method. What, if anything, needs to be done would need to be determined on a case-by-case basis.
Rebuilding the Partially Collapsed Arch
Rebuilding the partially collapsed center arch of Pudden Bridge would begin with retrieving as many of the fallen stones as possible. It is quite likely, even probable, that not all of the stones that fell from the bridge are usable, if found, which means that custom-cut stones would need to be supplied to replace any of the original stones that can’t be used. Those stones then would be placed in their proper locations on the falsework.
Existing stones in the arch that are badly skewed or otherwise out of position would be removed and reset into the arch. Again, replacing the arch stones would need to be done on a case-by-case basis. It may be desirable in some cases to remove some existing arch stones to allow for easier overall repair of the arch; that would just depend on the circumstances.
The Trouble with the Arches
The bad thing about Pudden Bridge is the fact that mortar was largely used to make the arch angles. This is not considered good practice, and, while the arch could be restored back to its original specs using this method, it would not be a first-class job.
Professional personnel would have to make the decision as to whether or not rebuilding the bridge in this fashion would be prudent for supporting road traffic, or if it would be best to simply leave the bridge as a roadside monument and bypass it.
While the facing stones of the arch were more precisely cut — these being primarily what would need to be replaced — they would still need to be bonded with the loosely-cut inner arch stones, which rely on mortar for the angles.
One simple method of strengthening these mortar arch angles would be to add rock fragments with the mortar (basically this would create a sort of concrete), but it would possibly be necessary to reinforce the arches in some fashion.
A common method of reinforcing weak arches is to form a reinforced concrete arch below the existing arch. While not exactly the most beautiful solution in the world, it does work. Whether or not this solution would be suitable for a structure on the National Register of Historic Places would need to be determined by a professional.
Another solution is to set tie-rods into the arches to firmly clamp them together. Another possibility is to pour a reinforced concrete slab atop the bridge to help distribute loads over the arch.
Repointing the Interior Mortar Joints
Regardless of how the arches were repaired, repointing the interior mortar joints of the bridge as needed would be a wise plan. A potential solution would be the injection of grout into the joints. This is done by pumping grout into the bridge to fill in any unseen cavities in the mortar.
Repairing the Spandrel Walls
After the arches were restored as necessary, the spandrel walls would need to be put back onto the bridge. Of course, any of the spandrel walls removed during construction could be replaced back to how they were, but for the section that fell with the collapsed portion of the middle arch, missing or unusable stones would be replaced with custom-cut stones.
In some cases, the fill needs to be added with the spandrel walls. This just depends on how the interior of the spandrel walls were designed; sometimes the fill is important for these walls’ stability.
The fill, of course, could be the original composition of rocks and soil, but lightweight concrete is a potential solution with many advantages. It is much less susceptible to being waterlogged, which is greatly beneficial for keeping those critical mortar joints that make the arch angles from deteriorating. As the concrete is a solid mass, this would also tend to make the bridge much stronger and would help distribute loads.
Repairing Exterior Mortar Joints
Once the fill is in the bridge is basically completed. The mortar throughout the bridge would need to be repointed. Injecting the mortar as described above would be highly advantageous for the structure as a whole.
Before the mortar on the exterior joints could be reworked, however, the tree(s) growing on the bridge would need to be cut down, and herbicide applied to the stumps. Usually the trees are best left in place, as their removal may dislodge masonry, while if killed and left, they will gradually decay over time, allowing the masonry to settle back into place.
In extreme cases, where masonry is already badly dislodged by a tree, a partial teardown of the affected area is required. Pudden Bridge, however, does not appear at a glance to be suffering badly from the small trees which have taken root, so they can probably just be cut, killed, and left as they are.
Basically, at this point the forms could be removed and the bridge called completed; of course gravel or some form of roadbed would need to be added, but there are still a few other considerations.
Do the arches need reinforced? If so, how?
Depending on the solution, this may have been already dealt with while the arches were being reworked, though if, say, a reinforced concrete slab was considered a good addition to the top of Pudden Bridge, this would have to wait until the interior work of the bridge was completed.
Another consideration with Pudden Bridge is the guard rail — or lack thereof. The original guard rail consisted of gas-pipe railing, judging from all appearances, but this is hardly considered sufficient for preventing modern cars from driving off a bridge. Besides, the railing is long gone!
Probably a modern, crash-proof guard rail would be added; after all, the bridge is rather too long, high, and narrow to go without a railing.
There are many ways to restore a stone arch bridge, and a broad array of options available for repairing such a structure.
The concept we have outlined is pretty basic, but it is necessary to have a team of professionals examine the bridge to determine what exactly can and should be done.
Certainly there is an excellent restoration potential for many deteriorated stone arch bridges. One such example is the oldest bridge in the United States: the Frankford Avenue Bridge in Philadelphia, Pennsylvania.
While this bridge was not partially collapsed, it was in a somewhat shabby state before restoration. It was professionally restored in 2018. The restoration job was outstanding, preserving the old bridge while ensuring it was suitable for modern traffic requirements.
Many of the methods used to restore that bridge could be used on Pudden Bridge. It is our hope that a way to restore Pudden Bridge, the only remaining triple-arch stone bridge in Cowley County, Kansas, can be found.