The Stone Arch Bridge in Minneapolis is set to reopen this fall after years of extensive study, design, and preservation work to restore its structural integrity and historic character.
The bridge is one of Minnesota’s most iconic structures. Built in 1883 by James J. Hill, it originally served as a railroad crossing over the cataracts of the upper Mississippi River, supported by 21 stone masonry arches. The bridge carried its last passenger train in 1978, and the tracks were officially abandoned in 1987. It was rehabilitated in 1992 and converted for use by pedestrians and cyclists. Today, it ranks among the city’s most popular attractions.
However, time takes its toll, and by 2017, nearly three decades after its celebrated conversion, the bridge was showing signs of significant wear due to weather and water flow. Routine underwater inspections were identifying progressing riverbed scour, deep mortar loss, and potential movement of the stone piers. MnDOT, the state agency that owns and maintains the bridge, hired LHB to conduct a thorough review of its condition.
A Deep-Dive Assessment
Our team began with a detailed scoping study in 2018, sometimes using snoopers (inspection trucks that hang over the side of bridges) to inspect the structure. This information, coupled with the most recent underwater inspection, was compiled into a comprehensive condition report. The findings were clear: significant repairs were needed to maintain the bridge’s stability and historic integrity.
Citing our recommendations, MnDOT sought funding from the state legislature to make repairs. Restoring a pedestrian bridge presents unique funding challenges, as non-vehicular bridges do not qualify for traditional funding sources.
Utilizing Modern Technology
In 2020, with funding in hand to begin design, MnDOT asked LHB to develop a scope of work and detailed construction documents aimed at preserving the bridge. Our design process incorporated cutting-edge technology, including the deployment of drones to create a photorealistic model of the bridge. This digital model was used in the field to record condition assessment notes, and became an invaluable tool, allowing the team to virtually visit the bridge during design without the need for travel and specialty inspection equipment. The model was also used to prepare detailed construction documents, using the images from the model as photo backgrounds in the construction plans.
Additionally, when the U.S. Army Corps of Engineers planned a drawdown of the Upper St. Anthony Falls Lock and Dam in 2020, we seized the opportunity to inspect sections of the piers normally submerged underwater. This 48-hour window provided crucial insights that influenced our final design decisions.
Mortar and Masonry Repairs
These insights revealed that the original mortar, critical to holding the bridge’s stones in place, had deteriorated extensively. Additionally, damaged stone faces needed replacement. Given the bridge’s historical significance, sourcing authentic stone was crucial.
The original construction had drawn from multiple quarries, so LHB took the unusual step of identifying historically accurate stone sources rather than leaving it to the contractor. The project team visited quarries, collected samples, and analyzed their durability before selecting three sources that matched the bridge’s original materials. This proactive approach ensured the repairs would blend seamlessly with the original stone of the bridge, unlike previous repairs that used concrete or stone that didn’t match the bridge.
Protecting the Bridge’s Foundation
Along with this, the bridge’s piers, subjected to constant water flow and freeze-thaw cycles, required reinforcement, and added to the project’s complexity. LHB brought on a specialty subconsultant to collaborate on the scope of work and aid in the design of the pier repairs. Due to their history with the bridge and expertise in underwater inspection and construction, Collins Engineers designed the underwater repairs. In many areas, repointing alone was not sufficient to repair the deteriorated and missing mortar — some sections required pressure-grouting of the mortar to ensure stability of the bridge and a long service life of the repair.
Massive riprap was also placed — stones the size of cars — around the piers to protect them from further erosion and stabilize the foundations. A Dewatering and Diversion Study was conducted to review constructability of the design. Ultimately, the specifications gave the contractor flexibility to perform repointing work using a cofferdam or by underwater methods. The contractor elected to perform the repairs underwater.
Overcoming Scheduling & Funding Challenges
Scheduling was another critical consideration. As planning got underway for repairing the Stone Arch Bridge, the nearby Third Avenue Bridge was also undergoing rehabilitation, and closing both bridges simultaneously was not an option. LHB worked closely with MnDOT and the Minneapolis Park and Recreation Board to minimize disruption, providing pedestrians and cyclists with alternative ways to cross the river during construction.
As mentioned, funding pedestrian bridges, especially extremely large ones, is complicated. The 2018 study estimate was $13 million and was based on pre-pandemic construction cost assumptions and without the benefit of detailed design. With the surge in specialty bridge construction costs after 2020, and the uniqueness of the work, an independent cost estimator was brought on to aid in preparing the Engineer’s Estimate, which estimated a construction cost of $25 million. Despite the efforts to mitigate the risk of construction costs, the low construction bid was $40 million. Eventually, however, MnDOT was able to secure the funding to rehabilitate the bridge.
Ready to Reopen
LHB’s work on this historic bridge is considered Phase I of an ongoing preservation effort: MnDOT eventually hopes to add waterproofing to the buried faces of the bridge under the trail to further protect the bridge from freeze-thaw cycles. But that effort is pending the condition of the trail surface and funding.
LHB’s improvements to the bridge won’t be particularly visible — in fact, the true mark of success is a bridge that looks like it’s been untouched for a century. But our team still takes immense pride in knowing that we’ve played a crucial role in the rehabilitation of the Stone Arch Bridge, extending its life for decades — or even another century — to come. ∎
Lisa Karlgaard, PE, is a Senior Project Manager with LHB’s award-winning bridge design group. Learn more about the work of LHB’s Bridges & Structures team here.
