Accelerating the Rehabilitation of a Masonry Arch Bridge
William Goulet, PE, SE, Senior Structural Engineer, STV Incorporated
Rehabilitating and upgrading the aging bridges that comprise our nation’s infrastructure cannot be executed with a one-size-fits-all approach, especially when it applies to unique structures that are more than 160 years old and in need of an expedited, but effective, solution.
Elevation view of the existing CRML 47.90 Bridge
The Connecticut River Mainline (CRML) Bridge 47.90 in Northfield, MA is a single-track, single-span masonry stone arch that was originally constructed in 1848 as part of the Boston & Maine Railroad in New England. The bridge consists of the main arch span and approximately 35 feet or retaining wall on each side. The existing deck consisted of timbers set on the masonry with concrete infill.
For years, the CRML was privately owned by various freight railroad companies and primarily served as a freight line with minimal passenger service until it was acquired by the Massachusetts Department of Transportation (MassDOT). Over the past decade, MassDOT has been engaged in upgrading the CRML corridor – also known as the “Knowledge Corridor” – between Massachusetts and Vermont, as part of a larger effort to increase passenger service in the region. So, with that context in mind, it was crucial to develop a long-term solution for both the structural and site drainage deficiencies to support the rail service expansion in this region.
STV was contracted by MassDOT in 2020 to perform an in-depth bridge inspection of Bridge 47.90. During this inspection, some problematic areas were discovered that were recommended for repair. If left unaddressed, these deficiencies would result in railroad speed and/or weight restrictions. In most of these instances, water intrusion or standing water was observed, indicating that freeze and thaw cycles were the likely cause of small cracks and fissures in the arch ring, deck, and spandrel wall and wingwall. Upon discovery of these structural deficiencies, STV’s design team coordinated closely MassDOT Rail & Transit Division to develop a solution that could be fast-tracked over a 72-hour window in order to minimize disruptions to existing freight rail traffic.
STV evaluated several short- and long-term recommendations, including replacement of the entire bridge, a possible shoofly alignment, as well as a rehabilitation with the replacement of the deck. These recommendations also considered alternatives to facilitate directing drainage away from the bridge in order to reduce future risk of freeze/thaw damage to the structure.
Setting a precast deck section on the grout bed
There were several factors that were considered in order to determine whether the bridge required a total structural replacement or a partial replacement/rehabilitation. One major influencing factor was was that the CRML typically only has one or two 72-hour work windows per year during which rail traffic could be diverted in order to accommodate construction. Additionally, the horizontal clearances at the existing bridge vary 14’-1” to 16’-7”. While the current roadway that passed under the structure primarily provides access to a farm and there are alternate access roads for emergency vehicles around it, a new bridge structure would need to meet federal guidelines for vertical and horizontal clearance. A replacement also had to consider available detours during construction and the extensive impacts to bridge approaches.
Through either a full replacement or rehabilitation scenario, STV determined that a similar 75-year service life could be provided. While the rehabilitation would have additional long-term maintenance requirements, the replacement would impact surrounding properties and would require multiple closures.
With these factors in mind, the rehabilitation option was selected, primarily because it could be constructed under an accelerated schedule and would require minimal approach work and property acquisition. The goals of the design were to replace the existing deck with an alternative that would address drainage, incorporate a ballasted deck to provide flexibility for future track geometry, and to provide a safety walk for inspection and maintenance. The developed plan worked within the constraints of the horizontal and vertical profiles of the existing approach track, repaired the cracks in the arch ring, provided repairs to the spandrel wall and wingwall, and addressed site drainage and slope erosion.
An extended weekend was identified where work could be performed starting Thursday night through Monday morning. To construct the concrete deck, precast sections were used to allow for quick erection. The deck sections were designed to span transversely across the supporting masonry walls. Since the width of the existing masonry walls was unknown, conservative assumptions for support locations near the edges of the walls were used. The depth of the concrete deck was dictated by the track and existing masonry elevations while providing a minimum 12” ballast depth. Rapid set concrete was used for closure pours which used hooked bar splices to minimize closure pour width. The closure pour concrete was required to meet 2500 psi at 4 hours and 5000 psi at 30 hours to provide sufficient strength during ballast placement and regular train service respectively. The precast concrete deck panels were set in a grout bed. Surveyed shim packs were used to set the panel elevation and excess grout was squeezed out by the weight of the panel during erection.
A spray applied waterproofing system was utilized in combination with a series of scuppers in the precast sections to address drainage. A transverse slope was incorporated into the precast deck panels combined with the longitudinal slope of the existing structure channeled stormwater runoff into scuppers that were located at the corner of the upturned ballast wall. Scuppers were tied into a drainpipe mounted below the safety walk and directed to the northwest corner of the project site into an area of rip rap to limit the potential for future erosion.
Elevation view of the rehabilitated CRML 47.90 Bridge
The overall project schedule from the start of design through construction presented several challenges, with 3 months for design and 5 months for procurement and construction. Although reviews were done with the contractor leading up through final design, only 7 of the 11 scuppers originally included in the design could be procured based on availability at the time. This led to the design team changing the scupper locations to the most effective locations. Some of the precast deck sections were cast within 28 days of the accelerated construction weekend so additional cylinders were tested to ensure the full 5000 psi strength was achieved. Due to the accelerated weekend schedule, the spray-applied membrane was applied to the precast prior to the weekend shutdown in order to mitigate the chances of rain producing unacceptable moisture content levels within the concrete for the membrane application. During the weekend closure, a temporary PVC drainage pipe was provided. The installation of the safety walk and final drainage system were scheduled to be completed after the weekend closure since these could be constructed without impacting rail service and were not critical to restoring service.
The rehabilitated structure was successfully constructed and has been serving MassDOT and the CRML for the past two years. Despite several challenges encountered during the accelerated construction, the bridge was ready to be opened for rail service ahead of the planned schedule on Monday morning. The project serves as a great example of what a cooperative partnership (amongst the client, contractor, and design team) and adaptability looks like over the course of an aggressive, but necessary, design and construction program.