Maffa Way and Mystic Avenue Superstructure Replacement Design Build Project – Phase 1
Michael Sullivan, PE, Senior Project Manager, CHA (left), Frank Salmon, Senior Project Manager, Skanska (center), and Theodore Boyle, Design-Build Project Manager MassDOT (right)
Construction of Phase 1 of the Maffa Way and Mystic Avenue Superstructure Replacement Design Build (DB) Project is nearing completion. Construction of Phase 2 is expected to begin this fall with final project completion anticipated in late fall 2025/early winter 2026. The project was advertised by MassDOT in August 2022 and bids from the three shortlisted teams were opened in November. The Skanska/CHA DB team was selected as the Best Value. Following notice to proceed from MassDOT in April 2023, the DB team advanced major design milestones including finalized Bridge Type Selection Worksheets and Sketch Plans, 75% Highway/First Structural, 100%/Second Structural and final Issued for Construction (IFC) plans. The IFC submission was accepted by MassDOT in April 2024, with construction of Phase 1 commencing shortly after.
The project involves the replacement of both bridge superstructures and rehabilitation of substructures of the two bridges carrying Maffa Way and Mystic Avenue over the MBTA Orange Line, Keolis Commuter Rail, and CSX train tracks. In addition to the bridge work, the project scope involves construction of moment slabs, retaining walls, significant utility relocations, and roadway reconstruction within the project limits. The City of Boston, City of Somerville, and FHWA are collaborating partners for developing and implementing multi-modal accommodations and design decisions.
Existing Conditions
Both bridges were built in the early 1950’s when the superstructures were previously replaced and the substructures were widened/rehabilitated. Maffa Way and Mystic Avenue both serve as one-way urban principal arterials connecting traffic between Somerville and Sullivan Square in Boston, are classified as emergency evacuation routes and have an Average Daily Traffic (ADT) of 27,600 and 22,800, respectively. These bridges provide vital links to bicyclists and pedestrians accessing the MBTA’s Sullivan Square Station.
The existing Maffa Way Bridge No. B-16-067 is a two-span steel stringer and reinforced concrete deck bridge with a total length of approximately 150 ft, an out-to-out width of approximately 84 ft, and a skew of 28 degrees. The bridge substructures consist of concrete gravity wall abutments and a solid stem reinforced concrete pier. All substructure elements are founded on spread footings. The existing minimum vertical clearance of the bridge to the top of railroad tracks below is approximately 16.9 ft. The existing bridge carries significant utilities including both abandoned and active electrical lines, telephone and communications conduits, both abandoned and active gas lines, highway lighting, MBTA power lines, and a 24 in. diameter insulated water line, owned and maintained by BWSC.
The existing Mystic Ave Bridge No. B-16-068=S-17-028 is a three-span steel stringer and reinforced concrete deck bridge with a total length of approximately 207 ft, an out-to-out width that varies between 65 and 74 ft, and a skew of 49 degrees. The bridge abutments consist of a stone masonry wall with concrete beam seat on the west side of the MBTA tracks and a reinforced concrete gravity wall supported on a spread footing at the east side. The two piers consist of reinforced concrete solid stem walls founded on spread footings. The existing minimum vertical clearance of the bridge to the top of railroad tracks below is approximately 16.6 ft.
Proposed New Superstructures
The project was initiated by MassDOT to achieve four primary goals: (1) to address structural deficiencies of the two bridges by providing new, code compliant and low-maintenance bridges, (2) to address multimodal accessibility by maintaining the general purpose travel lanes on the bridges, providing separate bicycle facilities, improving the sidewalks for pedestrians, maintaining the existing bus lane on Maffa Way, and adding a new bus lane on Mystic Ave, (3) to integrate the project with future developments, such as providing connections to the new Assembly Square, private developments in the project area, and integration with the future Rutherford Avenue improvements currently in design by the City of Boston, and (4) to increase vertical underclearance of the bridges as feasible.
The project is being constructed in two phases. Phase 1 includes the demolition and reconstruction of both the south side of the Mystic Ave Bridge and the north side of the Maffa Way Bridge (see Figure 1). The existing Maffa Way Bridge cross section consists of a sidewalk, three vehicular travel ways (two carrying I-93 off-ramp traffic and one carrying Broadway Street traffic), a dedicated bus lane, and a sidewalk (see Figure 2). To facilitate phase 1 construction, one travel lane from the I-93 off-ramp and one travel lane from Broadway Street are currently maintained, along with a bike lane and a sidewalk. The final proposed cross section will feature three travel lanes, a dedicated bus lane, a separated bike lane and a sidewalk (see Figure 3).
Figure 1 - Phase 1 construction
Figure 2 - Existing Maffa Way cross section
Figure 3 - Proposed Maffa Way cross section
On the Mystic Ave Bridge, the existing cross section consists of a sidewalk and four vehicle lanes. During phase 1 construction, the existing sidewalk and two travel lanes are currently maintained (see Figure 4). The final proposed cross section will feature a multi-purpose path, two travel lanes, and a dedicated bus lane (see Figure 5).
Figure 4 - Existing Mystic Ave cross section
Figure 5 - Proposed Mystic Ave cross section
The proposed bridge work includes new continuous-span composite weathering steel and reinforced concrete superstructures. The existing substructures and foundations are being retained with new reinforced concrete pier caps and abutment beam seats. Weathering steel was chosen to minimize long-term maintenance work over the active railroad tracks. The bridge vertical clearances were raised to the maximum extent possible during design to provide additional clearance for the railroads. Raising the profiles proved to be challenging, especially on the Maffa Way Bridge, as the I-93 Viaduct spans over the west side of the bridge.
For the existing bridge substructures, a 5% overstress waiver was granted by MassDOT, to allow the existing substructures to pass stability checks for updated loadings. The original base technical concept at the Mystic Ave Bridge proposed Expanded Polystyrene (EPS) geofoam blocks behind the west abutment and tiebacks drilled through the east abutment. However, thru the Alternative Technical Concept (ATC) process, Skanska/CHA refined our design and utilized innovative construction techniques to provide an alternative to the use of tiebacks to minimize the impact that the installation would have on the adjacent railroad tracks and foul time.
Beneath the bridges are seven active railroad tracks owned and operated by three different entities. Tracks 1 and 2 are owned and operated by CSX and are primarily utilized to carry freight. Tracks 3 through 5 are electrified and owned by the MBTA to operate the Orange Line. Tracks 6 and 7 are owned by Keolis and used for Commuter Rail operations. Removing and reconstructing two heavily used bridges over such an active railroad corridor has required significant coordination between Skanska/CHA, MassDOT, MBTA, CSX, and Keolis.
Figure 6 - Track ownership beneath the bridges (Looking north)
Phase One Construction
To facilitate Phase 1 demolition of the Maffa Way Bridge, the existing 24 in. diameter water main was relocated. Skanska/CHA worked closely with the Boston Water and Sewer Commission to coordinate relocation of this temporary water line. No utility relocations were required on the Mystic Ave Bridge, which simplified Phase 1 demolition for that structure. Demolition of the existing bridge superstructures was performed in different sections utilizing the various windows afforded by the different railroad entities. Skanska worked with CSX to arrange full shut downs of tracks 1 and 2 to facilitate the work. For work over the MBTA tracks 3 through 5, a full 9-day shutdown of the Orange Line was provided for the Phase 1 work performed in the summer of 2024. Another 9-day shutdown will also be provided in summer 2025 for Phase 2. Finally, work over Tracks 6 and 7 was facilitated with Keolis during non-revenue hours and weekend shutdowns of the Commuter Rail.
Skanska was able to take advantage of additional selective Orange Line shutdowns in the spring to further accommodate demolition activities, including removing portions of the existing superstructures and cutting down and reconstructing the cast-in-place reinforced concrete beam seats. A high early strength concrete mix was utilized at the abutments to facilitate quicker placement of the superstructure steel during the surge. CHA worked diligently with Skanska, as excavation and demolition efforts revealed substructure as-built conditions that differed from the original 1950’s construction plans. Using cast-in-place concrete at the abutments worked to the team’s advantage as it afforded more flexibility to modify the geometry of the structure based on the differing field conditions.
Due to the tight work windows and schedule constraints allotted by the railroads, Skanska and CHA decided that the new pier caps should be precast. During the 9-day shutdown of the Orange Line and following demolition of the existing superstructures, the existing piers were cut to select elevations. Precast pier cap pieces were placed and grouted. Once the grout was cured, new bearings were placed, and Phase 1 steel and stay-in-place forms were installed. This allowed for re-activation of the Orange Line and Skanska the ability to continue work safely above the active railroad corridor, to install utilities and place the concrete deck. Skanska chose to erect the steel girders in pairs to reduce the time needed during the Orange Line shutdown. Crane placement proved challenging at the project site due to the constrained area and the I-93 Viaduct.
Skanska is planning to place Phase 2 temporary traffic setup in December. Phase 2 construction will include the demolition and reconstruction of the north side of the Mystic Ave Bridge and the south side of the Maffa Way Bridge. The project is anticipated for completion in late 2025/early 2026.
Figure 7 - Superstructure demolition with low clearance to I-93 Viaduct
Figure 8 - Precast pier caps
Figure 9 - Steel erection using girder pairs