R. Zoppo Corp. Reconstructs 17th Century Cradock Bridge
The Mystic River Bridge is undergoing a $12.8 million renovation and reconstruction under Massachusetts' Accelerated Bridge Program. However, this is not the 2-mile long river span built in 1950 to connect Boston with Chelsea and re-named the Maurice J. Tobin Bridge, but the very first span built over the Mystic called Cradock Bridge.
Built in 1637 and diminutive by comparison to the Tobin, the Cradock Bridge was nevertheless as critically important to settlers, farmers, traders and others who lived north of the river as the Tobin is today to travelers going to and leaving Boston each day. Located north of, and flowing parallel with, the Charles River, the 7-mile Mystic River originally was a tidal estuary of Boston Harbor.
According to the Medford Historical Society and Museum, in the early part of the 17th century, most of what is now the city of Medford was a private plantation of Governor Matthew Cradock. In 1637 Cradock's men built the first bridge across the Mystic, which charged a toll for crossing the privately owned span. And for the next 150 years, until 1787 when a second bridge was built elsewhere over the river, Cradock and subsequent private owners collected tolls from all traffic north of the river - unless people chose to ford the river or take ferries to get in and out of Boston. People who could not afford the bridge toll continued for years to use the free ford. That could be dangerous when the tide was running high.
Two Bridges in One
R. Zoppo Corp. of Stoughton, Massachusetts, has the contract for the rehabilitation of the Cradock Bridge, which carries Route 38 (Main Street) over the Mystic River. At this point, the river flows west to east while the bridge is oriented south/north. Thus the west side of the bridge is the upstream face and the east side the downstream face, for reference purposes. The 100-foot-long bridge is actually formed of two joined structures, a stone arch span and a steel stringer span. The stone arch structure was built in 1637 featuring two arch spans and was reconstructed in 1880, while the conjoined bridge is a 20-foot-wide reinforced concrete beam structure constructed in 1908 and reconstructed in 1934.
The two original stone arch spans (labeled Span #2 and Span #3 on construction drawings) each provide a clear width of approximately 26 feet and were built of 20-inch granite stones with a 26-inch keystone. They are supported by stone piers resting in turn on timber cribbing with underlying timber piles. Complicating the design of the rehabilitation for engineers AECOM is the joint ownership of the bridge. The City of Medford owns the original stone-arch bridge, while the Massachusetts Department of Conservation and Recreation (DCR) owns the conjoined bridge over a boat canal and the entire west side of the bridge which it had widened by approximately 20 feet. At some time in the bridge's lengthy history a dam was built which provides support for the DCR-owned widened west side.
Complicated Tasks and Traffic Patterns
Zoppo's contract is an exceptionally complicated one, requiring construction to be completed in multiple stages and five different traffic patterns to be set up to minimize disruption for the heavy volume of traffic through this area. Route 38, classified as an Urban Minor Arterial, experiences an average daily traffic of 42,000, including transit busses. According to MassDOT's contract scope of work, the project's key elements include:
Rehabilitation of the city-owned original stone masonry arch spans
· Replacement of the DCR-owned steel stringer bridge over the boat canal at the south end by an 8-inch concrete structural slab
· Demolition of the DCR-owned west side widened superstructure of the bridge supported by a dam structure below
· Replacement of the entire west side widened structure with a concrete arch system that replicates the dimensions of the stone arch structure
· Rehabilitation of the river and canal walls immediately upstream and downstream from the bridge on the south end
While the project will replace most of the existing bridge structure, the original stone masonry arch bridge of spans #2 and #3 (of three spans) remains. The reconstructed bridge will follow the same alignment and profile, and maintain the same end bearing positions, as the existing bridge. Specifically, the overall width of the reconstructed bridge will be approximately 75 feet, 4 inches. There will be two 11-foot-wide travel lanes in both the north and south direction, with 4-foot shoulders, a 7-foot, 4-inch raised sidewalk on the east side and an 11-foot, 5-inch raised sidewalk with granite curbing on the west side.
Along the edge of the sidewalks adjacent to the travel lanes, a low profile BR-2 steel rail crash barrier will be mounted on a concrete foundation which acts as the curbing for the sidewalks on the bridge. And for the exterior edges of the bridge, existing granite stone pilasters will be reset and an ornamental steel railing will complete the outside bridge railing.
Numerous utilities to be carried by Cradock Bridge will be located on each side of the bridge within utility bays underneath the roadway and sidewalks, and made less conspicuous by positioning them within the depth of steel stringers. The final roadway deck surface will be asphalt over a waterproofing membrane, for the entire width and length of the bridge.
The overall budget for the project is $12.8 million, with Michael Winnick serving as the Resident Engineer.
Innovative Assembling of Temporary Bridge
To accommodate current traffic volumes and utility relocations, a temporary bridge was assembled to provide unimpeded northbound vehicle traffic during all of the five phases of construction. The temporary bridge spans the river from the intersection of Route 38 (Main Street) with the Mystic River Parkway/Route 16 on the south to Clippership Drive on the north. In line with this traffic plan, the Route 16 westbound off Ramp at Medford Square (Main Street), Exit 31, has been closed for the duration of the Cradock Bridge reconstruction project.
According to Andrew "Andy" Greenlaw, Project Executive for R. Zoppo Corp., the contractor chose an Acrow 700 XS Panel Bridge for this application.
Greenlaw explained that the most common method of erecting temporary panel bridges is to clear an area large enough to erect the entire bridge on land along the final alignment, build the bridge complete with a lightweight launch nose, push the bridge on rollers across the river to the rollers on the other abutment, remove the launch nose and jack the bridge down onto its bearings.
However, he noted, that procedure was not available for this project since there was only enough room to build 20 feet of the 134-foot bridge along the alignment, so they had to develop an alternative method. The only area suitable to construct a 134-foot structure was along the alignment of the closed Route 16 ramp which is perpendicular to the final bridge alignment. What's more, due to the bridge ends being skewed, it would be necessary to build a 150-foot- long structure to be able to set the bridge on all four rollers. Next, they chose the largest capacity mobile crane they could to the site, a Terex AC 500-1 from Hallamore, to perform the lift.
Working with the crane chart and the weight of the various Acrow bridge components, the construction team designed a structure that was 150 feet long and weighed 110,000 pounds, with a center of gravity located 50 feet from the heavy end. The crane chart allowed them to make the pick at a 72-foot radius from the center pin of the crane, with a 25 percent factor of safety. The geometry of the bridge and crane allowed them to set the crane at a radius of 71 feet - very close to the maximum capacity of the crane.
The tight tolerances required that the crane position and the buildout location be established using high-precision survey instruments.
Greenlaw said the entire process was executed successfully and the lift was carried out without mishap, allowing them to begin the process of completing the structure. With a small hydraulic crane and a crew of six they completed 20-foot sections of the bridge at a time (i.e., installing floor plates, etc.) then pushed the bridge across the rollers and removed a 20-foot section from the launch nose on the other end.
The process continued until the entire bridge was built out. It was then jacked off the rollers and lowered into place on the bearings.
Mini-Piles for Alternative Cofferdam Construction
Zoppo also suggested, and received approval for, an alternative method of constructing the cofferdams required for the demolition and reconstruction of abutments and piers that would take place below the river's normal pool elevation.
After a required dive inspection of the river bottom and substructure had been carried out, it was decided that an alternative to the original cofferdam design would be beneficial. Originally, the project design called for the temporary cofferdams required for construction dewatering to be placed on top of, and bolted to, existing concrete slabs on the river bottom. However, Zoppo, city officials and the engineers agreed on an alternative design due to the uneven character and questionable stability of the existing concrete slabs.
This alternative called for installing temporary mini-piles for lateral support, then attaching horizontal timber wales to the mini-piles and installing oak sheeting vertically in order to follow the uneven profile of the bottom.
Mini-piles, also called micro-piles, pin piles, needle piles or root piles, are deep foundation elements consisting of small diameter (usually 12-inch maximum) high strength steel pipe containing steel rebar and cement grout. They are designed to transfer axial loading to the earth via sidewall friction.
Once complete the cofferdams were sealed with a 4-foot deep concrete tremie pour. Also, since the contractor needed future access to these areas when they demolished the old bridge deck, the mini-piles were designed to carry estimated vertical loads. After demolition, they constructed pile caps made of steel HP sections and spanned them with 12-inch-thick wooden mats. This provided workers with a stable platform to support the mini-pile rig for the installation of the permanent piles.
The outermost piles will also be used to support necessary scaffolding for workers as they install the granite facing stones on the spandrel walls of the new arches on the west (upstream) side of the reconstructed bridge.
Zoppo's contract also covers roadway approach work north and south of the Cradock Bridge involving resurfacing, pavement marking and signing, landscaping, and sidewalk construction. Once construction is complete, a proposed landscaping design along with native restoration seed mix will be applied to exposed soils within the project limits.