By applying modem testing and rehabilitation techniques to a structure more than 160 years old, engineers have ensured the historic Canton Viaduct's place in the high-speed rail service of the future.

Custom-designed scaffolding over the Neponset River allowed workers to repair masonry and replace cast-in-place arches without closing the railway on the Canton Viaduct.

The Canton Viaduct in Canton, Mass., built in 1835 and in continuous service ever since, is the second-oldest multiple-arch masonry viaduct in the country. Serious deterioration had slowed the movement of trains over the viaduct and threatened the viability of the historic structure. A fast-track restoration that included arch replacements and a new, independent superstructure has helped save it from further deterioration and ensure its place along Amtrak's upcoming high-speed rail line through the Northeast.

The Canton Viaduct carries more than 50 trains daily, including Amtrak's Northeast Corridor trains, Massachusetts Bay Transportation Authority (MBTA) commuter service and occasional Conrail freight trains. Located on the main line between Boston and New York, the bridge was built in the 19th century by the Boston & Providence Railroad and in 1984 was placed on the National Register of Historic Places. Although the structure was serviceable, there were a number of physical limitations. These included substandard horizontal clearances between the two tracks that forced severe speed restrictions; inadequate ballast shoulders to contain the thermal effects from continuous welded rail; and an inability to accommodate the high-speed rail tracks. In addition, there were cracked granite blocks, deteriorated masonry joints and badly spalled concrete spandrel arches.

VIADUCT HISTORY

When constructed in 1834-35, the viaduct was the last link in the railroad from Boston to Providence, R.I. It spanned the Neponset River valley, one of the largest geologic depressions traversed by a railroad at that time. For this major undertaking, the railroad hired Captain William Gibbs McNeill as chief engineer and Major George Washington Whistler, both noted engineers and West Point graduates. Constructed entirely of locally quarried granite, the viaduct was built with one track and carried significantly lighter railroad equipment than today.

Increased traffic caused the railroad to double the track in 1860. The railroad installed transverse wood floor beams that supported a continuous wood fence at the face of the deck. This assembly retained the ballast on the structure. In 1880, steel beams and a heavy steel railing replaced the wood floor and rail systems.

In 1910, cracks appeared in the masonry spandrel arches. Workers cast secondary concrete arches just below the masonry arches. In 1952, a second roadway arch was cut into the structure. There is evidence of various masonry repairs over the years, but it appears that detailed inspections were conducted only in 1912 and 1980.

Although MBTA owns the Canton Viaduct, Amtrak has operating rights for its Northeast Corridor service (Boston-New York-Washington) and is making capital investments to implement new, high-speed rail service between Washington and Boston. This system includes an overhead catenary along the right-of-way. Amtrak is paying SO% of the cost for the $10 million Canton Viaduct Rehabilitation Project.

STRUCTURE DESCRIPTION

The viaduct is 188 m long and follows a 1 deg. curve. It is 6.7 m wide and the deck is approximately 18 m above the Neponset River. There are 21 spandrel arches, which spring from solid granite masonry piers spaced 8.4 m on center. A unique aspect of the viaduct is that the space between the piers contains longitudinal granite walls. Two parallel, 1.5 m thick walls separated by a 1.2 m wide chamber fill the space between the piers.

Where the viaduct spans the river, six smaller arches penetrate the longitudinal walls and allow the river to flow beneath. There are also two roadway arches: one that was part of the original construction and one added in 1952. The viaduct deck is ballasted to support two railroad tracks.

Three key project objectives of the design and construction of the track upgrade were strengthening and widening the structure for future high-speed rail service; restoring and preserving the historic fabric of the structure; and facilitating fast-track construction without hampering railroad operations.

The rehabilitation design focused on supporting an accelerated construction schedule while working adjacent to high-density railroad operations. Only one track could be taken out of service at a time, and the second track could be worked on only from 2 a.m. to 5 a.m. Both tracks had to be operational within 17 months.

To widen the structure for high-speed rail service, the distance between the two sets of tracks ("track center") was increased from 3.6 m, which was less than current railroad standards, to 4.0 m. The rehabilitated structure also needed to accommodate an improved track alignment and increased superelevation.

Designers based strengthening requirements on the characteristics of both current and proposed rail equipment. Current commuter rail requires a maximum axle loading of 31.7 metric tons at 177 km/h, and Conrail freight trains require axle loads of 36 metric tons at 80 km/h. A maximum axle load of 22 metric tons at 240 km/h was used for Amtrak's new high-speed rail equipment. The criteria also included an increase for impact loading and a seismic upgrade.

To maintain the historic fabric of the structure, extensive masonry restoration was needed. The painted steel railing, dating from 1880, was historically significant and had to be restored.

Engineers had to incorporate poles for the new overhead electrical distribution system without detracting from the structure's historic integrity. This required eight closely spaced poles because of the structure's curvature and the need to keep the catenary wires close to the track centerlines.

The contractors-the Middlesex Corp., Littleton, Mass.-had to complete their work in a tightly constrained area, with limited access, adjacent to ongoing railroad operations.

DESIGN INVESTIGATIONS

The first step was an extensive search for plans or other records of the original construction and subsequent engineering reports. Although our team made inquiries at numerous institutions-among them the Smithsonian, Harvard University, ASCE, the Historic American Engineering Record, and various railroads and local agencies-the team from HDR Engineering, Boston, never located the original plans, so we began extensive investigations of the structure in 1994.

We mapped the exterior of the structure and recorded the locations of deteriorated masonry. At the same time, we obtained cores to compression-test the granite and mortar joints, and contractors removed two granite blocks to conduct interior inspections in two chambers between the piers.

Divers also inspected the piers under the river surface, and we used ground penetrating radar and Fathometer surveys to evaluate the profile of the river bottom and assess scouring. Based on all of these inspections, we prepared a load rating of the existing structure.

Test pits exposed several pier footings. Although previous engineering reports indicated that the footings were 2.4 m below grade, some were only 1.4 m below grade. Rather than solid blocks, the footings consisted of rubble and random fill.

Test borings for the underlying soils, taken in four locations, showed that the soils generally consisted of a 0.3 to 1.8 m stratum of fill; some pockets of alluvium; a very dense glacial till; and a hard bedrock stratum about 4 to 10 m below grade.

Built on a 1 deg. curve, the structure is subjected to horizontal centrifugal forces and overturning moments from high-speed trains. Allowable bearing pressures were 0.8 MPa for footings founded on the glacial till and 0.5 MPa on the fills or alluvium. A geotechnical analysis determined that differential settlements from the proposed loading could approach 51 mm. Also, pressures from seismic events could exceed the ultimate bearing capacity of the soils, and there could be uplift on the foundation. For these reasons, we recommended that a supplemental deep foundation be provided for the structure.

We evaluated different deep foundation types, including drilled shafts, grouted minipiles and driven steel H piles. The use of drilled shafts and H piles proved problematic because they would require new footings adjacent to the masonry piers. In addition, the piers in the river would require cofferdams for construction. We also considered alternative strengthening methods such as compaction grouting, chemical grouting and jet grouting-but grouting provides no tensile strength and we were concerned about spillage into the river during installation.

Minipiles offered a number of advantages. They could be installed by coring from the top of the structure through the granite blocks and overburden, minimizing disruption to the river. Minipiles also provide compression and uplift resistance, and their cost is competitive with other strengthening methods.

We conducted a minipile load test to determine the capacity of the soils at the site and to assess the feasibility of coring through the granite masonry without damaging it.

The test confirmed that coring vertically from the deck was both more feasible and more cost effective than inclined drilling through the face of the masonry. This approach also avoided a variety of harmful effects to the historic masonry and the fragile environment of the river.

In addition to strengthening the structure, we designed a complete replacement of the concrete spandrel arches and a new, independent superstructure using precast, prestressed concrete (PPC) deck beams. In addition, the historic masonry was repaired, repointed and cleaned, and the steel railings were removed, rehabilitated, galvanized, shop painted and reinstalled.

BRING DOWN THE NOISE

We were asked to perform a noise and vibration study to measure and predict the impacts of future high-speed rail operations on the community.

We placed vibration sensors at the top and bottom of the viaduct to measure structural vibrations from passing trains and to assess the level of vibrations transmitted to the adjacent community. Our goal was the preconstruction vibration levels, since the structure hasn't sustained any major damage over its many years of operation. In addition, we used Federal Transit Administration (FTA) criteria to establish vibration thresholds for abutting buildings, as well as thresholds for cosmetic structural damage to historic and fragile buildings.

Modeling of the future high-speed train operations showed that there could be excessive vibrations in the structure and that the FTA annoyance criteria could be exceeded in a number of locations in the community. As a result, we modified the structural design by adding supports between the piers.

We also installed rubber ballast mats under the tracks to absorb noise and vibrations and designed the new track structure with concrete ties and continuous welded rail (CWR). Prior to construction, the track on the viaduct was jointed rail on wood ties.

CWR had not been installed because of the inability of the original ballast retainer assembly to restrain its thermal movements. We designed the new PPC deck to accommodate the concrete ties and the CWR. This combination of approaches greatly reduced the anticipated vibration levels on the structure and the ground-borne vibration in the community.

The PPC beams were cast of 45 MPa concrete with 13 mm diameter low-relaxation steel strands. Other reinforcing steel in the PPC beams was galvanized. We selected PPC beams because they could be erected quickly and with minimal impact on railroad operations. The dynamic clearance envelope for the trains on the adjacent track governed the geometric design of the PPC beams, of particular concern because the PPC beams installed for the first track would be 0.8 m higher than the existing track.

We specified that embedded plates be cast into the PPC beams to attach the historic railing. The railing was repaired, galvanized, shop painted and reattached to the structure on a new steel support truss. The truss also supported a 1 m wide steel grating safety walkway. Base plates for the new electrification poles were fabricated as part of the refurbished railing assembly.

Contractors demolished all of the secondary concrete spandrel arches and replaced them while the track was out of service. We designed the new arches with 28 MPa concrete and epoxy reinforcing steel. The construction contractor was not permitted to apply any new loads to the structure until the concrete for the arches had cured.

CAREFUL CONSTRUCTION

Only one track could be taken out of service at a time, and the second track could be used for construction only from 2a.m. to 5a.m.

Work on top of the structure, above, was conducted close to live rails (inset) as trains passed by.

The design included a temporary track crossover just west of the viaduct to permit single-track railroad operations, as well as a steel "ballast retainer" plate between the two tracks. This allowed workers to remove the stone ballast at the out-of-service track prior to deck replacement while providing continuous support to the operational track.

Because of its proximity to the live track, the ballast retainer could be installed only during the early morning window, when no trains were running.

We assessed various drill rig configurations with respect to clearance from the live track, adequate power capacity to drill through solid masonry, and maneuverability through the constricted work zone. Extreme care was taken during this operation to minimize the vibrations from the drilling because of concerns about damaging the existing masonry joints and causing a loss of the minipile grout.

The PPC beams were delivered by truck and erected with conventional hoisting equipment. Even so, the beams could be erected only between trains during the day-or at night, when they had to be hoisted over live tracks. To facilitate beam erection at the river, we provided a pile-supported work platform in the river.

Because this was a one-of-a-kind project, the MBTA convened a value engineering team to review and comment on the design approach. The group comprised six internationally recognized experts in railroad engineering, including representatives from the U.K., France and Germany. The team suggested a number of refinements; however, it endorsed the basic design approach developed by HDR design engineers.

Because our team focused on the design objectives, contractors encountered few unknowns during the construction phase. Meanwhile, we achieved our schedule objectives and maintained railroad operations throughout all phases.

Vahid Ownjazayeri, P.E., is deputy director of construction contracts for the Massachusetts Bay Transportation Authority, Boston. David A. Peters, P.E., is project manager for HDR Engineering Inc., Boston.

Copyright American Society of Civil Engineers Oct 1998

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