BACK ON TRACK
Rehabilitation of Providence and Worcester Railroad Bridge over the Coginchaug River
150 YEAR OLD RAILROAD BRIDGE STANDS STRONG
This Connecticut Department of Transportation (CTDOT) Emergency Declaration Project consisted of the rehabilitation of a historic railroad bridge located in the Wadsworth Falls State Park in Middlefield, Connecticut. Over time, the steel truss had shifted northward from its original position and pinned itself against the back face of the north abutment wall. The north abutment and the pier were also exhibiting distress in the form of cracking, voids, missing and loose joint mortar, and settlement and tilting of the bridge seats. CTDOT deemed the bridge insufficient and stopped all traffic on the bridge in April 2015. H.W. Lochner and Geo
Design were retained to design a fix.
The two-span bridge was reportedly constructed around 1860, is about 135 feet long, and generally spans over the Coginchaug River in the north-south direction. The superstructure consists of a single-span steel truss section that is supported on a north abutment with wingwalls and a central pier; and a masonry arch span supported on the central pier and a south abutment with wingwalls.
The bridge truss was temporarily shored with two prefabricated towers allowing the bridge to be reopened under limited service in June 2015. The south tower was supported on crane mats bearing on natural Glacial Till. A cofferdam consisting of concrete blocks and sand bags lined with polyethylene was constructed around the south tower area to facilitate foundation construction and prevent scour. Due to the presence of unsuitable soils, shallow groundwater levels, and close proximity to the Coginchaug River, the north tower was constructed on a concrete slab supported on micropiles socketed into bedrock.
A multi-media grouting program was designed by GeoDesign. The grouting was used to improve the north abutment backfill soils, and to fill existing voids and fractures within the masonry of the north abutment and the pier.
Grouting was performed in stages to allow final selection of grout locations, mixture, and pressures based on initial grout takes and other factors. The grouting program was verified using a combination of continuous observation and documentation (e.g. volumes, flow rates, pressure behavior, travel distance, etc.) during grouting and confirmation sampling and testing (e.g. coring and penetration testing).
The north abutment backfill was grouted using vertical tube-a-manchette (TAM) pipes that were installed at 18 locations using a split spacing of 4 feet on center. The TAMs were about 30 feet long with 22 vertical ports and were installed in 3-inch diameter predrilled holes using a lean (weak) sheath grout mixture. The TAMs were generally pressure grouted to approximately 10 to 80 psi and a total of about 14,200 gallons of grout was pumped into the TAMs. Standard penetration N-Values increased about 100 to 1,000% in the grouted zone resulting in a significant decrease in the lateral earth pressure on the north abutment.
A total of 43 and 32 vertical and near horizontal holes were cored or drilled into north abutment and pier, respectively, using a split spacing ranging from 2 to 8 feet on center. The holes were about 3-inches in diameter and ranged in length from 7 to 30 feet. Gravity grouting techniques were used to grout the masonry structures. Approximately 6,500 and 7,000 gallons of grout were pumped into the north abutment and pier, respectively.
Reinforcing bars were installed in the masonry grout holes immediately after grouting to serve as passive dowels and structurally tie the masonry blocks together.
Based on the consistently high initial (primary locations) grout takes followed by lower subsequent (secondary locations) grout takes in the masonry, confirmation cores, and overall high grout take volumes, it was concluded that the masonry grouting successfully filled the major voids and cracks in the stone masonry of the north abutment and pier.
After grouting was successfully completed, the masonry was repointed, existing bridge seats were replaced with reinforced concrete, the expansion bearing was replaced, and the steel bridge truss was longitudinally jacked into its original position. The temporary shoring towers were then removed and the bridge was reopened for full service in November 2015.
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View to the east after grouting of abutments, replacement of bridge seats, and re-pointing of exterior.