Crews in Boston are removing in sections a monolithic, 43,000-sq-ft, 4-in.-thick reinforced-concrete ceiling off an Interstate 90 tunnel using specialized trucks equipped with shipping containers and hydraulic lifts.
In the predawn hours of April 6, three flatbed trailer trucks, side by side, crept out of the Prudential Tunnel in three of the four eastbound lanes carrying a 33-ft-wide, 45-ft-long slab weighing 75,000 lb—the largest of 106 sections to be removed to date, says Cory Brett, project manager for Simpson Gumpertz & Heger (SGH), the prime design contractor.
The move was part of an $8.5-million Massachusetts Convention Center Authority (MCCA) project. It was the third of four scheduled weekend closures to remove the spalling ceiling, which has reached the end of its useful life, says Scott DiFiore, SGH structural engineer. The ceiling originally was installed as a sound barrier and to provide fire protection for the steel girders of the Hynes Convention Center directly overhead but became obsolete when fire protection was applied to the girders in 2011, says Jack Haley, MCCA chief facilities officer. A limited number of panels will be replaced, if necessary, to serve as sound barriers.
Crews installed 200 support hangers on each side of the tunnel to allow the ceiling to be cut into panels, most about 12 ft wide, 30 to 50 ft long and weighing about 30,000 lb, says DiFiore. "Iron- workers had to drag 450-lb structural-steel beams across the [suspended] ceiling—above the train tracks—through a 2-ft-wide, 3-ft-tall access door crawling on their hands and knees," Brett says.
The panels were detached from the tunnel structure and lowered onto trucks through a lift-platform system fabricated by LM Heavy Civil Construction (LMH).The 40-ft flatbed trailer trucks include a 10-ft by 40-ft frame system on which an 8-ft by 40-ft steel shipping container sits, with four jacking legs in each corner of the frame, says LMH Vice President Pete Williamson, adding that Transition Engineering designed the legs to telescope up independently, which allowed for adjustments due to varying elevations.
Using a manifold on the frame and a removable hydraulic pump, crews raised the container so it fit snugly against the ceiling before cutting the supporting hangar rods. In the case of the 75,000-lb slab, an escalator pit in the convention center is 18 in. above the ceiling, so rather than cutting the ceiling into four pieces parallel to each of the lane lines, crews could cut the ceiling only at the south and north walls and at the edge of the northernmost lane, Brett says. This left the three-lane-wide slab section to be removed.
Chris Blanchard, project manager for J.F. White, which has the $8.5-million construction manager-at-risk contract, says the project is ahead of schedule despite obstacles on March 21-23, the first of the four weekend closures.
Blanchard recalls work in the west- bound lanes in which "the bottom flange of structural-steel girders for the convention center was four inches above the top of the concrete, so there was no room to saw-cut. We had to demolish the concrete with jackhammers to break it up."
Crews have removed all but four of the 110 ceiling pieces from the 430-ft-long tunnel. Work on the westbound lanes was completed after two weekend closures in March, and work on the east-bound tunnel is 90% complete after the third weekend closure, on April 4-6.
Traffic management was atypical for a busy urban turnpike, says Robbie Burgess, senior civil engineer with Howard/Stein-Hudson Associates, traffic engineering consultant for SGH. "Running bi- directional traffic in an interstate tunnel with daily weekend traffic volumes of over 30,000 vehicles is not something I have seen before," he says.
The last tunnel closure was to be on April 25-27, following the April 21 Boston Marathon. But discussions with the Massachusetts Dept. of Transportation led to that closure's cancellation, says Williamson. Instead, the last four panels will be removed in two- or three-lane closures over a weekend night. Final project completion is slated for the fall.