Last June TMC submitted the successful $31.7M bid for “Shoreline Stabilization and Yard Improvements” at the MBTA’s largest bus maintenance facility. The project is located along the tidally influenced Mystic River, separated from the water by a deteriorating sheet pile seawall. Behind the seawall exists a 345kV buried transmission line that is a major electrical trunk line for eastern MA. Considering the transmission line, the poor soils, and the life cycle costs of a new steel bulkhead, the engineers ruled out a replacement of the seawall and designed a new natural shoreline constructed over “stabilized” subsurface soils. The stabilization consists of a vast grid of soilcrete columns installed via jet-grouting techniques into the deep clay layers under the river. For the specialty subcontractor, this project is one of the largest jet grouting jobs on the east coast. The contract also includes upgrades to the drainage system, dredging of the river bottom, and the construction of a new flood wall to provide hurricane resiliency to the facility.
During the bid phase, the TMC Estimating team recognized opportunities to deviate from the original design concept and still achieve the final intent. Much of the construction occurs within an 1100 foot long cofferdam, and this provided an opportunity to utilize our extensive marine expertise and equipment to add value. Rather than maintain a dry cofferdam for 8+ months, we’ve planned the grouting operation to take place in the wet, from barges inside the cofferdam that will rise and fall with the daily 10-12 foot tidal fluctuations. When dewatering is eventually necessary to construct the new shoreline, the cofferdam will be segmented into cells to greatly reduce the pumping effort and duration. Another key strategy of the bid was a cofferdam reinforced by soilcrete columns rather than conventional steel bracing. This eliminated not only the major expenses for materials and installation of the bracing, but also the inefficiencies of working among the bracing. Neither of these two creative schemes were incorporated into any of our competitors’ bids, so hats off once again to our resourceful Estimating Team!
Notice to Proceed was granted in October and currently the project is fully mobilized on site and underway installing cofferdam sheeting with the barge-based pile driving crew. This operation will continue through the duration of the winter in order to meet the mandatory Feb. 15 date for suspending in-water work. Work taking place within the enclosed cofferdam is allowed after that date and throughout the five-month restricted period. The job will run through summer of 2019.
The TMC team steering the Charlestown project includes: General Superintendent Nathan Batchelder, Project Superintendent Dan Nash, Safety Director Darren Hohn, Marine Superintendent Rick Ronan, Foreman John Ricker, Office Engineer Owen McCaffrey, Surveyor Tim O’Leary, Scheduler Mona Mahmoud, and Sr. Project Manager Evan McCormick.
Before After (rendering)