Some 51 buildings are ‘too fragile’ to withstand Second Avenue Subway tunnelling works. MTA Capital Construction has completed its survey of the 225 buildings on Second Avenue, New York that could be affected by tunnel works, assessing that one in four of the buildings require reinforcement.
The work will be undertaken by MTA itself, at an anticipated cost for both works and relocation of residents of around USD 6 to 8 million.
The tunnel itself isn’t the issue, said MTA spokesman Aaron Donovan. “There has been no indication of any vibration or settlement associated with the TBM operation. The area in which the building reinforcement has occurred is in the TBM Launch Box area, which is a braced cut and cover excavation and is approximately 70ft wide by 70ft deep by 800ft long (21m by 21m by 244m). The outside walls are either slurry wall (where the 96th St Station will be built) or secant piles (where the walls are temporary for TBM launch purposes only). The adjacent 4-5 story brick masonry buildings are typically on shallow foundations supported on soil or timber piles.”
Repairs have been completed on 12 of the 80-100 year old buildings already and involved cellar ties between front foundation walls and bearing walls where none existed already, though this is only undertaken if a certain level of settlement is reached said Donovan. He added that in some cases where façade tilting is of concern, the front walls are being ‘anchored’ by steel plates and ties to floor joist trusses.
The tunnel is being bored by a 485t, 140m hard rock TBM. Phase one is scheduled for completion at the end of 2016. However, MTA initially predicted a progression of 50ft (15m) per day. Actual progress is slower than this. TBM excavation has only achieved a rate of 40ft (12m) per day.
The subway will serve approximately 200,000 people per day upon completion.
