Holes drilled at a 30-degree angle rather than vertically, plus inadequate decking, contributed to the Aug. 21 subway station project excavation blast that broke several store windows on the east side of Manhattan in New York City, according to preliminary investigation results. The information was released by the Metropolitan Transportation Authority, which is building the $4.45-billion Second Avenue Subway project.
A joint venture of Shea, Schiavone and Kiewit had conducted the "controlled" blast as part of its contract to create a 970-ft-long, 70-ft-wide, 50-ft-high station cavern. The blast was to make room for a future escalator with only sound and vibration noticeable from the street.
Instead, smoke and debris escaped and shot up into the air and window glass shattered. Nobody was injured, but work will be halted until the team provides a decking system review and recommendations for how it will proceed.
The MTA's Capital Construction Co. plans to retain an independent safety consultant, add protective rubber matting and "be much more involved in understanding what the various methods are and how [the contractor] is going to be blasting," said president Michael Horodniceanu at an Aug. 23 press briefing.
"When you drill vertically, the energy that is created should go laterally," he said. "In this case, it went vertically and actually aimed directly to the southeast corner of the 72nd Street deck. The decking lifted and allowed rock to actually spread into the street." He added that the cover was supposed to absorb all of the pressure, but it did not.
The barrier plate that was to contain the blast was 1500 lb, Horodniceanu said. "I believe that it was not anchored down at the edge. This is a question we are asking now," he said.
The team had conducted 72 blasts previously without problems, he noted.
A call to the contractor was not immediately returned.