A steel plate that was secured along only one side “opened up like a trapdoor†and sent rocks and other debris flying through the air when explosives were set beneath it on Tuesday at the Second Avenue subway excavation site, a transit official said.
Shrapnel from the blast broke windows in a building across the street and drew a fresh criticism of the Metropolitan Transportation Authority's oversight of the project.
At most of the construction site at the northwest corner of Second Avenue and East 72nd Street, the inch-thick steel plates that cover the big hole in the ground sit atop steel beams and are anchored into the beams, Adam Lisberg, a spokesman for the authority, said Wednesday.
But at the edge of the site, Mr. Lisberg said, there are plates that just sit loose on the concrete or are anchored on one edge but not others. It was 40 feet beneath one of these plates, at the southeastern corner of the sidewalk, that the explosives were set, Mr. Lisberg said.
He said it was not yet clear why the explosives were placed there or where they should have been placed, but that when the blast hit the 7-by-6-foot plate, “the side that was anchored stayed anchored and the side that was not anchored opened up like a hinge.â€
Another factor in the mishap, in which no one was injured, was that all the force of the blast was directed straight up, Mr. Lisberg said, though he said it was not yet clear where the force should have been directed.
Mr. Lisberg said that before a blast, traffic and pedestrians are cleared out of the area. But on Tuesday, a retired engineer who was crossing Second Avenue was close enough that he snapped some spectacular photos of the eruption even as a construction worker tried to shield him from the falling debris.
“Neither he nor anybody else should have been allowed to stand right there,†Mr. Lisberg said. “We're going to re-examine our blasting protocols.â€
Work at the site remained suspended Wednesday as the authority continued its investigation.
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