While accidents involving large circular saw blades failing can happen, it is highly unusual for a blade to become dislodged from its saw and fly off into the air, says one industry expert, referring to such an incident that occurred at a Manhattan job site on May 27.

The 3-ft-diameter blade appears to be from a slab saw, or flat saw, which is typically used to cut thick concrete structures and roadways and is tough enough to cut through steel reinforcements, says Russell Hitchen, a staff member of the non-profit Concrete Sawing & Drilling Association (CSDA), which provides training, forums and tool box safety guidelines to the concrete industry.

“Typically, the blade is secured to the saw, and there is some kind of locking mechanism,” says Hitchen, who has no connection to or information beyond media reports of the May 27 incident. Some contractors also use a guard that covers a certain percentage of the blade on the saw, “so if the core fails and the blade shatters or cracks that guard is there for protection,” he adds.

While video clips of the blade flying through the air are grainy and hard to read, the blade appears to be intact—not shattered or cracked, which are among the main reasons why a blade would fail, Hitchen says.

If it is intact, a likely scenario is that the locking pins securing the blade to the saw failed, he says. “This is just speculation, though, because we don’t know if this is a core failure or something that happened to the blade collar,” he adds.

The blade reportedly flew 10 ft in the air Tuesday morning, traveling down part of a city block and ultimately striking a woman’s leg. She was brought to Bellevue Hospital with what appeared to be minor lacerations and has been released, officials say.

The investigation into the incident is ongoing, says a spokesman at the Dept. of Design and Construction (DDC), which is heading up the project at Ninth Avenue and West 48th St. where contractors are doing trunk and water main work. Under the NYC Dept. of Environmental Protection, DDC is managing 10 of 11 sites throughout the borough as part of a multi-year $427-million project to connect the water distribution system to the Manhattan leg of Water Tunnel No. 3.

“Our street construction projects in Manhattan have had excellent safety records, despite the fact that we are working in densely populated areas,” the DDC spokesman said in a statement. “Our thoughts are with the pedestrian who was hurt; we would like to assure her that we will do everything we can to make sure that nothing like this happens again.”

He adds that DDC has “directed the contractor’s on-site supervisor to inspect these machines every time they’re put in use—each day before work begins and again as needed after any interruption of work.”

Waterworks, a joint venture of Judlau Contracting Inc. and its parent firm OHL USA, is the GC on the project. Judlau did not respond to calls for comment by press time.