Andover emergency crews responded quickly to a report of a man trapped under a tractor trailer truck after trying to connect a trailer to the truck on Lupine Road.
Fire Chief Michael Mansfield said that a worker's legs were caught underneath the double tires of the trailer while working on the bridge project just south of the utility yard on Lupine Road.
Mansfield said they had the man freed within 25 minutes, using a pneumatic tool to lift up the tires.
The man, who Mansfield would not identify, was flown to Beth Israel in Boston with life-threatening injuries after a helicopter was able to land off Dascomb Road.
Mansfield said he was conscious and alert when they took him to Boston.
The State Police and the Occupational Safety and Health Administration (OSHA) will be investigating the incidents.