A tractor trailer driver delivering construction trailers to a Rice Road work site on Tuesday suffered serious injuries when he was hit in the chest by one of the trailers. The 37-year-old man, who was not identified, was taken by a medical rescue helicopter to a Boston hospital to be treated for “serious injuries” after the 11:45 a.m. incident, Fire Chief Neil McPherson said. The man was driving a flatbed and delivering several smaller construction trailers to the work site near Oak Street and was helping to unload them onto the ground. “They were offloading it when somehow it ended up swinging and striking him in the upper chest area,” McPherson said. The man was flung between two other trailers that were still on the flatbed and was trapped there until firefighters and workers could figure out how to get him out. “The extrication was a little bit difficult due to his size and the placement of the various trailers,” McPherson said.