BOSTON, Nov. 2 (UPI) -- Federal investigators allege Boston's Big Dig tunnel ceiling was designed with a smaller-than-normal margin of safety.
A preliminary report obtained by the Boston Globe says there was nothing to prevent heavy concrete slabs from falling on a passing car when ceiling bolts fell out.
The drop ceiling was held up by steel hangers that were suspended from bolts that had been glued into the tunnel roof.
"No redundancy was built into the ceiling in the event the hangers failed," the Globe said the National Transportation Safety Board states in its report.
The NTSB report said the board has "researched other tunnels throughout the country and has found that significant redundancy is built into the ceiling design" so the ceilings would not collapse when bolts fall out.
The newspaper said the investigation by the NTSB "will carry enormous weight in assessing what went wrong and who should be punished."
