NAPLES — A 120-foot boom on a crane pitched over backward, falling partially into the water next to the Naples causeway swing bridge on Route 302 Tuesday morning. No one was injured.

“As far as we can tell, the operator inadvertently hit a lever,” said Kim Suhr, vice president of Wyman and Simpson of Richmond, owner of the crane and prime contractor on the nearly $9 million bridge project.

Nothing was on the end of the boom, and it was not being used to lift anything at the time, he said. Only the tip of the boom was submerged in the water, he said.

Suhr said they are still investigating the exact cause of the mishap and by early afternoon, work had begun on disassembling the boom.

“We’ll bring it back to our headquarters, to try to determine exactly what happened,” he said.

Craig Hurd, Maine Department of Transportation project resident engineer, said he was just headed out of his rented office near the scene when he heard loud bang that sounded like a dump truck dumping its load.

“We hear a lot of loud noises, so I didn’t think too much about it,” he said.

George LaPointe of Sebago, bridge tender on duty Tuesday, said, “I just saw it come down, and that was it.”

Naples firefighters and emergency personnel, and a deputy from the Cumberland County Sheriff’s Office went to the scene.

Damage to the equipment is not expected to slow down the project, Suhr said. Either the repaired crane or a new one will be brought in to complete the work.

Work on the bridge, which replaces one of the few remaining swing bridges in the state, began last October. The new span will open May 15, 2012, Hurd said.

The swing bridge opens eight times a day in the summer to allow the Songo River Queen paddle boat to go between Long Lake and Brandy Pond. Tuesday’s mishap did not delay the Queen’s departure, LaPointe said.

The last time the swing bridge will be opened will be on Sept. 17, when the Queen will take the last cruise of the year on the two bodies of water. After that, it will only cruise around Long Lake.

The center section of the old bridge will be removed during a two-week period after Sept. 17. The project includes extensive landscaping, a boardwalk with new lighting, and reconfigured parking which provides a wider and safer area for motorists backing out onto Route 302.

The final paving, removal of the rest of the old bridge, and some landscaping will not be finished until the spring or summer of 2013, Hurd said.

He and project inspector Lisa Cole are the only two MDOT employees working at the site. The rest of the workers are employed by Wyman and Simpson and the subcontractors the company has hired.

“Our job is just to make sure everything is done according to the specifications,” Hurd said.

The total cost for the project, most of which is paid for with state and federal money, is expected to come in at $8,827,000, Hurd said.

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