The failure of bolts holding together two sections of a 10-foot-wide pipe most likely caused a massive 2010 water main break that forced 2 million Boston area residents to begin boiling their drinking water, an independent panel has found.
The panel appointed by the Massachusetts Water Resources Authority looked at two possible causes and concluded the bolt failure was the most likely reason.
A report released Wednesday pointed to cracks in the bolts' threads that likely were created during or shortly after installation. The report said that the bolts were then loaded beyond acceptable levels.
The panel said the cracks probably grew until one or more failed, transferring the load to the remaining bolts. The cascading effect caused a blow-out of the rubber O-ring helping join together the two sections of pipe.
The bolt and bolt halves recovered from the scene showed evidence of cracks, said the panel.
While testing concluded the bolts met the necessary material hardness to clamp the two sections of the coupling together, the bolts themselves "exhibited characteristics that suggest poor fabrication," according to the report.
"Cracks were present on essentially all of the thread `crowns.' Additionally, cracks were present at many of the thread `roots,'" the report said.
The May 1, 2010 water main break forced about 2 million residents of 30 Massachusetts cities and towns, including Boston, to boil their drinking water for three days to ensure safety.
It also prompted a months-long hunt for the massive coupling that proved to be a key piece of forensic evidence in determining the cause of the break. Crews also recovered bolts and the rubber gasket used as part of the coupling assembly.
The panel discounted a second theory under which some outside event might have thrown the two halves of the pipe out of alignment, forcing the coupling to become separated from the pipe.
At a Statehouse hearing last year, Fred Laskey, executive director of the Massachusetts Water Resources Authority, said the coupling was not the first choice to hold together the two sections of pipe.
Laskey said initial plans called for the pipe sections to be welded together. The decision to switch to the coupling was made at the request of the contractor during construction partly because of time pressure, he said.
That request was approved by engineering and design firms working on the project, according to Laskey, who said the use of a coupling wasn't unusual to join together sections of a pipe. He said that a first coupling didn't work, and a second coupling eventually was installed.