Progressive bolt failure likely caused Boston’s massive water-main break in 2010 involving a 10-ft-dia, 1,400-lb steel pipe and insulated coupling in a reservoir supply tunnel, says a May 25 report by an independent panel appointed by the Massachusetts Water Resources Authority. The incident led to a 53-hour “boil water” edict that affected two million people. The report points to progressive failure based on bolt stresses that were significantly beyond code standards and to limited evidence retrieved from the site. MWRA suggests that pre-existing cracks in some bolt thread roots occurred during coupling installation or after initial pipe pressurization. Additional cracks in the wet environment would have transferred more load to remaining bolts, the panel notes. Testing of recovered bolts indicated poor fabrication, the report adds. Steve Remsberg, MWRA’s general counsel, declined to comment, noting litigation resulting from the event.