Defence mechanisms used to prevent explosions failed at the UBB mine in West Virginia, then owned by Massey Energy.
West Virginia Office of Miners’ Health, Safety and Training’s long-awaited final report into the blast followed a 20-month investigation, which included underground examinations, interviews and a review of thousands of documents.
WVOMHST’s findings concur with those already released, including reports from the United Mine Workers of America and the US Mine Safety and Health Administration.
The state concluded a machine cutting through sandstone to reach the coal created the heat or spark that methane needed to ignite.
Broken water sprayers then failed to stop the fireball from turning into a much more powerful explosion which was fuelled by coal dust.
Two possible sources of ignition were listed as the cause of the explosion which ripped through the mine on April 5, 2010.
One source of ignition was a sandstone roof rock falling behind the longwall shields ignited gas by rock-on-rock impact or rock-on-steel impact.
However, WVOMHST found the most likely cause of ignition happened on the longwall as the shearer was cutting sandstone roof, igniting gas by steel-on-rock impact along the shearer bits, producing hot smears as it cut.
An accumulation of methane which was ignited by the longwall shearer was “not detected by the required mine examinations or by the required machine-mounted methane monitors,” the report said.
The methane explosion quickly transmitted into a coal dust blast which damaged ventilation controls, conveyor belts and electrical systems.
The report, which was released on Thursday, found the amount of rock dust being maintained on mine surfaces at the time was insufficient to stop a coal dust explosion.
“The region where the dust explosion started does not appear to have had rock dust periodically applied over the fine coal dust,” the report said.
“Periodic applications of rock dust over accumulating fine coal dust are necessary to render such dust harmless.”
One of the most “disturbing facts” yielded by the state’s findings was the #21 Tailgate side of the longwall was never rock dusted after the longwall started production in September 2009.
Approximately 5400ft of the 6700ft-long panel had been mined “without any record of rock dust being applied.”
The report said safety inspection logs had been signed to say the company was aware coal dust accumulation had resulted in the need for rock dusting, but there was no evidence of the issue being addressed.
WVOMHST’s investigation also determined log books were severely neglected.
“Obvious problems observed while reviewing the pre-shift and on-shift books was the lack of clarity and full disclosure of findings,” the report said.
“Extreme brevity of information was used on a daily basis in these records.”
The report said the lines of defence used to prevent an explosion, including adequate ventilation, detection of explosive gases, removal of ignition sources and clean-up of fine coal dust accumulations and rock dusting all failed.
WVOMHST recommended additional defences need to be implemented to prevent propagation of a methane explosion into a coal dust explosion.
“Research is needed to demonstrate the practical application of water barriers, rock rubble barriers and other explosion mitigation strategies as supplemental protection, with generalized rock dusting, to prevent explosion propagations,” it said.
During the course of the investigation into the blast, the WVOMHST issued 253 violations.
The report stressed the need for justice to be done for the miners who lost their lives.
“Individuals involved in the day to day decision making at the mine must be held accountable regardless of their title,” the report said.
The report comes a day after federal prosecutors charged Upper Big Branch mine former superintendent Gary May with felony conspiracy.
He was charged with conspiring to impede the efforts of federal investigators by using code words to provide pre-notification of the presence of inspectors on mine property, as well as to conceal safety and health violations and falsify examination record books.
If convicted, he faces up to five years imprisonment.