The project will be backed by a federal fund for the reclamation of abandoned mine lands – in turn funded by coal mining royalties.

The fire is spread over 58 acres near North Union Township, Farette Country.

GAI will inject low-permeability cementitious (LPC) material into the mine at a series of boreholes to block the fire from spreading and remove air voids that feed the fire.

The grout, made from coal combustion ash and cement, will be used for the first time in a mine fire application, but has been used widely as a low-cost structural fill material.

The product is delivered to the site in a moist state, eliminating dust problems, and is mixed with water and cement to form slurry, which is pumped directly into the mine workings where it hardens to become low-strength cement.

Because the material is blended on-site, the contractor can adjust the blend to control how LPC material flows into the abandoned mine workings. The wet material will lower the fire temperature and flow into voids and cracks, eliminating air sources that can feed the fire. In addition, LPC material will fill the mine void and eliminate future mine subsidence at the site.

The Percy fire was first discovered in July 1974, and is believed to have been started by burning rubbish. The Department of Environment Protection’s Bureau of Abandoned Mine Reclamation and the US Office of Surface Mining, Reclamation and Enforcement (OSMRE) have conducted several emergency projects to slow the fire's movement to protect the nearby village of Youngstown, but have not previously attempted to extinguish the fire.

There are currently 112 underground mine fires burning nationwide, 39 of which are in Pennsylvania.

"Pennsylvania's mineral riches have fuelled our nation's economy for 300 years, but the unregulated mining practices of the past have left us with the largest abandoned mine land problem in the United States," environmental protection secretary Kathleen McGinty said.