Opinion ID: 170445
Heading Depth: 1
Heading Rank: 1

Heading: introduction

Text: Plateau operated the Willow Creek Mine, an underground coal mine in Carbon County, Utah. The coal was extracted using a method known as longwall mining, which permits a mine operator to remove a seam of coal without leaving behind pillars of coal for support. Before longwall mining begins, a set of tunnels  called entries  is constructed around a large, rectangular block of coal, called a panel. (See schematic drawing below.) The two entries running along one long side of the block of coal are known as the headgate entries. A conveyer belt is installed in one of these entries. The entries on the other long side are known as the tailgate entries. A machine called a shearer is placed at one end of the block of coal, known as the face. The shearer takes passes across the longwall face, severing approximately a 30-inch slice of coal from the seam with each pass. The severed coal falls onto a chain conveyer that runs along the face, which feeds the coal onto the conveyer belt on the headgate side for transport out of the mine. Temporary roof supports called shields advance behind the shearer as the panel of coal retreats. The shields support the roof above the miners and the equipment. Behind the shields, the roof is allowed to collapse. This mined area behind the shields is known as the rubble zone. An area where mining has been completed is known as a worked-out area. 30 C.F.R. § 75.301. The mined area and the entries immediately adjacent to it are referred to as the gob. Some mines, such as Willow Creek, are considered gassy mines because their operation liberates a significant amount of methane. Methane is trapped within the pores of a coal bed and is released when the coal is broken up during mining. Gassy mines present a challenge because methane-air mixtures are explosive at concentrations of 5 to 15% methane. Mine operators use a ventilation system, called a bleeder system, to dilute methane in worked-out areas. The methane is released from the coal at a concentration of 100%; the purpose of the bleeder system is to dilute the methane so that only small quantities are in the explosive range and concentrations are well below 5% near likely ignition sources, such as areas where miners are present and work is being done. At the longwall panel involved in this case (the D-3 panel), a fan blew fresh outside air into one of the headgate entries. The air was forced across the longwall face, which was 815 feet wide, and also through the rubble zone. Streams of fresh air  air from the headgate side that had not ventilated the gob and air from the tailgate side brought in to maintain appropriate ventilation pressure  joined the air coming off the rubble zone, diluting the methane further. From there the air entered tunnels on the tailgate side, known as bleeder entries, and eventually traveled out of the mine. After mining on the D-3 panel had progressed approximately 350 feet, the bleeder system would be supplemented with gob vent boreholes, which are holes from the earth's surface down to the ceiling of the gob. The holes had been drilled, and once the coal under them had been removed, they would ventilate methane directly upward to the surface of the mine at high concentrations. At the time of the accident, Plateau believed that it would reach the first gob vent borehole within a few days.