Page 311 - Advanced Mine Ventilation

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cross-measure boreholes varies from 30% to 70% of the total emissions from the
longwall panel. Cross-measure boreholes can also be drilled on the headgate side
of the longwall panel, but they are generally not as effective as those drilled on the
tailgate side of the longwall panel.

17.1.2.2 Vertical Gob Wells Completed in Lower Coal Seams

Vertical gob wells are commonly used to degas longwall gobs in the US coalfields, but
they are completed, typically, 50e100 ft above the coal seam being mined. They effi-
ciently capture gas emissions from the overlying coal seams but are not very effective
in capturing emissions from the floor. About 20 years ago, they began to drill these gob
wells to a depth of about 240 ft below the coal seam being mined, as shown in
Fig. 17.4.
The portion of the borehole below the mineable coal seam is filled with coarse sand
and small gravel, and a solid cement plug is put across the working coal seam. When
longwall face is mined through the borehole and moved 100e150 ft outby, the floor
heaves. The partially filled gob well provides a channel for gas to come out in the
gob at a point that is 100e150 ft inby of the longwall face. Such gob wells can be
placed at close intervals (150e500 ft apart) on the longwall panels that are prone to
floor gas emissions. Gas production is assisted by vacuum pumps creating 1e5 psi
negative pressure.

Figure 17.4 Gob well completed 200 ft below working seam.