Page 297 - Advanced Mine Ventilation
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274 Advanced Mine Ventilation
The technique is generally more successful for advancing longwall panel than it is
for retreat faces. The flow from individual boreholes is typically 20 CFM, but occa-
3
sionally, it can go up to 100 ft /min for deeper holes. Sealing of the surface casing
is very important and is usually done with quick-setting cement. Sometimes, a liner
(a pipe of smaller diameter than the borehole) is inserted in the borehole and sealed
at the mouth to preserve the production from the borehole even if the borehole is
sheared by rock movements.
16.2.3 The Superjacent (or Hirschback) Method
This technique is mainly used for retreating longwall faces in very gassy seams.
Fig. 16.6 shows a typical layout.
A roadway is driven 60e100 ft above the longwall face, preferably, in an unwork-
able coal seam. The roadway is sealed and vacuum pressures up to 120 in. of water
gage are applied. To improve the flow of gas, inclined boreholes in the roof and floor
Figure 16.6 Methane drainage by superjacent method.
