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Floor Gas Emissions and Gas Outbursts 289

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Figure 17.5 Short 10 e15 deep borehole for grouting on the longwall face.
17.1.3 Injection Boreholes on Longwall Faces
In many mines, the floor gas emission problem is not very serious. There may not be
any major coal seam in the gas emission space below the coal seam being mined, but
gas may be originating from lenses of coal or other gas-bearing strata, such as carbo-
naceous shale or tight sandstone with low porosity. These emissions do not last more
than a few hours and are best controlled by drilling short (6e10 ft deep) boreholes on
the longwall face next to the point of gas emission (Fig. 17.5).
A mixture of micromatrix (very fine) cement and water is pumped into these bore-
holes to plug the channels of gas entry. Equally good results have been obtained with
other mixtures, such as the polyurethane foam used for sealing fractures in the mine
roof. Many contractors provide this service to mines on a routine basis. Although
not always true, the floor grouting with cement appears to have a compounding effect
on floor emissions, i.e., it makes the emissions less severe as the longwall retreats
outby.

17.2 Gas Outbursts

Lama [4] provides a number of definitions of “gas outburst,” but, in essence, it is a sud-
den ejection of large volumes of gas (methane, CO 2 or both) accompanied by a large
mass of very fine coal from the working face. A classic definition by Hargraves [5] is
perhaps the most appropriate and reads as follows:

The sudden disintegration of coal, and its projection from the seam, without deliberate
initiation and accompanied by, and followed by, enormous gas emissions. The gas has
the effect of carrying the broken coal for considerable distances. The projected coal is
of fine size. The gas pressures and volumes associated are sometimes sufficient to
penetrate the intake airways for considerable distances and blow up stoppings.

Campoli [6] and Lama and Bodziony [3] list the primary factors contributing to gas
outburst as the following:
1. High gas contents and corresponding high gas pressures.
2. Low permeability creating high pressure gradients close to the working face.