Evolution and distribution of macroscopic gas channels in an overburden strata

The evolution of gas bearing channels in the roof, and their spatial distribution, was studied. A complete consideration of gas flow changes through the stress–strain changes in the roof near a working face is made. The theoretical abutment pressure distribution using displacement monitors and borehole visual recording instruments allow a theoretical analysis. Field test research determined the conditions for formation of macroscopic gas channels. These appear along the working face roof, normally distributed to it. These results show that the coal rock stratification becomes a macroscopic gas channel boundary if its deformation is less than the lower layer, or greater than the layer above it. At the same time the stability is greater than the distance from the roof for hanging dew conditions. The working face advances and the roof gas channels experience a cycle of development. Microscopic channels dominate the initial stage then macroscopic gas channels form, develop, and close. The evolution of the macroscopic channels depends on the ratio between the distances from the new compaction area in the goaf to the initial stress area in front of the working face. The amount of daily advance of the face also affects channel development. The experimental observations in one mining area showed that the main gas channels are located about 2 and 6.2 m above the lower surface of the roof and that they have an evolution period 7 to 11 days long.