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In Table 1 examples of values of permeability coefficient of gobs for different type of roof rocks and different distance from the front longwall (for lenght of the goaf with caving is 400.0), calculated based on dependences 3-4 are given.
The aim was to determine distribution of physical parameters of air stream flowing through these gobs.
Way of air distribution in the gobs for real system of roof rocks is presented in Fig.
4 and 5 there are presented characteristics of pressure changes in gobs for tested variants of roof rocks.
The desorbed (released) methane in the gobs is extracted by utilizing the negative pressure at the well bottom produced by a water-ring vacuum pump.
The methane flow in abandoned mine gobs can be described by Darcy's law :
The desorption rate of the residual coal in abandoned mine gobs initially decreases sharply with increasing desorption time, after which it decreases more gradually .
At this point, it is advisable to cease drainage for a period to allow the methane in the coal pillar, roadway, or other gobs to gradually accumulate in the gob where the well is located; over time, the residual coal in this gob can thus gradually return to the initial methane pressure.
In order to analyse the load-carrying characteristics of the combined natural arch structure of multi-gob roofing of Jurassic close distance coal strata in Datong minefields, the natural arch structure of the gobs after the excavation of a single working face of coal strata is analysed first.
The geometric shape of the natural arch structure of the roofing of the gobs is generally elliptical (Miao, 1990; Wang et al., 2007; Du et al., 2011; Wang and He, 2006).
In order to calculate the self-weight of the upper roof structure on the coal pillars in the gobs, the shaded area in Figure 4 can be calculated as:
According to the principle of area complementation (as shown in Figure 5), the self-weight of the upper roof structure on a single coal pillars in the gobs can be calculated as:
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