allow to clearly determine the impact of type of the roof rocks on the air parameters flowing through the gobs. However, the credibility of these results depends on determined physical parameters of rocks forming the goaf with caving and on the measurements of parameters of air flow and methane content of seam.
Part of air stream supplied to the longwall is separate into two streams--one flows into the longwall, second goes to the gob (Fig.
Because laminar flow, transition flow and turbulent flow coexist in the goaf, the flow through the gob is treated as a non-darcy flow.
The porosity can be obtained by K, which is the coefficient of the bulk increase of caving rocks in the gob:
The determination of a co -efficient of longwall gob permeability, Archives of Mining Science 46: 451-468.
Occasionally, incomplete or inaccurate abandoned gob information may lead to a well being mistakenly drilled into the coal pillar around a gob.
During methane drainage, the methane pressure in an abandoned gob becomes relatively low, at which point the gob has a low free methane volume, as indicated by (2).
The bottom of well #1 is located in the gob fractured zone approximately 40 m above the roof of coal seam #3, whereas the bottom of well #2 is located approximately 5 m below the floor coal seam #3.
Therefore, it can be concluded that the methane extraction is better the closer the well bottom is to the bottom of the abandoned mine gob.
The average methane volumetric flow rate at a well whose bottom is close to the gob bottom is 2.5 times greater than that of a well located in the gob fractured zone.
Therefore, in the condition of identical excavating range of coal strata, the average load of coal pillars and flooring in the gobs of lower coal stratum can also be calculated by respectively Equations 9 and 10.
As the compressive ratio of waste rocks in the gobs is relatively large, the weight of overlying rocks in the gobs will be sustained by natural arch of both coal strata, while the weight of waste rocks of the two coal strata applies directly on the bottom as static load.
Similarly, according to vertical force equilibrium of natural arch in the gobs, the load of lower coal stratum flooring on the coal pillars in the gobs can be calculated as:
The loading of flooring of lower coal strata due to the static load of waste rocks of the gobs of the two coal strata can be calculated according to the principle of hydrostatic pressure as:
Similarly, the load of waste rocks in the gobs of lower coal stratum on the coal stratum flooring can be calculated as: