In this study the special tests of gas permeability of coal have been conducted in order to make clear the effect of internal crack pressure and external stresses on gas permeability of coal. Practically, gas permeation rate has been measured by changing the differential pressure as a parameter under a constant mean pressure and a constant hydrostatic stress.
As the results of these tests, the relation between the gas permeation rate and the differential pressure had the linearity determined from Darcy's law. The gas permeability of coal decreased almost exponentially in response of an increase of the internal mean pressure and the external hydrostatic stress. From these considerations it became clear that the gas permeable cracks closed corresponding to the effective stress. Particularly, hydrostatic stress closed the permeable cracks irreversibly.
