The seam gas content at Westcliff Colliery is 12-15m3/tonne of coal.Composition ranges from 99% CH4 to 52% C02 in one area._x000D_
Desorption rates are proportional to gas contents and CH4 is preferentially desorbed. A crushed zone of reduced permeability advances with the face - at Westcliff ribside crushed zones are 2.5 to 6.5m wide. Poor strata control widens crushed zones. 12 months of pre-drainage allowed 2 heading development and longer pillars to be used in a virgin area instead of 3 or more headings without drainage. Machined coal may lose as much as 1.6m3/tonne being conveyed out of Westcliff Colliery. Experience differs from mine to mine. Permeation is affected by time, strata movements, changing water content, and changing stress. Permeabilities vary from lOmd for Pittsburgh Seam to less than 1 and for the Bulli Seam. All these data and differences await further investigation.