Handbook for Methane Control in Mining - AMMSA

Figure 5–2.—Simplified illustration of a flow-through systemwith bleeder entries.65controls to maintain a pressure differentialacross the pillared area.Figure 5–2 is a simplified illustrationof a flow-through system withbleeder entries. The coal barriersreplace many of the ventilation controlsand can provide additional protectionfor the bleeder entries fromthe ground pressures created as aresult of extracting the coal pillars.These simplified illustrations do notshow all of the ventilation controlssometimes necessary to direct theairflow, nor do they show the additionalmultiple entries that may benecessary to carry the required quantityof air.Ground control and pillar design.Ground control and pillar design arecritical to the overall stability ofbleeder systems. Some factorsto be considered include miningparameters (e.g., mine openingdimensions, pillar size, panel size,bleeder geometry, and intersectiondimensions), overburden andhorizontal stresses, roof controlsystems, and multiple-seam interaction.Irregular shapes in thebleeder system that concentrateabutment loads generally requireadditional attention. Groundpressures developed as a result of mining can be magnified in such areas and may necessitateadditional support.The life expectancy of the bleeder system must be considered during the design process. Thenecessity of maintaining access and protecting primary internal airflow paths within the pillaredarea cannot be overstated. The long-term nature of bleeder entries and their close proximity topillared areas usually require that supplemental roof support be installed in them. The additionalsupport should be installed in advance of overburden stresses from pillaring because, typically,limited access to bleeder entries makes remedial actions difficult or impossible.Roof support requirements are mine-specific. Wood cribbing has long been recognized as anappropriate supplemental roof support for many bleeder systems. Technology and innovativedesign have provided alternative materials. Concrete donuts, canned cribs, pumpable supports,yielding posts, metal supports, cable bolts, and trusses have been used.