Abstract:

Conventional room-and-pillar or bord-and-pillar mining geometry (CMG) is typically associated with pillars of uniform size. Though this layout lends itself to simplified mining cut sequences, the ground control and extraction ratio are sub-optimal. In such geometry, pillars around the panel center have lower safety factors while pillars around the barrier pillars have higher safety factors. To overcome the disadvantage of lower extraction ratio and reduced pillar and floor safety factors in the central belt entries, an alternate mining geometry (AMG) concept has been developed. This is a novel concept which utilizes unequal pillar sizes, such that central entries have larger pillars as compared to ones near panel barriers. This allows equalizing pillar safety factors (PSF) and floor safety factors (FSF) across the entire width of a mining panel while simultaneously achieving a higher extraction ratio. This paper discusses the concepts of alternate geometry and their field demonstration in two mines in Illinois.

1. CONCEPTS OF ALTERNATE GEOMETRY

A conventional room-and-pillar mining geometry (CMG) has pillars of uniform size (Figure 1). Such a layout is easy to administer from operations and training points of view but the ground control and extraction ratio are sub-optimal. For such geometry, pillars around the panel center have lower safety factors based on coal and floor strengths while pillars around the barrier pillars have higher safety factors. The term PSF refers to failure of pillar based on coal strength while FSF refers to failure of floor underneath the pillar or foundation failure. Safety factor for roof failure in bending or shear failure is considered for roof stability. This is because pillar size is based on the highest expected load (tributary area theory) while pillar loading increases from edge pillars (lowest) to central pillars (highest). Thus, there is a higher risk of roof falls or floor heave in the central entries where the conveyor belt and other critical infrastructure are located. These disadvantages can be overcome through development of a novel alternate mining geometry (AMG) concept which utilizes unequal pillar sizes across a mining panel, such that central entries have larger pillars as compared to ones near panel barriers. This allows equalizing pillar safety factors (PSF) and floor safety factors (FSF) across the entire width of a mining panel while simultaneously achieving a higher extraction ratio. Furthermore, cut sequencing in a panel with AMG can be optimized to realize higher production rate and improved mine ventilation and dust control at a lower production cost. As a result of improved panel stability, AMG has the potential to reduce ground control costs. Though no accurate economic analyses to assess gains due to better ground control have been conducted yet, it is expected that major economic benefits would accrue due to the lower incidence of roof falls, associated clean-up costs, and productivity delays due to falls in belt entries and travel ways. Over the last 7–8 years, the above concept has been demonstrated in two (2) mines in Illinois in cooperation with coal companies.

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