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December 27, 2023 Page 3-1 <br />Agapito Associates, Inc. <br />3 PILLAR EXTRACTION METHODS <br />Previous research has shown that sudden massive collapse of coal pillars is associated with slender <br />pillars (i.e., width-to-height [w/h] ratio <3) and a pillar safety factors (SF) <1.5 (Mark et al. 1997). <br />At low w/h ratios (<3), overloaded coal pillars tend to fail in a brittle, uncontrolled fashion, <br />whereas at greater w/h ratios (>4), the overloaded pillars demonstrate a more plastic form of <br />deformation. Significant displacement may still take place in the form of roof-to-floor <br />convergence, as well as rib spall. However, the pillar core remains confined and tends to retain its <br />load carrying ability, generally without failing in the commonly understood sense. This behavior <br />was shown by Das (1986) in tests on Indian coals (see Figure 3-1). <br /> <br />Figure 3-1. Complete Stress-Strain Curves for Indian Coal Specimens Showing Increasing <br />Residual Strength with Increasing w/h Ratio (Das 1986) <br />There are typically three design approaches utilized during coal pillar extraction to help control <br />massive pillar collapse. These design approaches are summarized below: <br /> Prevention—The resulting pillars are designed so that after partial pillar extraction, pillar <br />collapse is highly unlikely. This is achieved by stipulating a minimum w/h ratio of 4 and a <br />minimum SF of 1.5 for the remnant pillars. <br /> Containment—High extraction ratios are practiced within individual compartments that are <br />separated by barrier pillars. <br /> Full Extraction—Mining all or most of the coal of the pillars removes the support to the <br />main roof, thereby limiting the potential width of the pressure arch and, in doing so, <br />limiting the load on the surrounding pillars. <br />