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December 27, 2023 Page 4-1 <br />Agapito Associates, Inc. <br />4 PILLAR STABILITY CRITERIA <br />4.1 Design Standards <br />In keeping with the pillar design standards outlined for the full extraction approach, the following <br />has been used as a minimum baseline for the pillar extraction plans in the Sunset Mains South: <br /> Following extraction of the mains pillars, the remnant pillars must not remain in a load <br />bearing condition and, as such, no more than 10 ft of coal is left in the remnant. <br /> For those pillars where a substantial amount of the pillar will remain after lifting (i.e., the <br />gateroad and chute road pillars), the pillars Safety Factor (SF) must be a minimum of 1.5. <br /> Following pillar extraction, the pillars next to the life-of-mine roadways shown in Figure 5 <br />must retain a minimum SF of 1.5. <br /> A minimum stand-off distance of 50 ft between a life-of-mine roadway and the inby extent <br />of the pillar extraction should be applied. <br /> Pillar extraction will be conducted by the outside-lift Christmas tree method. <br /> The lifts will be a maximum of 37 ft long. <br />4.2 Pillar Strength Methodology <br />As shown in Figure 2-1, most of the existing pillars are rectangular in shape. The strength of <br />rectangular pillars can be significantly greater than square pillars due to the greater confinement <br />generated within them (Mark et al. 1995). On this basis, the following Mark-Bieniawski three- <br />dimensional (3D) formula will be used to calculate the strength of the pillars: <br /> 𝜎௣ ൌ𝑆௜ ቂ0.64 ൅ 0.54 ቀ௪ <br />௛ ቁെ0.18 ቀ௪ మ <br />௛௟ ቁቃ(Eqn. 4-1) <br />  <br />where p = pillar strength (pounds per square inch [psi]) <br /> Si = in-situ coal strength (taken as 900 psi) <br /> w = pillar width (ft) <br /> l = pillar length (ft) <br /> h = pillar height (ft) <br /> <br />4.3 Pillar Loading for Development Conditions <br />As the width of the pillar extraction panels in Sunset Mains South will be less than the overburden <br />thickness (H), full Tributary Area loading has been assumed (see Figure 4-1). Tributary Area <br />loading assumes that the pillar carries its proportional share of the full overburden load up to the <br />surface and can be defined as follows: <br /> <br />