The Holland-Gaddy relationship.

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A coal mine’s entry and breakthrough centers are set at 55 ft with 20-ft-wide openings. The 5.5-ft coal seam lies 850 ft below the surface, and the specific gravity of the overburden is 2.4. If the compressive strength of 4-in. cubic coal specimens is 5,000 psi, determine the safety factor for compressive failure of the pillars utilizing the Holland-Gaddy relationship. Is this value within the recommended range for the safety factor as proposed by Holland-Gaddy?