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African gold mines in the 1950's and 60's using concrete plugs that would withstand a hydrostatic pressure in excess of 6,000 psi. The following equation • was developed for a parallel plug (installed parallel to the roof rib and floor without recessing or tapering): L = a b L = Length of plug, ft. a+b fc a = Width of entry, ft. b = Height of entry, ft. p = Hydrostatic pressure, psi fc= Allowable compressive strength of rock or concrete, whichever is the lesser, psi. The height and width,.. a and b, will be the primary variable in determining the length, 1, of the plug. The roof, ribs and floor will be cleaned to compe- tent material but will not be recessed into solid rock or coal. The maximum calculated hydrostatic pressure, p, for the No. 1 Mine is 76' of water or 33 psi, page 4-115b. To maintain a conservative approach to the plug design, a hydrostatic pressure of 100 psi will be used. On the No. 3 determine the hydrostatic head, p, is more difficult because of the unknown source of inflows. Mining in the No. 3 Mine was done within 280' of the old Spring Gulch Mine. The largest inflows are from a suspected fault zone at a point closest to the Spring Gulch Mine. It is unknown whether the inflow is hydrologically related to the old mine or to fault connections with the surface. The water level in the old mine is unknown but its portals are 800' higher than the No. 3 Mine portals. The inflows show seasonal variation which would indicate a surface connection. A conservative approach to plug design in the No. 3 Mine is to use the potential 800' of water between the portals or 350 psi. Coal will have the limiting compressive strength on approximately half of the perimeter and concrete will have the limiting compressive strength on half of the peri- meter. In triaxial tests on coal in the No. 1 Mine the compressive strength 4-66a New 5/11/87