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BLUE RIBBON COAL COMPANY <br />NOV C-89-008 ABATEMENT PLAN <br />MINE BENCH SLUMP <br />HISTORY <br />The original mine bench was constructed in the late seventies by <br />casting material over the Pace of the bench from excavation <br />required to face up the portal. The mine bench wsa probably <br />unstable from that time on. During late 1984 Blue Ribbon began a <br />bench stabilization project which involved the placement of com- <br />pacted coal waste, underdrains, toe buttresses etc. By mid 1985 <br />all of the coal waste had been placed. Late in 1985, four feet <br />of nontoxic cover material was placed on the face of the bench. <br />The entire mine site was reclaimed during 1986. This work in- <br />cluded placing cover material on the top of the bench and on the <br />lower south portion of the bench. <br />Late in 1986 it became apparent that the top of the bench was not <br />graded appropriately to allow water to freely drain from the <br />area. In June of 1987 the top of the bench was graded to provide <br />positive drainage. <br />The cover material on the face of the bench remained stable <br />during 1986 and 1987. Early in 1988 a superficial slump was <br />noted on the north portion of the bench. This slump was repaired <br />in 1988. Early in 1989 another slump was noted in the center <br />portion of the bench. <br />MONITORING <br />The stabilized mine bench has three types of monitoring devices; <br />1) stability monuments; 2) piezometers; and 3) underdrains. The <br />stability monuments are placed in the coal waste material so they <br />will detect movement in the coal waste material but not in the <br />cover material. Results of the quarterly surveys of the <br />stability monuments have not given anp indication of instability <br />in the compacted coal waste material. <br />Four piezometers have been installed in the mine bench. Three of <br />the piezometers, P-1, P-2 and P-3, are located on the lower bench <br />and one of the piezometers, P-4, is located on the top on the <br />bench. All of the piezometers are completed in the coal waste <br />material so they show the static water level in the fill. <br />Piezometer P-4 gives an indication of the effectiveness of the <br />drainage from the top of the bench. <br />A network or gravel underdrains was installed under the coal <br />waste as it was being placed. These underdrains surface in three <br />1 <br />