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• 1993 MINE BENCH REPAIR PLAN HISTORY The original mine bench was constructed in the late seventies by casting material over the face of the bench from excavation required to face up the portal. The mine bench was probably unstable from that time on. During late 1984 Blue Ribbon Coal Company began a bench stabilization project which involved the placement of compacted coal waste, underdrains, toe buttresses etc. By mid 1985 all of the coal waste had been placed. Late in 1985, four feet of nontoxic cover material was placed on the face of the bench. The entire mine site was reclaimed during 1986. This work included placing cover material on the top of the bench and on the lower south portion of the bench. Late in 1986 it became apparent that the top of the bench was not graded appropriately to allow water to freely drain from the area. In June of 1987 the top of the bench was graded to provide positive drainage. The cover material on the face of the bench remained stable during 1986 and 1987. Early in 1988 a superficial slump was noted on the north portion of the bench. This slump was repaired in 1988. Early in 1989 another slump was noted in the center portion of the • bench. This slump was repaired in 1989. Early in 1993 another more extensive slump occurred at the location of the 1989 slump and a minor slump occurred at the location of the 1988 slump. MONITORING The stabilized mine bench has three types of monitoring devices; 1) stability monuments; 2) piezometers; and 3) underdrains. The stability monuments are placed in the coal waste material so they will detect movement in the coal waste material but not in the cover material. Results of the quarterly surveys of the stability monuments have not given any indication of instability in the compacted coal waste material. Six piezometers have been installed in the mine bench. Three of the piezometers, P-1, P-2, and P-3, are located on the lower bench. Two of the piezometers, P-5 and P-6, were installed on the bench face near the 1989 slump in 1989. One of the piezometers, P-4, is located on the top on the bench. All of the piezometers are completed in the coal waste material so they show the static water level in the fill. Piezometer P-4 gives an indication of the effectiveness of the drainage from the top of the bench. Piezometers P-5 and P-6 are located on the face of the bench in the area of the 1989 cover material slump. P-5 is 28.2 feet deep with • a bentonite seal at 8 feet. It is designed to measure the water 113v (Rev. 06-25-93)