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V. DISCUSSION AND CONCLUSIONS • During the first and second quarters of 1988, underground mining activity was concentrated in the headgate and tailgate of Longwall (LW) Panel Number 1. Subsidence monuments were installed along 600 feet of the west portion of LW Panel No. 1 and perpendicular to the east-west line. The north - south monument line extends beyond the projected angle of draw. As indicated in the 1987 Annual Subsidence Survey Report, underground development has progressed beneath Foidel Creek without any measurable surface subsidence taking place and underground development has pr•ogr•essed in 1st Panel South with subsidence taking place in the full pillar extraction area. Monuments SM-15 through SM-31 have experienced subsidence movement from 0.4 feet to 3.2 feet with a maximum subsidence of 3.2 feet on monument SM-26. The data indicate subsidence occurred during the third quarter of 1987; full pillar extraction was conducted during June and July of 1987. First and second quarter data of 1988 demonstrates that movement has stabilized. The spoils slope south of the mine facilities are currently stable at a 2H:1V slope. As noted previously, there is visible evidence of ground movement near monuments SM-19 through SM-26; one inch wide cracks up to twenty feet long were evident. Also, structural damage had occurred to the former Northeast Water Tank; this may have been due to subsidence of the 1st Panel South. • Since the 1987 for.rrth quarter data and the 1988 first and second quarter data leave shown that subsidence has stabilized, Twentymile Coal Company proposes to discontinue monitoring of sites SM-7 through and including $M-41 plus the tlrr•ee water tanks. This proposal will be presented to the Division as a revision in the near future. Tlie next scheduled resurvey of subsidence monuments and surface structures will be conducted in the third and fourth quarter of 1988.