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<br />Powerline Subsidence Mitigation <br />Power structures that start to Ult due to subsidence will be guyed to prevent excessive tilting. These <br />guys will be constructed to limit the tilt, not to eliminate it. Those structures that undergo a permanent <br />tilt will be examined after all subsidence is complete and evaluated to determine if any further mitiga- <br />tion is necessary. Those poles within the panel outline will undergo a net tilt of approximately zero, <br />and will simply be lowered. <br />Powerline Monitoring <br />Power structures in areas of active subsidence were monitored periodically for operational purposes <br />until essentially all residual subsidence was complete. No problems were identified and all appreciable <br />subsidence should have occurred by now. Future monitoring of the powedine due to subsidence <br />concerns is no longer warranted. <br />III.A.7.1 Subsidence Monitoring Program. A program was designed to yield subsidence data that <br />would be used to verify the accuracy of initial predictions under actual ground conditions, and which <br />could be used to predict future effects after monitoring the first longwall and the first room and pillar <br />panel. <br />• BME monitored subsidence over the first longwall panel (LW1) using state-of-the-art analytical <br />photogrammetry. LW1 was mined in calendar year 1987 and the first two months in 1988. <br />Four flights were flown to record and monitor the surface over LW1. The first flight was flown during <br />August, 1986 to establish baseline conditions and base drawings for both LW1 and RP5 for <br />comparison with the future flights. The second and third flights occurred in May 1987 and September <br />1987. Data and drawings from these flights were presented to Colorado Mined Land Reclamation <br />Division (CMLRD), now DMG, in a report in December, 1987 for review. The fourth flight occurred in <br />June, 1988. The fourth flight information and drawings were assembled into a final report and <br />presented to DMG in October, 1988. <br />Residual subsidence may occur after June, 1988, above panel LW1. It is important, for purposes of <br />verifying the accuracy of subsidence projections contained within the permit document, to accurately <br />document the magnitude of subsidence which occurs above this test panel. Conventional semi-annual <br />survey monitoring of the five control points located within the zone of subsidence (see description <br />below) continued until cessation of subsidence was accurately documented. In the event that any of <br />these conventional survey monuments experienced more than 1.0 foot of residual subsidence, an <br />additional aerial photographic flight, subsidence map and report would have been prepared for <br />• t9rr~'~rm Rzdr~as (8;2^JS2} tt',-22 <br />