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hBVI` agrleUll Ur:d significance weal uF tha 1011th meridian. Federal and stag laws rcyuirc that mining activi- ties cannot disturb AVF areas, m- cludinq mine subsidence of such FIG. 2 features. The firs[ stop was to work with the state reeulatory agency to set up subsidence monitoring for AVF im- pacts. As shown in Fig, 5, the west- ern longwall panels bordered on the Fish Creek AVF. They were pro- puscd to be extended under the AVF to lengthen the panels and to provide a test area with the state agency to monitor longwall subsid- ence in an AVF area. Subsidence and h_vdruloeic monitorin, with de- tailed surveying was put in place [o monitor three panels that affected the AVF area during athree-year period. The other major element for handling the two AVF areas in the proposed EMD was to pursue the option of having the areas reclassi- fied from AVF designation. This proved to be successful through de- tailedsurveying ofthe areas and the fact that most of the AVF areas did not meet AVF crite- ria. The resulting subsidence data from the monitoring of the three longwall panels in the west proved to be beneficial, showing minimal impacts to [he hydrology of the stream over time. I[ is important [u note that al- though the AVF areas in the EMD were reclassified, the study allowed the permit process to continue and ulti- mately be approved with [he AVF and now non-AVF ar- eas over the long panels. Power lines. The three major power lines that bisect the long panels consisted of 340, 240 and 135 kV lines and structures. The 340 kV line is a lattice structure. The other two lines are H-frame pole structures. A 69-kV power line crossed two of the longwall panels in the western area and was subsided without damage. This was important [o show the owners in the EMD [hat wooden structures could be undermined with minimal damage. The 340 k V power line and steel structures caused the most concern to the owner and the mine. Several meetings were conducted early to educate the owners on the effects of longwall mine subsidence. These meetings were beneficial in establishing the engineering criteria required to show the impact to the structures. A test area was established over one of the western longwall panels with steel structure foundations, such as those installed on the actual power lines. The steel lattice was not installed because the survey monitoring of the inundations was believed to be adequate to show the impacts. Figure 6 shows the foundation locations relative to the longwall panel where the impacts of maximum subsidence and strain would occur. A remote total station was purchased to allow for continuous monitoring of the foundations so that no data would be excluded as the longwall subsided the foundations. A structure was assembled to house the in- strumentand computer for the continuous monitoring to protect i[ from weather and animals. The data collected was instrumental in establishing the magnitude of how the foundations would move in the x, y and z planes. As was anticipated. the differential settling between the four legs of the towers was the criti- calelement that would affect the structure most. A mitigation plan was put together based on [he data collected and the experiences of the owners in deal- ing with towers affected by ground movement in other areas. The primary portion of the mitigation plan is the daily surveying of the tower legs for movement and a vi- sual inspection Eor damages. The mitigation plans were submitted as part of the permit process. They were ap- proved based on all parties acceptance of the ability to subside the power line structures without catastrophic impact to utility service. Railroad spur. The railroad spur that feeds the mine operation traverses the EMD across the long panels for about 6 km (4 miles) (Fig.4). The major factor in obtain- ing the mitigation plan for railroad subsidence for the mining permit started with educating the railroad about longwall subsidence. Detailed mapping of the existing railroad was ac- quired from the owner and then transferred into a com- puter-aid design system to establish premining and postmining subsidence profiles of the railroad spur. Sub- sidence prediction and profiles were calculated based on several panels of actual subsidence data from the west- ern mining district. The data was then used to determine where grades in the railroad spur would exceed the maximum of +J- 2°f°. Mitigation plans were then devel- oped to reestablish the proper grades. The plan was ac- MINING ENGINEERING ^ DECEM9ER 1998 Z$ Original eastern mining district at Twentymile, shown to the right of the western mining district.