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SUBSIDENCE MDNITORIN6 ANALYSIS EYALUATIDN OF EXISTING MDNITDRIN6 MIDELY SPACED GRID Page -2- CONCLUSIONS Based on the information gathered from the widely spaced grid, surveying accura- cy, as determined by human and instrument errors, has a tolerance of +0.10 ft. This appears to be the limit of our surveying instruments. Any variation in el- evation of greater than 0.2 ft, should be attributed to subsidence or other sur- face movement such as mass wasting or land slides. Data evaluated from Monument No, 11 over the 8E-25 panel shows that subsidence- induced surface movement takes place quickly, Approximately three months were required for the subsidence effects to be seen at the surface, No long term subsidence effects are indicated based on continued surveying of this monument, No subsidence is experienced over advance-only entries, Except in a few iso- lated monuments, no movement outside of surveying tolerance was indicated. The subsidence only takes place after secondary pillar mining, RECOMMENDATIONS CWI recommends that Station Nos. 1, 2, 3, 4, 5, 9, 10, 11, and 16 be deleted from further monitoring. The reason for the request is that Station Nos. 3, 4, and 5 have not been undermined and CWI does not intend to undermine these monu- ments. The remaining above-mentioned monuments were surveyed initially after secondary retreat mining. CWI does not feel that additional information may be obtained from any of these monuments, because no movement has been detected. CWI recommends suspension of widely spaced monitoring over advance-only areas. This would only delay monitoring because CWI would again monitor once areas be- low these monuments would be retreat-mined. Those points are Point Nos. 6, 7, 8, 12, 17, 18, 19, and 26. Sufficient information has been obtained on these monuments 5o that future effects can be analyzed. Currently, Point Nos. 14, 15, 22, 23, 24, and 25 are not being monitored. No advance mining has taken place below these monuments and none is anticipated for the near future. The monuments will be monitored again in the future based on actual mining requirements. When CWI submitted Map No. 3-1 in the Permit Revision Application, CWI was min- ing at a rate of approximately 1.3 million tons per year. Because of the re- duced coal demand, CWI plans to mine approximately 300,000 tons for the next few years. The installation of Point Nos. 71, 72, 73, 75, 16, 78, and 79 will not be impacted as soon as was anticipated. For that reason, CWI recommends delayed installation and monitoring of these monuments. None of these monuments will be undermined in 1985 or 1986. Probably none will be undermined in 1987. CWI recommends that normal monitoring proceed on Point Nos. 19 and 27. Mining will definitely take place in the 5N-1W panel below Point No. 19, and in the 9E- 1N-lE section below 27.