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• subcritical width, some valuable data such as maximum <br />possible subsidence and maximum possible surface strains may <br />not be measured even from this closely-spaced network. A <br />maximum of useful information will be obtained by continuing <br />monitoring until settlements have ceased, including <br />settlements due to mining panels other than that which the <br />monitoring network directly overlies. The monitored panel <br />should be close to the central portion of an extraction area <br />to provide as much information as possible on maximum <br />subsidence and final subsidence profiles. <br />Any tension cracks which are observed during mining <br />should be surveyed and accurately located on a map showing <br />the surface topography and the underlying mine layout. The <br />progress of crack development should be plotted with and <br />correlated to the mining sequence. This provides a means of <br />verification through field measurement of the assumed angle <br />of break. Regular visual observations above all mined areas <br />should be aimed at early identification of subsidence <br />cracks. <br />If mining is extended beyond the current lease area, a <br />widely-spaced monitoring network similar to that existing on <br />the current lease should be extended to cover all mined <br />• areas. <br />Regular and accurate monitoring of subsidence provides <br />the only means of verifying subsidence predictions. Re- <br />evaluation of subsidence predictions in light of reliable <br />monitoring results can provide a basis and justification for <br />a less conservative mine design in which more coal is <br />recovered from protective pillars. <br />June, 1963 <br />Michal Bukovansky <br />Senior Geotechnical Engineer <br />Gra2me Major <br />dining Engineer <br />• <br />__% _ <br />