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-63- permit a 400 foot barrier pillar width (adjoining the mains) and a subsidence test panel in excess of 500 feet. The wider barrier pillar will reduce the possibility of overriding loading on the life-of-mine mains being developed in the area." WECC originally proposed a basic subsidence control scenario emphasizing data collection during the first five-year permit period. The results of the first five year's collection would then be utilized to finalize design of subsidence control specifics in later permit periods. WECC acknowledges that subsidence would occur but proposes to prevent damage to significant permit area features such as the Dry Fork of Minnesota Creek, and Beaver and Minnesota Reservoirs, by utilizing a limited extraction plan beneath these critical features. This limited extraction will ensure that no adverse effects of subsidence would occur. The subsidence evaluation prepared by WECC reflects the current state-of-the-subsidence-prediction-art. It included an analysis of pillar strength in areas proposed for protection by limited extraction without pillar recovery upon retreat. The originally approved WEX'C plan also included the installation and monitoring of the subsidence monitoring network over the first panel to be fully mined, during the third year of the first five-year permit period. Data from this monitoring network was to be utilized to verify the design of the buffer zones proposed beneath the Dry Fork of Minnesota Creek and to predict the magnitude of subsidence and subsidence phenomena to be expected throughout the remainder of the lease property. Additional future subsidence monitoring networks shall also be proposed for the Dry Fork area to assure that subsidence impacts are prevented. As a portion of its November, 1984 permit revision application to add 320 acres,WECC proposed a relocation of the originally required subsidence test panel and accompanying subsidence monitoring network. Reconfiguration of the underground mine plan, nxessitated by an approved reorientation of the mains to avoid an area of bad coal and roof conditions, resulted in a need to relocate the first panel to be subsided. The relocation was not significant, allowing the originally installed ground water monitoring wells to serve their original purpose, monitoring ground water response to subsidence of the test panel. However, the surface subsidence monument locations required amendment. The amended monitoring network is depicted in Figures 7, 8 and 9 of Appendix A to WECC's November, 1984 permit revision application. A row of monuments installed parallel to the axis of the test panel will be spaced on 100-foot centers. Two rows of monuments would transect the test panel and would be spaced on nominal 50-foot centers. These transverse monument rows would extend to the east of the panel centerline above the mains and to the west for at least 750 feet to the west of the panel centerline. PIIX.'C would tie the monitoring to a network of more permanent surveying triangulation bench marks to be established beyond the area of potential subsidence influence. Resurveying of triangulation monuments will be performed annually, to allow correlation of semi-annual aerial photogrammetric surveys discussed hereafter.