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-53- <br />prepare and submit a subsidence evaluation which properly addressed <br />the requirements of Rule 2.05.6(6). WECC prepared, submitted and <br />subsequent]y revised a report entitled "Subsidence Eva]uation for <br />Mt. Gunnison No. 1 Mine", prepared by WECC's Mine Engineering Staff <br />and included as exhibit 3.4.8.A within the amended permit <br />application. <br />As a portion of its November, 1984, "Application to Revise the <br />Mining and Reclamation Plan for the Mt. Gunnison No. 1 Mine", WECC <br />submitted a revised subsidence monitoring plan. WECC states in the <br />application that the requested 320 acre permit area expansion <br />" ..is, in part, to accommodate a revision to the underground <br />mining sequence necessitated by geologic conditions encountered <br />during mining which required that the mains be relocated". <br />Further, the requested area expansion will allow the relocation of <br />the required subsidence test panel. WECC observes, and the <br />Division concurs, "Relocating the 5-year permit boundary westward <br />will permit a 400 foot barrier pillar width (adjoining the mains) <br />and a subsidence test panel in excess of 500 feet. The wider <br />barrier pillar will reduce the possibility of overriding loading on <br />the life-of-mine mains being developed in the area." <br />WECC originally proposed a basic subsidence control scenario <br />emphasizing data collection during the first five-year permit <br />period. The results of the first five year's collection will then <br />be utilized to finalize design of subsidence control specifics in <br />later permit periods. WECC acknowledges that subsidence will occur <br />but proposes to prevent damage to significant permit area features <br />such as the Dry Fork of Minnesota Creek, and Beaver and Minnesota <br />Reservoirs, by utilizing a limited extraction plan beneath these <br />critical features. In addition, extraction will be limited to <br />development beneath the landslide body delineated above the <br />Somerset townsite. Exhibit 3.4.8.A, in its amended form states "a <br />detailed analysis of the slope stability, incorporating the <br />influence of subsidence on the stability of the landslide, will be <br />performed prior to pi]lar robbing beneath the ]andslide. This <br />analysis shall be submitted to MLRB for approval before pillar <br />recovery begins". (page 30, exhibit 3.4.8.A) <br />The subsidence evaluation prepared by WECC reflects the current <br />state-of-the-subsidence-prediction-art. It includes an analysis of <br />pillar strength in areas proposed for protection by limited <br />extraction without pillar recovery upon retreat. The originally <br />approved WECC plan also included the installation and monitoring of <br />the subsidence monitoring network over the first panel to be fully <br />mined, during the third year of the first five-year permit period. <br />Data from this monitoring network is to be utilized to verify the <br />design of the buffer zones proposed beneath the Dry Fork of <br />Minnesota Creek and to predict the magnitude of subsidence and <br />subsidence phenomena to be expected throughout the remainder of the <br />lease property. Additional future subsidence monitoring networks <br />shall also be proposed for the Dry Fork area to assure that <br />subsidence impacts are prevented. <br />