1982-08-24_GENERAL DOCUMENTS - C1981013 (5)

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CF&I will perform a transit survey to an appropriate fixed benchmark (such as a tagged nail, corner of a foundation, or similar point ) for each of structures 14, 27 , 28 , 31 and 32 . These structures are identified in Table 17 and on Map 17 of the permit application. The surveys will be made prior to the area of influence of underground workings expanding to include these structures. The survey will establish the location, elevation, and orientation (by offsets or additional point ) of the structure. These surveys will be repeated on an annual basis to monitor for the effects of subsidence . Should access to any structure noted in the two preceding paragraphs be denied to CF&I for monitoring purposes , the Division will be notified and CF&I will request relief from the monitoring requirement for that structure . CF&I will perform all monitoring proposed in this section and maintain records of them at the mine and in the Pueblo office. The subsidence monitoring monuments to be installed above the First North Mains shall be revised as follows: The location and orientation of the line formed by the monuments will be as shown on Map 17 of the permit application. The spacing between monu- ments will be revised to approximately 100 feet. This new spacing will require 15 monuments installed along the alignment to locate the end points outside the 30-degree angle of draw influence area. All the subsidence monitoring monuments will consist of 2-in. diameter, 6-ft long iron pipes driven into 9-inch diameter, 4-ft deep hole . A 3-in. diameter, flat metal disk will be set on the 2- in. iron pipe and the hole will be backfilled with Portland Cement and sand-concrete. In areas where rock is encountered at a depth of less than 4 ft, the pipe will be driven as far as possible to provide maximum stability for the monument. The attached sketch shows a cross-section of a typical monument . A theoretical calculation of pillar stability with respect to size and percent extraction has not been made for the Maxwell Mine operations . The best verification that the mining methods and pillar configurations that will be employed will not have adverse effect with respect to subsidence is demonstrated by the past mining in the same circumstances and conditions which exist at the Allen Mine. The Allen has been in operation for approximately 30 years and extensive workings underlie the Purgatoire River. Any adverse subsidence effect would have been evidenced by excessive water entry in the mine or effects on the highway and railroad. This has not occurred. Mining recovery rates at the Maxwell Mine will not approach the 50 percent level within the renewable resource area during the five- year permit term. CF&.I has requested a copy of the technique of pillar stability calculation from the Division noted in the draft findings document, for our review.