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<br />Tom Gillis -3- February 19, 1982 <br />Backfill and Cradinq <br />The application includes an acceptable request for variance from approximate <br />original contour restoration requirements for steep slope mining, in <br />conformance with Rule 2.06.5. V.u~i~mceis requested for 3 surficial disturbace <br />sites, the Hubbard and Bear Canyon ventilation portal facilities and the <br />Bear Canyon B-seam Degas sites. The 26-acre Elk Creek surface facility <br />area will be reclaimed to approximate original contour. Mining has been <br />conducted on this site since 1902, making pre-mining contours difficult to <br />ascertain. Ilowever, the proposed post-mining configuration appears to be <br />consistent with natural surrounding terrain. The applicant states that slopes <br />will not erceed 2:I (horizontal to vertical). If the operator exercises <br />appropriate care in placement and compaction of the backfilled materials, it <br />should not be difficult to achieve the required static slope safety Factor of <br />1.3, in conformance with Rule 9.19.2(1)(b). <br />Sealing of Mine Portals (a portion of Findings Section r.YII) <br />a <br />The applicant proposes to seal all mine openings in accordance with MSHA <br />regulations, as described in the Code of Federal Regulations, Chapter 30 <br />CFR 75.1711-2. Solid concrete block stoppings will be built and a minimum of <br />25 feet of non-combustible material will be placed outby those block stoppings. <br />The block stoppings will be keyed into the walls and roof and founded upon a <br />concrete footer keyed into the floor of the passage. The surface entrance of <br />the mine portals will be graded to approximate original contour upon <br />completion. This proposal conforms with the requirements of our regulations. <br />Subsidence (Findings Section XXIII) <br />The applicant has inventoried structures and renewable resource lands within <br />and adjoining the proposed permit area. That inventory has determined that <br />structures which exist within the area potentially affected by subsidence are <br />limited to the support facilities at the Blue Ribbon mine in Hubbard Creek <br />Canyon. The applicant also has determined that a portion of the Hubbard Creek <br />Valley and its contained alluvial aquifer, which constitutes a renewable <br />resource Iand, exists within the potentially affected area. <br />The applicant proposes to prevent damage to these areas and structures through <br />a plan of limited extraction. An 800-foot wide barrier of coal will be left <br />beneath the potentially affected canyon and the adjoining Blue Ribbon mine <br />structures. The only extraction proposed to occur within that barrier pillar <br />would be the 5th West Main entries. Extraction of the six 20-foot wide entries <br />on 90-foot centers with 20-foot wide cross cuts on 100-foot centers would amount <br />to an overall extraction ratio of 37.8. The applicant completed a pillar <br />stability calculation, based upon a technique developed by Obert and Duval, <br />which determined a pillar stability safety Factor of a minimum of 3.69 for this <br />extraction configuration. A safety factor of that magnitude normally implies <br />conservative factoring against pillar failure induced subsidence. <br /> <br />