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~... iiiiiiiiiiiiiiiiiii 5I nI P GI ~_Oi U~+ni)O m ~~..wi u. ~ ,. ~. ~. .v. ~~. ~ 999 DEPARTMENT OF NATIIR AI RESOURCES D. Mome Pascoe, Eve<utwe O~recror MIN1~U LANU RECLAR~A'I`ION 423 Centen coal Build mg, 1313 Sherman Street Denver. Colorado 80203 Tel.(30~) 839-3567 David C. Shelton Director M E M O R A N D U M 30 October 1991 TO: Tom Gillis FROM: Jim Pendleton l~ RE: Supplemental "tabil.yty and Subsidence Submissions for D1ue/Ribbon Coal Pfine Pursuant to your request, I have reviewed the supplemental materials submitted by Western Associated Coal Corporation for the Blue Ribbon Coal Mine. Z forward the following comments For your consideration. Portal and We11 Sealing: The sealing techniques descriptions and drawings are acceptable, with one deficiency in the companys' presentation. The operator fails to discuss whether mine drainage will occur at the location of the poztals following reclamation and sealing. The hydrology section should be amended to specifically discuss this topic. Portal Bench and NighwaZl Stabiliti The supplemental materials contain discussions of boCh operational and backfilled portal bench and highwall slope stability. The applicant and the applicants' consultants both recognize the existence of a slope instability problem on the portal bench facial slope. The applicant has submitted a remedial plan to eliminate this problem in response to an inspection notice of violation (OC•tober 2?, 1981 letter by Ute Engineering & Surveying Company). Ttris design includes installation of a perforated interceptor drain the and a regrading of the bench surface, both intended to decrease infiltration of moisture into the bench which the applicant believes is the cause of the problem, The applicant further corrmrits, in the amended application, to the completion of appropriate yeotechnical and stability analyses and the installation of three standpipe piezometers to monitor the phreaL'ic surface and degree of saturation of the portal bench. This proposal and the applicants' indication of completion of the remedial construction by February of 198,', can be approved immediately in order to expedite correction of the problern .rnd completion of the geotechnical analyses. The amended application also commits the applicant to completion of a stability analysis of the proposed final backfilled configuration, including shear strength testing and definition of the phreatic surface. The application incorporates a request for variance from AOC based upon the existence of steep slope mining conditions within the portal area.