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t <br />1- III IIIIIIIIIIIII III <br />sss <br />STATE OF COLORADO <br />DIVISION OF MINERALS AND GEOLOGY <br />Department of Natural Resources <br />1313 Sherman 51., Room 215 I+~`~ <br />Denver, Colorado 80203 I I` <br />Phone. (303) 866-3567 <br />FAX: (303) 832 8106 <br />DEPARTMENT OF <br />NATURAI. <br />DATE: September 7, 1995 RESOURCES <br />~-~ Roy Romer <br />TO: Susan Burgmaier c°~er"°r <br />dames 5 Lochhead <br />~(~ Executive Direcor <br />FROM: Jim Pendleton - ~~^^~~ Michael B. Long <br />Division Director <br />RE: Edna Mine C sure Moffat Area Stability <br />Addend ermit No: C-80-001) <br />Following upon our discussion of this morning, I commit the <br />following additional suggestions to writing for your consideration. <br />My memo of yesterday expressed my continued concern regarding a <br />pore pressure precipitated destabilization of the backfilled pit <br />similar to that which occurred at Center Ridge. This morning you <br />informed me that P & M has already commenced backfilling of the pit <br />in anticipation of their revised post-mining topography approval. <br />In doing so they have eliminated the opportunity to easily <br />perforate the parting between the Wadge and the Lower Wadge seams. <br />I continue to believe it would be prudent to enhance the drainage <br />below the parting. <br />At the moment the pit and surrounding area is disturbed. If the <br />monitoring wells were to document a rise in the phreatic surface <br />beneath the parting in the future, the Division would direct P & M <br />to perforate and weep the parting. This would result in <br />significant redisturbance and expense. Redisturbance would extend <br />the operator's liability period and prevent bond release. It is a <br />lot more practical to complete the required excavation now. I <br />recommend, as an alternative to excavating windows in the base of <br />the pit, that we direct P & M to excavate and construct several <br />linear french drains parallel to the slope fall line. These french <br />drains should be located to cross the subcrop of the parting. They <br />should be approximately ten feet wide, ten feet deep and one <br />hundred feet in length. After excavation they should filled with <br />durable rock and covered with a filter layer, subsoil and topsoil. <br />A gravel toe weep should be created at the downgradient end and <br />drained into the ditch. <br />If P & M prefers not to complete the installation of the toe <br />drains, I believe we should direct them to analyze the stability of <br />their proposed revised post-mining configuration. This analysis <br />should employ sensitivity variation techniques directed at <br />