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,~ <br />. Tif/° <br />STATE OF Cv~viv-~~O <br />DIVISION OF MINERALS AND GEOLOGY <br />Depanmem of Natural Resources <br />1313 Sherman St., Room 215 <br />Denver, Colorado 80203 <br />Phone: 130318663567 <br />FA%:1303)832-8106 <br />May 25, 1995 <br />I~~ <br />DEPARTMENT OF <br />NATURAL <br />RESOLIIZCES <br /> Ray Rumer <br /> Governor <br />MI. Jim 1{i er <br />g <br />Senior Environmental Engineer <br />Colowyo Coal Company lames 5. Lochhead <br />Executive Director <br />Michael B. Long <br />Division Director <br />5731 State Highway 13 <br />Meeker, CO 81641 <br />RE: NOV C-90-005 <br />Colowyo Coal Mine (C-81-019) <br />Dear Mr. Kiger: <br />Following review of aerial and on-the-ground observations of the West Pit area during <br />recent inspections, the Division has determined that a modification of the abatement <br />measures for NOV C-90-005 is warranted. We feel additional analysis and design work for <br />the hillside next to the West Pit pond and fill is warranted because instability within the <br />disturbed portion of the hill has continued to express itself despite previous efforts at <br />control. The modified abatement measures are necessary to address the stability of the <br />pond and the slope above the pond which has exhibited increasing signs of instability during <br />recent weeks. The NOV modification is enclosed, and additional information is provided <br />in this letter. The modification will essentially allow the NOV to be terminated upon <br />completion of analysis and design work by Colowyo, and commitments to implement any <br />necessary repairs in a timely manner. <br />The Division is requiring submittal and approval of a technical revision to address the <br />stability of the slope near the pond. The adequacy of the pond's capacity should be <br />reevaluated after any remedial measures have been completed. In order to assess the <br />stability of the slope, a triaxial shear analysis of representative material on the slope and/or <br />pond in-slopes would be appropriate. This information, in conjunction with identification <br />of the phreatic surface in the slope, and identification of slip surfaces will facilitate the <br />design of appropriate remedial measures for stabilizing the slope and pond. The design <br />documentation should include an appropriate slope stability analysis. Piezometers and slope <br />movement monuments should be installed to verify conditions following slope repair. <br />Installation of the piezometers prior to remedial efforts would be most beneficial since the <br />effects of repair work on the phreatic surface could actually be verified. <br />