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iii iiiiiiiiiiiuiii 999 STATE OF COLORADO DIVISION OF MINERALS AND GEOLOGY Depanmem of Natural Resources 1313 Sherman 51 , Room 215 Denver, Colorado 80203 f'Fhone' 0031 8h6-3567 FA%.11031 R32-8106 DATE: May 15, 1995 TO: Wally Erickson FROM: Jim Pendleton RE: Marr Mine, (] ~~~~'-. r`l0 Pit Stability C-80-006) II~~ DEPARTMENT OF NATURAL RESOURCES Roy Romer Goverrwr lames S. Lochhead Executive Dnecror Michael B Lang Div is~an Director Analysis Pursuant to your request, I have reviewed the stability analysis you forwarded to my attention, TerraMatrix, Inc. has back calculated an angle of internal friction based upon the stable overburden storage pile slope angle. Because it was stable, the 2:1 configuration of the storage pile had to have achieved a slope stability factor of at least 1.0. Using that assumption, TerraMatrix calculated a minimum angle of internal friction for the overburden spoil of 26.0 degrees. This approach is acceptable depending upon the degree of precision required for the final slope stability evaluation. Using the 26 degree angle of internal friction, a cohesion intercept of zero, and an assumed phreatic surface 30 feet below the surface at the head of the backfilled slope, TerraMatrix calculated a static slope stability safety factor for the backfilled Pit 720 slopes of 1.5. Each of these assumptions appears to be relatively conservative. The required slope safety factor for general backfilled slopes is 1.3. In my opinion this approach is acceptable in this instance, in light of the significant exceedence of the required threshold stability value. I suspect, based upon my experience with sloping reclaimed pit walls, that the cracking observed in late 1994 probably resulted from consolidation of the backfilled spoil. In contrast to the mechanical stability of the intact pit highwall, the consolidation of the spoil results in differential cracking parallel to the highwall. If no symptomatic evidence of rotational movement exists, the movement is not due to slope failure. Simple redressing and sealing of the cracks to avoid infiltration of surface runoff will normally prevent the development of rotational failure problems.